netlink/

neighbors.rs

// Copyright 2026 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! A module for managing neighbor information by receiving RTM_*NEIGH Netlink
//! messages and maintaining neighbor table state from Netstack.

use std::collections::{BTreeMap, HashMap, HashSet};
use std::net::IpAddr;
use std::num::NonZeroU64;

use crate::Errno;
use crate::client::{ClientTable, InternalClient};
use crate::logging::{log_debug, log_warn};
use crate::messaging::Sender;
use crate::multicast_groups::ModernGroup;
use crate::netlink_packet::UNSPECIFIED_SEQUENCE_NUMBER;
use crate::protocol_family::ProtocolFamily;
use crate::protocol_family::route::NetlinkRoute;
use crate::util::respond_to_completer;
use derivative::Derivative;
use futures::StreamExt as _;
use futures::channel::oneshot;
use linux_uapi::rtnetlink_groups_RTNLGRP_NEIGH;
use net_types::ip::IpVersion;
use netlink_packet_core::{
    NLM_F_APPEND, NLM_F_CREATE, NLM_F_EXCL, NLM_F_MULTIPART, NLM_F_REPLACE, NetlinkMessage,
};
use netlink_packet_route::neighbour::{
    NeighbourAddress, NeighbourAttribute, NeighbourFlags, NeighbourHeader, NeighbourMessage,
    NeighbourState,
};
use netlink_packet_route::route::RouteType;
use netlink_packet_route::{AddressFamily, RouteNetlinkMessage};
use thiserror::Error;

use fidl_fuchsia_net as fnet;
use fidl_fuchsia_net_ext as fnet_ext;
use fidl_fuchsia_net_interfaces_ext as fnet_interfaces_ext;
use fidl_fuchsia_net_neighbor as fnet_neighbor;
use fidl_fuchsia_net_neighbor_ext as fnet_neighbor_ext;

/// NetlinkNeighborMessage conversion related errors.
#[derive(Debug, PartialEq)]
pub(crate) enum NetlinkNeighborMessageConversionError {
    /// Interface id could not be downcasted to fit into the expected u32.
    InvalidInterfaceId(u64),
}

/// A wrapper type for the netlink_packet_route `NeighbourMessage` to enable conversions
/// from [`fnet_neighbor_ext::Entry`].
#[derive(Clone, Debug, Eq, PartialEq)]
pub(crate) struct NetlinkNeighborMessage(pub(crate) NeighbourMessage);

impl NetlinkNeighborMessage {
    pub(crate) fn optionally_from(
        neighbor: fnet_neighbor_ext::Entry,
    ) -> Option<NetlinkNeighborMessage> {
        match neighbor.try_into() {
            Ok(message) => Some(message),
            Err(NetlinkNeighborMessageConversionError::InvalidInterfaceId(id)) => {
                log_warn!("Invalid interface id found in neighbor table entry: {}", id);
                None
            }
        }
    }

    /// Wrap the inner [`NeighbourMessage`] in an [`RtnlMessage::NewNeighbour`].
    pub(crate) fn into_rtnl_new_neighbor(
        self,
        sequence_number: u32,
        is_dump: bool,
    ) -> NetlinkMessage<RouteNetlinkMessage> {
        let NetlinkNeighborMessage(message) = self;
        let mut msg: NetlinkMessage<RouteNetlinkMessage> =
            RouteNetlinkMessage::NewNeighbour(message).into();
        msg.header.sequence_number = sequence_number;
        if is_dump {
            msg.header.flags |= NLM_F_MULTIPART;
        }
        msg.finalize();
        msg
    }

    /// Wrap the inner [`NeighbourMessage`] in an [`RtnlMessage::DelNeighbour`].
    pub(crate) fn into_rtnl_del_neighbor(self) -> NetlinkMessage<RouteNetlinkMessage> {
        let NetlinkNeighborMessage(message) = self;
        let mut msg: NetlinkMessage<RouteNetlinkMessage> =
            RouteNetlinkMessage::DelNeighbour(message).into();
        msg.finalize();
        msg
    }
}

impl TryFrom<fnet_neighbor_ext::Entry> for NetlinkNeighborMessage {
    type Error = NetlinkNeighborMessageConversionError;

    fn try_from(
        neighbor: fnet_neighbor_ext::Entry,
    ) -> Result<NetlinkNeighborMessage, NetlinkNeighborMessageConversionError> {
        let mut header = NeighbourHeader::default();
        let fnet_ext::IpAddress(addr) = neighbor.neighbor.into();
        header.family = match addr {
            IpAddr::V4(_) => AddressFamily::Inet,
            IpAddr::V6(_) => AddressFamily::Inet6,
        };
        header.ifindex = neighbor.interface.get().try_into().map_err(|_| {
            NetlinkNeighborMessageConversionError::InvalidInterfaceId(neighbor.interface.get())
        })?;
        header.state = match neighbor.state {
            fnet_neighbor::EntryState::Delay => NeighbourState::Delay,
            fnet_neighbor::EntryState::Incomplete => NeighbourState::Incomplete,
            fnet_neighbor::EntryState::Probe => NeighbourState::Probe,
            fnet_neighbor::EntryState::Reachable => NeighbourState::Reachable,
            fnet_neighbor::EntryState::Stale => NeighbourState::Stale,
            fnet_neighbor::EntryState::Static => NeighbourState::Permanent,
            fnet_neighbor::EntryState::Unreachable => NeighbourState::Failed,
        };
        // Unlike Linux, Netstack3 only keeps unicast addresses in its neighbor
        // tables so there's no need to derive this from the address and/or
        // interface properties.
        header.kind = RouteType::Unicast;

        let mut attributes = vec![];
        attributes.push(NeighbourAttribute::Destination(match addr {
            IpAddr::V4(addr) => addr.into(),
            IpAddr::V6(addr) => addr.into(),
        }));
        if let Some(mac) = neighbor.mac {
            attributes.push(NeighbourAttribute::LinkLocalAddress(mac.octets.into()));
        }
        // TODO(https://fxbug.dev/488135156): Include the `CacheInfo` attribute
        // with the last update time set.

        let mut msg = NeighbourMessage::default();
        msg.header = header;
        msg.attributes = attributes;
        Ok(NetlinkNeighborMessage(msg))
    }
}

fn neighbor_fidl_ip(
    family: AddressFamily,
    address: Option<&NeighbourAddress>,
) -> Result<fnet::IpAddress, RequestError> {
    match family {
        AddressFamily::Inet => match address {
            Some(NeighbourAddress::Inet(addr)) => Ok(fnet_ext::IpAddress(IpAddr::V4(*addr)).into()),
            Some(_) => Err(RequestError::AddressFamilyMismatch(family)),
            None => Err(RequestError::MissingIpAddress),
        },
        AddressFamily::Inet6 => match address {
            Some(NeighbourAddress::Inet6(addr)) => {
                Ok(fnet_ext::IpAddress(IpAddr::V6(*addr)).into())
            }
            Some(_) => Err(RequestError::AddressFamilyMismatch(family)),
            None => Err(RequestError::MissingIpAddress),
        },
        _ => Err(RequestError::InvalidAddressFamily(family)),
    }
}

/// Arguments for an RTM_GETNEIGH [`Request`].
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(crate) enum GetNeighborArgs {
    Dump { ip_version: Option<IpVersion>, interface: Option<NonZeroU64> },
    Get { ip: fnet::IpAddress, interface: NonZeroU64 },
}

impl GetNeighborArgs {
    // Attempts to convert a netlink_packet_route `NeighbourMessage` into
    // `GetNeighborArgs`.
    pub(crate) fn try_from_rtnl_neighbor(
        message: &NeighbourMessage,
        is_dump: bool,
    ) -> Result<Self, RequestError> {
        if is_dump {
            Self::dump_request_from_rtnl_neighbor(message)
                .inspect_err(|e| log_debug!("{e} in dump neighbors request"))
        } else {
            Self::get_request_from_rtnl_neighbor(message)
                .inspect_err(|e| log_debug!("{e} in get neighbors request"))
        }
    }

    fn dump_request_from_rtnl_neighbor(message: &NeighbourMessage) -> Result<Self, RequestError> {
        let NeighbourHeader { family, flags, .. } = &message.header;
        if flags.contains(NeighbourFlags::Proxy) {
            // Netstack3 does not support ARP/NDP proxying.
            // TODO(https://fxbug.dev/42111873): Support ARP/NDP proxying.
            log_warn!("unsupported Proxy flag in dump neighbors request");
            return Err(RequestError::UnsupportedFlags(*flags));
        }
        // TODO(https://fxbug.dev/500013355): Support strict validation of dump
        // requests.
        let ip_version = match family {
            AddressFamily::Unspec => None,
            AddressFamily::Inet => Some(IpVersion::V4),
            AddressFamily::Inet6 => Some(IpVersion::V6),
            family => {
                return Err(RequestError::InvalidAddressFamily(*family));
            }
        };
        // Note that the interface index is pulled from the attribute here,
        // whereas it's pulled from the header for get requests. This is
        // intentional, in order to maintain consistency with Linux's behavior.
        let interface = message
            .attributes
            .iter()
            .find_map(|attr| match attr {
                NeighbourAttribute::IfIndex(ifindex) => Some(u64::from(*ifindex).try_into()),
                _ => None,
            })
            .transpose()
            // 0 is treated as a lack of filter.
            .unwrap_or(None);
        Ok(GetNeighborArgs::Dump { ip_version, interface })
    }

    fn get_request_from_rtnl_neighbor(message: &NeighbourMessage) -> Result<Self, RequestError> {
        let NeighbourHeader { ifindex, family, state, flags, kind } = &message.header;
        if *state != NeighbourState::None {
            return Err(RequestError::InvalidState {
                actual: *state,
                expected: NeighbourState::None,
            });
        }
        if *kind != RouteType::Unspec {
            return Err(RequestError::InvalidKind(*kind));
        }
        if flags.intersects(!NeighbourFlags::Proxy) {
            return Err(RequestError::InvalidFlags(*flags));
        }
        if flags.contains(NeighbourFlags::Proxy) {
            // Netstack3 does not support ARP/NDP proxying.
            // TODO(https://fxbug.dev/42111873): Support ARP/NDP proxying.
            log_warn!("unsupported Proxy flag in get neighbor request");
            return Err(RequestError::UnsupportedFlags(*flags));
        }

        let (address, unsupported) = message.attributes.iter().fold(
            (None, false),
            |(address_acc, unsupported_acc), attr| {
                match attr {
                    NeighbourAttribute::Destination(addr) => {
                        // Note: In the event the Destination attribute is
                        // provided multiple times, keep the first.
                        (address_acc.or(Some(addr)), unsupported_acc)
                    }
                    _ => {
                        if !unsupported_acc {
                            // Only log for the first invalid attribute to avoid spamming.
                            log_warn!(
                                "unsupported request attribute: {attr:?} in get neighbor\
                                request; only `DST` is supported"
                            );
                        }
                        (address_acc, true)
                    }
                }
            },
        );
        if unsupported {
            return Err(RequestError::InvalidAttribute);
        }
        let ip = neighbor_fidl_ip(*family, address)?;
        // Note that the interface index is pulled from the header here, whereas
        // it's pulled from the attribute for dump requests. This is
        // intentional, in order to maintain consistency with Linux's behavior.
        let interface =
            u64::from(*ifindex).try_into().map_err(|_| RequestError::MissingInterface)?;
        Ok(GetNeighborArgs::Get { ip, interface })
    }
}

/// Arguments for an RTM_NEWNEIGH [`Request`].
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(crate) enum NewNeighborArgs {
    CreateStatic { ip: fnet::IpAddress, interface: NonZeroU64, mac: fnet::MacAddress },
    ProbeExisting { ip: fnet::IpAddress, interface: NonZeroU64 },
}

impl NewNeighborArgs {
    // Attempts to convert a netlink_packet_route `NeighbourMessage` into
    // `NewNeighborArgs`.
    pub(crate) fn try_from_rtnl_neighbor(
        message: &NeighbourMessage,
        netlink_flags: u16,
    ) -> Result<Self, RequestError> {
        Self::try_from_rtnl_neighbor_internal(message, netlink_flags)
            .inspect_err(|e| log_debug!("{e} in new neighbor request"))
    }

    fn try_from_rtnl_neighbor_internal(
        message: &NeighbourMessage,
        netlink_flags: u16,
    ) -> Result<Self, RequestError> {
        let NeighbourHeader { ifindex, family, flags, state, .. } = &message.header;
        if flags.contains(NeighbourFlags::Proxy) {
            // Netstack3 does not support ARP/NDP proxying.
            // TODO(https://fxbug.dev/42111873): Support ARP/NDP proxying.
            log_warn!("unsupported Proxy flag in new neighbor request");
            return Err(RequestError::UnsupportedFlags(*flags));
        }

        // Read common attributes required for identifying the neighbor.

        let (ip_addr, ll_addr) =
            message.attributes.iter().fold((None, None), |acc @ (ip, ll), attr| match attr {
                // Note: In the event an attribute is provided multiple times,
                // keep the first value.
                NeighbourAttribute::Destination(addr) => (ip.or(Some(addr)), ll),
                NeighbourAttribute::LinkLocalAddress(addr) => (ip, ll.or(Some(addr))),
                _ => acc,
            });
        let ip = neighbor_fidl_ip(*family, ip_addr)?;
        let interface =
            u64::from(*ifindex).try_into().map_err(|_| RequestError::MissingInterface)?;

        // Determine the specific operation and check operation-specific
        // attributes.

        let new_neighbor_flags = NLM_F_CREATE | NLM_F_REPLACE | NLM_F_EXCL | NLM_F_APPEND;
        let set_flags = netlink_flags & new_neighbor_flags;
        if set_flags == NLM_F_REPLACE {
            // If the caller only specified `NLM_F_REPLACE`, the only case
            // Netstack3 supports is triggering an immediate neighbor probe.
            if *state != NeighbourState::Probe {
                return Err(RequestError::InvalidState {
                    actual: *state,
                    expected: NeighbourState::Probe,
                });
            }
            // Setting the link address while triggering a neighbor probe is
            // unsupported.
            if ll_addr.is_some() {
                return Err(RequestError::InvalidAttribute);
            }
            Ok(NewNeighborArgs::ProbeExisting { interface, ip })
        } else if set_flags == (NLM_F_CREATE | NLM_F_REPLACE) {
            // If the caller specified `NLM_F_CREATE`, Netstack3 only supports
            // addition of static neighbors.
            if *state != NeighbourState::Permanent {
                return Err(RequestError::InvalidState {
                    actual: *state,
                    expected: NeighbourState::Permanent,
                });
            }
            let mac = ll_addr.ok_or(RequestError::MissingMacAddress).and_then(|mac| {
                mac.clone()
                    .try_into()
                    .map_err(|_| RequestError::InvalidMacAddress)
                    .map(|octets| fnet::MacAddress { octets })
            })?;
            Ok(NewNeighborArgs::CreateStatic { interface, ip, mac })
        } else {
            Err(RequestError::UnsupportedOperation)
        }
    }
}

/// Arguments for an RTM_DELNEIGH [`Request`].
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(crate) struct DelNeighborArgs {
    pub(crate) ip: fnet::IpAddress,
    pub(crate) interface: NonZeroU64,
}

impl DelNeighborArgs {
    // Attempts to convert a netlink_packet_route `NeighbourMessage` into
    // `DelNeighborArgs`.
    pub(crate) fn try_from_rtnl_neighbor(message: &NeighbourMessage) -> Result<Self, RequestError> {
        Self::try_from_rtnl_neighbor_internal(message)
            .inspect_err(|e| log_debug!("{e} in del neighbor request"))
    }

    fn try_from_rtnl_neighbor_internal(message: &NeighbourMessage) -> Result<Self, RequestError> {
        let NeighbourHeader { ifindex, family, flags, .. } = &message.header;
        if flags.contains(NeighbourFlags::Proxy) {
            // Netstack3 does not support ARP/NDP proxying.
            // TODO(https://fxbug.dev/42111873): Support ARP/NDP proxying.
            log_warn!("unsupported Proxy flag in del neighbor request");
            return Err(RequestError::UnsupportedFlags(*flags));
        }

        let address = message.attributes.iter().find_map(|attr| match attr {
            NeighbourAttribute::Destination(addr) => Some(addr),
            _ => None,
        });
        let ip = neighbor_fidl_ip(*family, address)?;
        let interface =
            u64::from(*ifindex).try_into().map_err(|_| RequestError::MissingInterface)?;
        Ok(Self { interface, ip })
    }
}

/// [`Request`] arguments associated with neighbors.
#[derive(Copy, Clone, Debug, PartialEq, Eq)]
pub(crate) enum NeighborRequestArgs {
    /// RTM_GETNEIGH
    Get(GetNeighborArgs),
    /// RTM_NEWNEIGH
    New(NewNeighborArgs),
    /// RTM_DELNEIGH
    Del(DelNeighborArgs),
}

/// An error encountered while handling a [`Request`].
#[derive(Copy, Clone, Debug, PartialEq, Eq, Error)]
pub(crate) enum RequestError {
    /// Invalid state in neighbor header.
    #[error("invalid state; expected={expected:?}, actual={actual:?}")]
    InvalidState { actual: NeighbourState, expected: NeighbourState },
    /// Invalid kind in neighbor header.
    #[error("invalid kind: {0:?}")]
    InvalidKind(RouteType),
    /// Invalid flags in neighbor header.
    #[error("invalid flags: {0:?}")]
    InvalidFlags(NeighbourFlags),
    /// Unsupported flags.
    #[error("unsupported flags: {0:?}")]
    UnsupportedFlags(NeighbourFlags),
    /// Invalid address family.
    #[error("invalid address family: {0:?}")]
    InvalidAddressFamily(AddressFamily),
    /// Address family in request header doesn't match family of address.
    // In practice this should never be encountered:
    // `NeighbourAddress::parse_with_param` parses the address based on the
    // address family from the header, and a discrepancy between the expected
    // and actual address length results in a parsing failure.
    #[error("address family mismatch; expected={0:?}")]
    AddressFamilyMismatch(AddressFamily),
    /// Request doesn't specify required neighbor IP address.
    #[error("missing required `DST` attribute")]
    MissingIpAddress,
    /// Request doesn't specify required neighbor MAC address.
    #[error("missing required `LLADDR` attribute")]
    MissingMacAddress,
    /// Request doesn't specify required interface.
    #[error("missing required interface")]
    MissingInterface,
    /// Request specifies invalid attribute(s).
    #[error("invalid request attribute")]
    InvalidAttribute,
    /// No such neighbor.
    #[error("no such neighbor")]
    NeighborNotFound,
    /// Interface not found.
    #[error("no such interface")]
    InterfaceNotFound,
    /// Invalid neighbor IP address.
    #[error("invalid neighbor IP address")]
    InvalidIpAddress,
    /// Invalid neighbor MAC address.
    #[error("invalid neighbor MAC address")]
    InvalidMacAddress,
    /// Interface not supported.
    #[error("interface not supported")]
    InterfaceUnsupported,
    /// Neighbor link address unknown.
    #[error("link address unknown")]
    LinkAddressUnknown,
    /// Operation not supported.
    #[error("unsupported operation")]
    UnsupportedOperation,
}

impl From<RequestError> for Errno {
    fn from(value: RequestError) -> Self {
        match value {
            RequestError::InvalidState { .. } => Errno::EINVAL,
            RequestError::InvalidKind(_) => Errno::EINVAL,
            RequestError::InvalidFlags(_) => Errno::EINVAL,
            RequestError::UnsupportedFlags(_) => Errno::ENOTSUP,
            RequestError::InvalidAddressFamily(_) => Errno::EAFNOSUPPORT,
            RequestError::AddressFamilyMismatch(_) => Errno::EINVAL,
            RequestError::MissingIpAddress => Errno::EINVAL,
            RequestError::MissingMacAddress => Errno::EINVAL,
            RequestError::MissingInterface => Errno::EINVAL,
            RequestError::InvalidAttribute => Errno::EINVAL,
            RequestError::NeighborNotFound => Errno::ENOENT,
            RequestError::InterfaceNotFound => Errno::ENODEV,
            RequestError::InvalidIpAddress => Errno::EINVAL,
            RequestError::InvalidMacAddress => Errno::EINVAL,
            RequestError::InterfaceUnsupported => Errno::ENOTSUP,
            RequestError::LinkAddressUnknown => Errno::EINVAL,
            RequestError::UnsupportedOperation => Errno::ENOTSUP,
        }
    }
}

impl From<fnet_neighbor::ControllerError> for RequestError {
    fn from(value: fnet_neighbor::ControllerError) -> Self {
        use fnet_neighbor::ControllerError;
        match value {
            ControllerError::InterfaceNotFound => RequestError::InterfaceNotFound,
            ControllerError::InterfaceNotSupported => RequestError::InterfaceUnsupported,
            ControllerError::InvalidIpAddress => RequestError::InvalidIpAddress,
            ControllerError::MacAddressNotUnicast => RequestError::InvalidMacAddress,
            ControllerError::NeighborNotFound => RequestError::NeighborNotFound,
            ControllerError::LinkAddressUnknown => RequestError::LinkAddressUnknown,
            ControllerError::__SourceBreaking { unknown_ordinal: e } => {
                panic!("encountered unknown controller error: {e:?}")
            }
        }
    }
}

/// Trait abstracting the ability to check if an interface exists.
pub(crate) trait LookupIfInterfaceExists {
    /// Returns whether an interface exists at the provided index.
    fn exists(&self, interface: NonZeroU64) -> bool;
}

type InterfaceMap = BTreeMap<
    u64,
    fnet_interfaces_ext::PropertiesAndState<
        crate::interfaces::InterfaceState,
        fnet_interfaces_ext::AllInterest,
    >,
>;

impl LookupIfInterfaceExists for InterfaceMap {
    fn exists(&self, interface: NonZeroU64) -> bool {
        self.contains_key(&interface.get())
    }
}

/// A request associated with neighbors.
#[derive(Derivative)]
#[derivative(Debug(bound = ""))]
pub(crate) struct Request<S: Sender<<NetlinkRoute as ProtocolFamily>::Response>> {
    /// The resource and operation-specific argument(s) for this request.
    pub args: NeighborRequestArgs,
    /// The request's sequence number.
    ///
    /// This value will be copied verbatim into any message sent as a result of
    /// this request.
    pub sequence_number: u32,
    /// The client that made the request.
    pub client: InternalClient<NetlinkRoute, S>,
    /// A completer that will have the result of the request sent over.
    pub completer: oneshot::Sender<Result<(), RequestError>>,
}

/// A subset of `NeighborRequestArgs`, containing only `Request` types that can be pending.
#[derive(Clone, Debug, PartialEq, Eq)]
pub(crate) enum PendingNeighborRequestArgs {
    /// RTM_NEWNEIGH
    New(NewNeighborArgs),
    /// RTM_DELNEIGH
    Del(DelNeighborArgs),
}

#[derive(Derivative)]
#[derivative(Debug(bound = ""))]
pub(crate) struct PendingNeighborRequest<S: Sender<<NetlinkRoute as ProtocolFamily>::Response>> {
    request_args: PendingNeighborRequestArgs,
    client: InternalClient<NetlinkRoute, S>,
    completer: oneshot::Sender<Result<(), RequestError>>,
}

/// Errors related to handling neighbor events from Netstack.
#[derive(Debug, Error, PartialEq)]
pub(crate) enum HandleWatchEventError {
    /// An event indicated a neighbor was removed that was not previously known.
    #[error("Netstack reported removal of an unknown neighbor: {0:?}")]
    UnknownNeighborRemoved(fnet_neighbor_ext::Entry),
    /// An event indicated a neighbor was changed that was not previously known.
    #[error("Netstack reported change of an unknown neighbor: {0:?}")]
    UnknownNeighborChanged(fnet_neighbor_ext::Entry),
    /// An event indicated a neighbor was added that conflicts with a known
    /// neighbor.
    #[error(
        "Netstack reported addition of a neighbor that already exists: \
        existing={existing:?}, new={new:?}"
    )]
    ConflictingNeighborAdded { existing: fnet_neighbor_ext::Entry, new: fnet_neighbor_ext::Entry },
    /// An `Existing` or `Idle` event was received after collecting the initial
    /// neighbors from the event stream.
    #[error("Netstack reported unexpected event: {0:?}")]
    UnexpectedEventReceived(fnet_neighbor_ext::Event),
}

#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq, PartialOrd, Ord)]
struct NeighborKey {
    interface: NonZeroU64,
    neighbor: fnet::IpAddress,
}

impl From<&fnet_neighbor_ext::Entry> for NeighborKey {
    fn from(
        fnet_neighbor_ext::Entry { interface, neighbor, .. }: &fnet_neighbor_ext::Entry,
    ) -> NeighborKey {
        NeighborKey { interface: *interface, neighbor: *neighbor }
    }
}

/// Handles asynchronous work related to RTM_*NEIGH messages.
///
/// Can respond to RTM_*NEIGH message requests.
pub(crate) struct NeighborsWorker {
    neighbor_table: HashMap<NeighborKey, fnet_neighbor_ext::Entry>,
    neighbors_controller: fnet_neighbor::ControllerProxy,
}

impl NeighborsWorker {
    /// Create the Netlink Neighbors Worker.
    ///
    /// Panics if the existing neighbors cannot be retrieved from
    /// `neighbors_view` or if the response contains conflicting neighbors.
    pub(crate) async fn create(
        neighbors_view: &fnet_neighbor::ViewProxy,
        neighbors_controller: fnet_neighbor::ControllerProxy,
    ) -> (
        Self,
        impl futures::Stream<
            Item = Result<fnet_neighbor_ext::Event, fnet_neighbor_ext::EntryIteratorError>,
        > + Unpin
        + 'static,
    ) {
        let mut neighbor_event_stream = Box::pin(
            fnet_neighbor_ext::event_stream_from_view(neighbors_view)
                .expect("connecting to fuchsia.net.neighbors.View FIDL should succeed"),
        );
        let existing_neighbors: HashSet<fnet_neighbor_ext::Entry> =
            fnet_neighbor_ext::collect_neighbors_until_idle(neighbor_event_stream.by_ref())
                .await
                .expect("determining existing neighbors should succeed");
        let existing_count = existing_neighbors.len();
        let neighbor_table = existing_neighbors
            .into_iter()
            .map(|e| (NeighborKey::from(&e), e))
            .collect::<HashMap<_, _>>();
        assert_eq!(
            neighbor_table.len(),
            existing_count,
            "conflicting existing entry in neighbor table"
        );
        (Self { neighbor_table, neighbors_controller }, neighbor_event_stream)
    }

    pub(crate) fn handle_neighbor_watcher_event<
        S: Sender<<NetlinkRoute as ProtocolFamily>::Response>,
    >(
        &mut self,
        event: fnet_neighbor_ext::Event,
        clients: &ClientTable<NetlinkRoute, S>,
    ) -> Result<(), HandleWatchEventError> {
        let message_for_group = match event {
            fnet_neighbor_ext::Event::Removed(entry) => {
                match self.neighbor_table.remove(&(&entry).into()) {
                    Some(_) => Ok(NetlinkNeighborMessage::optionally_from(entry)
                        .map(NetlinkNeighborMessage::into_rtnl_del_neighbor)),
                    None => Err(HandleWatchEventError::UnknownNeighborRemoved(entry)),
                }
            }
            fnet_neighbor_ext::Event::Added(entry) => {
                match self.neighbor_table.insert((&entry).into(), entry.clone()) {
                    Some(existing) => Err(HandleWatchEventError::ConflictingNeighborAdded {
                        existing,
                        new: entry,
                    }),
                    None => Ok(NetlinkNeighborMessage::optionally_from(entry)
                        .map(|n| n.into_rtnl_new_neighbor(UNSPECIFIED_SEQUENCE_NUMBER, false))),
                }
            }
            fnet_neighbor_ext::Event::Changed(entry) => {
                match self.neighbor_table.insert((&entry).into(), entry.clone()) {
                    Some(_) => Ok(NetlinkNeighborMessage::optionally_from(entry)
                        .map(|n| n.into_rtnl_new_neighbor(UNSPECIFIED_SEQUENCE_NUMBER, false))),
                    None => Err(HandleWatchEventError::UnknownNeighborChanged(entry)),
                }
            }
            e @ fnet_neighbor_ext::Event::Existing(_) | e @ fnet_neighbor_ext::Event::Idle => {
                Err(HandleWatchEventError::UnexpectedEventReceived(e))
            }
        }?;
        if let Some(message) = message_for_group {
            clients.send_message_to_group(message, ModernGroup(rtnetlink_groups_RTNLGRP_NEIGH));
        }
        Ok(())
    }

    pub(crate) async fn handle_request<S: Sender<<NetlinkRoute as ProtocolFamily>::Response>>(
        &mut self,
        Request { args, mut client, sequence_number, completer }: Request<S>,
        interface_lookup: &impl LookupIfInterfaceExists,
    ) -> Option<PendingNeighborRequest<S>> {
        enum HandleResult {
            Done(Result<(), RequestError>),
            Pending(PendingNeighborRequestArgs),
        }
        let result = match args {
            NeighborRequestArgs::Get(args) => match args {
                GetNeighborArgs::Dump { ip_version, interface } => {
                    self.neighbor_table
                        .values()
                        .filter(|n| {
                            ip_version.map_or(true, |ip_version| match n.neighbor {
                                fnet::IpAddress::Ipv4(_) => ip_version == IpVersion::V4,
                                fnet::IpAddress::Ipv6(_) => ip_version == IpVersion::V6,
                            })
                        })
                        .filter(|n| interface.map_or(true, |i| n.interface == i))
                        .filter_map(|e| NetlinkNeighborMessage::optionally_from(e.clone()))
                        .for_each(|m| {
                            client.send_unicast(m.into_rtnl_new_neighbor(sequence_number, true));
                        });
                    HandleResult::Done(Ok(()))
                }
                GetNeighborArgs::Get { ip, interface } => {
                    let neighbor = self
                        .neighbor_table
                        .get(&NeighborKey { interface, neighbor: ip })
                        .map(|e| NetlinkNeighborMessage::optionally_from(e.clone()))
                        .flatten();
                    match neighbor {
                        Some(msg) => {
                            client.send_unicast(msg.into_rtnl_new_neighbor(sequence_number, false));
                            HandleResult::Done(Ok(()))
                        }
                        None => {
                            let err = if interface_lookup.exists(interface) {
                                RequestError::NeighborNotFound
                            } else {
                                RequestError::InterfaceNotFound
                            };
                            HandleResult::Done(Err(err))
                        }
                    }
                }
            },
            NeighborRequestArgs::New(args) => match args {
                args @ NewNeighborArgs::CreateStatic { ip, interface, mac } => {
                    let response = self
                        .neighbors_controller
                        .add_entry(interface.get(), &ip, &mac)
                        .await
                        .expect("sent neighbor controller request");
                    match response {
                        Ok(_) => HandleResult::Pending(PendingNeighborRequestArgs::New(args)),
                        Err(e) => HandleResult::Done(Err(e.into())),
                    }
                }
                args @ NewNeighborArgs::ProbeExisting { ip, interface } => {
                    let response = self
                        .neighbors_controller
                        .probe_entry(interface.get(), &ip)
                        .await
                        .expect("sent neighbor controller request");
                    match response {
                        Ok(_) => HandleResult::Pending(PendingNeighborRequestArgs::New(args)),
                        Err(e) => HandleResult::Done(Err(e.into())),
                    }
                }
            },
            NeighborRequestArgs::Del(args @ DelNeighborArgs { interface, ip }) => {
                let response = self
                    .neighbors_controller
                    .remove_entry(interface.get(), &ip)
                    .await
                    .expect("sent neighbor controller request");
                match response {
                    Ok(_) => HandleResult::Pending(PendingNeighborRequestArgs::Del(args)),
                    Err(e) => HandleResult::Done(Err(e.into())),
                }
            }
        };

        match result {
            HandleResult::Done(result) => {
                log_debug!("handled request {args:?} from {client} with result = {result:?}");
                respond_to_completer(client, completer, result, args);
                None
            }
            HandleResult::Pending(request_args) => {
                log_debug!("pending request {args:?} from {client}");
                Some(PendingNeighborRequest { request_args, client, completer })
            }
        }
    }

    /// Checks whether a `PendingRequest` can be marked completed given the
    /// current state of the worker. If so, notifies the request's completer and
    /// returns `None`. If not, returns the `PendingRequest` as `Some`.
    ///
    /// TODO(https://fxbug.dev/488124265): Use synchronization primitives to
    /// more robustly match requests to their corresponding watch events.
    pub(crate) fn handle_pending_request<S: Sender<<NetlinkRoute as ProtocolFamily>::Response>>(
        &self,
        pending_neighbor_request: PendingNeighborRequest<S>,
    ) -> Option<PendingNeighborRequest<S>> {
        let PendingNeighborRequest { request_args, client: _, completer: _ } =
            &pending_neighbor_request;

        let done = match request_args {
            PendingNeighborRequestArgs::New(NewNeighborArgs::ProbeExisting { ip, interface }) => {
                // Assuming the `ProbeEntry` call succeeds, this is guaranteed
                // to complete eventually, despite the fact that Netstack does
                // not generate a `Changed` event if the neighbor is already in
                // the expected state.
                //
                // If the neighbor was not in the expected state when Netstack
                // processed the request, then a `Changed` event is generated,
                // and since this method is called after each event that Netlink
                // processes, this condition must eventually be true.
                //
                // If the neighbor *was* in the expected state when Netstack
                // processed the request, then no `Changed` event is generated,
                // but it's necessarily true that the immediately preceding
                // `Added` or `Changed` event for the neighbor must contain the
                // expected state.
                //
                // Here there are two cases to consider: Netlink has either
                // already processed that event, or not.
                //
                // In the former case, the fact that there cannot have been
                // intervening events means that this check will succeed on the
                // call to this method that immediately follows the Controller
                // request in the event loop.
                //
                // In the latter case, the event will be processed in a later
                // iteration of the event loop, at which point this condition
                // will become true.
                self.neighbor_table
                    .get(&NeighborKey { interface: *interface, neighbor: *ip })
                    .map_or(false, |entry| entry.state == fnet_neighbor::EntryState::Probe)
            }
            PendingNeighborRequestArgs::New(NewNeighborArgs::CreateStatic {
                ip,
                interface,
                mac,
            }) => {
                // It's also true here that Netstack does not generate an event
                // if the neighbor is already in the expected state, but this is
                // nevertheless guaranteed to complete eventually by the same
                // logic as above.
                self.neighbor_table
                    .get(&NeighborKey { interface: *interface, neighbor: *ip })
                    .map_or(false, |entry| {
                        entry.mac.is_some_and(|m| m == *mac)
                            && entry.state == fnet_neighbor::EntryState::Static
                    })
            }
            PendingNeighborRequestArgs::Del(DelNeighborArgs { ip, interface }) => !self
                .neighbor_table
                .contains_key(&NeighborKey { interface: *interface, neighbor: *ip }),
        };

        if done {
            log_debug!("completed pending request; req = {pending_neighbor_request:?}");
            let PendingNeighborRequest { request_args, client, completer } =
                pending_neighbor_request;

            respond_to_completer(client, completer, Ok(()), request_args);
            None
        } else {
            // Put the pending request back so that it can be handled later.
            log_debug!("pending request not done yet; req = {pending_neighbor_request:?}");
            Some(pending_neighbor_request)
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::client::ClientTable;
    use crate::client::testutil::{CLIENT_ID_1, CLIENT_ID_2, new_fake_client};
    use crate::interfaces::testutil::FakeInterfacesHandler;
    use crate::messaging::testutil::{FakeSender, SentMessage};
    use crate::route_eventloop::{
        EventLoopComponent, EventLoopInputs, EventLoopSpec, EventLoopState, IncludedWorkers,
        Optional, Required, UnifiedRequest,
    };

    use super::*;

    use assert_matches::assert_matches;
    use fidl_fuchsia_net as fnet;
    use fidl_fuchsia_net_interfaces as fnet_interfaces;
    use fidl_fuchsia_net_neighbor::ViewRequest;
    use fidl_fuchsia_net_neighbor_ext::testutil::EventSpec;
    use fidl_fuchsia_net_root as fnet_root;
    use futures::channel::mpsc;
    use futures::{FutureExt as _, SinkExt as _, TryStreamExt as _, pin_mut};
    use maplit::hashset;
    use net_declare::{fidl_ip, std_ip_v4, std_ip_v6};
    use netlink_packet_core::NetlinkPayload;
    use netlink_packet_route::neighbour::{NeighbourAddress, NeighbourFlags};
    use std::collections::HashSet;
    use test_case::test_case;

    fn valid_neighbor_entry() -> fnet_neighbor_ext::Entry {
        fnet_neighbor_ext::Entry {
            interface: NonZeroU64::new(1).unwrap(),
            neighbor: fidl_ip!("192.168.0.1"),
            state: fnet_neighbor::EntryState::Reachable,
            mac: Some(fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] }),
            updated_at: 123456,
        }
    }

    #[test]
    fn netlink_neighbor_message_from_entry_invalid_iface_id() {
        let entry = fnet_neighbor_ext::Entry {
            interface: NonZeroU64::new(u64::MAX).unwrap(),
            ..valid_neighbor_entry()
        };

        assert_eq!(
            NetlinkNeighborMessage::try_from(entry),
            Err(NetlinkNeighborMessageConversionError::InvalidInterfaceId(u64::MAX))
        );
    }

    #[test]
    fn netlink_neighbor_message_from_entry_valid_iface_id() {
        assert_matches!(
            NetlinkNeighborMessage::try_from(fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(1).unwrap(),
                ..valid_neighbor_entry()
            }),
            Ok(NetlinkNeighborMessage(NeighbourMessage {
                header: NeighbourHeader { ifindex: 1, .. },
                ..
            }))
        );
    }

    #[test_case(fnet_neighbor::EntryState::Delay, NeighbourState::Delay; "delay")]
    #[test_case(fnet_neighbor::EntryState::Incomplete, NeighbourState::Incomplete; "incomplete")]
    #[test_case(fnet_neighbor::EntryState::Probe, NeighbourState::Probe; "probe")]
    #[test_case(fnet_neighbor::EntryState::Reachable, NeighbourState::Reachable; "reachable")]
    #[test_case(fnet_neighbor::EntryState::Stale, NeighbourState::Stale; "stale")]
    #[test_case(fnet_neighbor::EntryState::Static, NeighbourState::Permanent; "permanent")]
    #[test_case(fnet_neighbor::EntryState::Unreachable, NeighbourState::Failed; "failed")]
    fn netlink_neighbor_message_from_entry_state_converted(
        fidl_state: fnet_neighbor::EntryState,
        expected: NeighbourState,
    ) {
        assert_matches!(
            NetlinkNeighborMessage::try_from(fnet_neighbor_ext::Entry {
                state: fidl_state,
                ..valid_neighbor_entry()
            }),
            Ok(NetlinkNeighborMessage(NeighbourMessage {
                header: NeighbourHeader { state, .. },
                ..
            })) if state == expected
        );
    }

    #[test]
    fn netlink_neighbor_message_from_entry_ipv4() {
        let fidl_entry = fnet_neighbor_ext::Entry {
            neighbor: fidl_ip!("192.168.0.1"),
            ..valid_neighbor_entry()
        };
        let NetlinkNeighborMessage(message) =
            fidl_entry.try_into().expect("should be able to convert valid neighbor entry");

        assert_eq!(message.header.family, AddressFamily::Inet);
        let expected_address: NeighbourAddress = std_ip_v4!("192.168.0.1").into();
        assert_matches!(
            &message.attributes[..],
            [
                NeighbourAttribute::Destination(address),
                NeighbourAttribute::LinkLocalAddress(_)
            ] if *address == expected_address
        );
    }

    #[test]
    fn netlink_neighbor_message_from_entry_ipv6() {
        let fidl_entry =
            fnet_neighbor_ext::Entry { neighbor: fidl_ip!("fe80::1"), ..valid_neighbor_entry() };
        let NetlinkNeighborMessage(message) =
            fidl_entry.try_into().expect("should be able to convert valid neighbor entry");

        assert_eq!(message.header.family, AddressFamily::Inet6);
        let expected_address: NeighbourAddress = std_ip_v6!("fe80::1").into();
        assert_matches!(
            &message.attributes[..],
            [
                NeighbourAttribute::Destination(address),
                NeighbourAttribute::LinkLocalAddress(_)
            ] if *address == expected_address
        );
    }

    #[test]
    fn netlink_neighbor_message_from_entry_address_link_local_present() {
        let fidl_entry = fnet_neighbor_ext::Entry {
            mac: Some(fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] }),
            ..valid_neighbor_entry()
        };
        let NetlinkNeighborMessage(message) =
            fidl_entry.try_into().expect("should be able to convert valid neighbor entry");

        assert_matches!(
            &message.attributes[..],
            [
                NeighbourAttribute::Destination(_),
                NeighbourAttribute::LinkLocalAddress(addr)
            ] if addr == &[0, 1, 2, 3, 4, 5]
        );
    }

    #[test]
    fn netlink_neighbor_message_from_entry_address_link_local_absent() {
        let fidl_entry = fnet_neighbor_ext::Entry { mac: None, ..valid_neighbor_entry() };
        let NetlinkNeighborMessage(message) =
            fidl_entry.try_into().expect("should be able to convert valid neighbor entry");

        assert_matches!(&message.attributes[..], [NeighbourAttribute::Destination(_)]);
    }

    #[test]
    fn netlink_neighbor_message_optionally_from_failure() {
        assert_eq!(
            NetlinkNeighborMessage::optionally_from(fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(u64::MAX).unwrap(),
                ..valid_neighbor_entry()
            }),
            None
        );
    }

    #[test]
    fn netlink_neighbor_message_optionally_from_success() {
        let fidl_entry = fnet_neighbor_ext::Entry {
            interface: NonZeroU64::new(1).unwrap(),
            neighbor: fidl_ip!("192.168.0.1"),
            state: fnet_neighbor::EntryState::Reachable,
            mac: None,
            updated_at: 123456,
        };

        let mut expected_message = NeighbourMessage::default();
        expected_message.header = NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            state: NeighbourState::Reachable,
            flags: NeighbourFlags::empty(),
            kind: RouteType::Unicast,
        };
        expected_message.attributes =
            vec![NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into())];

        assert_eq!(
            NetlinkNeighborMessage::optionally_from(fidl_entry),
            Some(NetlinkNeighborMessage(expected_message))
        );
    }

    #[test]
    fn netlink_neighbor_message_into_rtnl_new_neighbor() {
        let message: NetlinkNeighborMessage = valid_neighbor_entry()
            .try_into()
            .expect("should be able to convert valid neighbor entry");
        let NetlinkNeighborMessage(payload) = &message;

        let expected_payload =
            NetlinkPayload::InnerMessage(RouteNetlinkMessage::NewNeighbour(payload.clone()));

        let result = message.clone().into_rtnl_new_neighbor(1, true);
        assert_eq!(result.payload, expected_payload);
        assert_eq!(result.header.sequence_number, 1);
        assert_eq!(result.header.flags & NLM_F_MULTIPART, NLM_F_MULTIPART);

        let result = message.into_rtnl_new_neighbor(1, false);
        assert_eq!(result.payload, expected_payload);
        assert_ne!(result.header.flags & NLM_F_MULTIPART, NLM_F_MULTIPART);
    }

    #[test]
    fn neighbor_keyed_by_interface_and_ip() {
        let entry = fnet_neighbor_ext::Entry {
            interface: NonZeroU64::new(1).unwrap(),
            neighbor: fidl_ip!("192.168.0.1"),
            mac: None,
            state: fnet_neighbor::EntryState::Reachable,
            updated_at: 123456,
        };

        let same_iface_and_ip = fnet_neighbor_ext::Entry {
            mac: Some(fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] }),
            state: fnet_neighbor::EntryState::Stale,
            updated_at: 654321,
            ..entry
        };
        assert_eq!(NeighborKey::from(&entry), NeighborKey::from(&same_iface_and_ip));

        let different_iface =
            fnet_neighbor_ext::Entry { interface: NonZeroU64::new(2).unwrap(), ..entry };
        assert_ne!(NeighborKey::from(&entry), NeighborKey::from(&different_iface));

        let different_ip = fnet_neighbor_ext::Entry { neighbor: fidl_ip!("192.168.0.2"), ..entry };
        assert_ne!(NeighborKey::from(&entry), NeighborKey::from(&different_ip));

        let different_iface_and_ip = fnet_neighbor_ext::Entry {
            interface: NonZeroU64::new(2).unwrap(),
            neighbor: fidl_ip!("192.168.0.2"),
            ..entry
        };
        assert_ne!(NeighborKey::from(&entry), NeighborKey::from(&different_iface_and_ip));
    }

    #[fuchsia::test]
    #[should_panic(expected = "determining existing neighbors should succeed")]
    async fn neighbors_worker_create_panics_on_view_protocol_error() {
        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let (view, view_server_end) = fidl::endpoints::create_proxy::<fnet_neighbor::ViewMarker>();
        // Close the channel without responding.
        drop(view_server_end);

        let (_worker, _remaining) = NeighborsWorker::create(&view, controller).await;
    }

    #[fuchsia::test]
    #[should_panic(expected = "determining existing neighbors should succeed")]
    async fn neighbors_worker_create_panics_on_event_stream_error() {
        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let (view, view_server_end) = fidl::endpoints::create_proxy::<fnet_neighbor::ViewMarker>();
        let mut view_request_stream = view_server_end.into_stream();

        let entry_iter_fut = view_request_stream
            .next()
            .then(|req| {
                match req
                    .expect("View request_stream unexpectedly ended")
                    .expect("failed to receive `OpenEntryIterator` request")
                {
                    ViewRequest::OpenEntryIterator { it, .. } => {
                        // Close the channel without responding.
                        drop(it);
                        futures::future::ready(())
                    }
                }
            })
            .fuse();

        let worker_fut = NeighborsWorker::create(&view, controller);

        let ((), (_worker, _remaining)) = futures::join!(entry_iter_fut, worker_fut);
    }

    #[fuchsia::test]
    #[should_panic(expected = "conflicting existing entry")]
    async fn neighbors_worker_create_panics_on_conflicting_entry() {
        let events: Vec<_> = [
            // Create two neighbors with the same `NeighborKey` but differing
            // fields; truly duplicate entries are ignored.
            fnet_neighbor_ext::Entry {
                state: fnet_neighbor::EntryState::Reachable,
                ..valid_neighbor_entry()
            },
            fnet_neighbor_ext::Entry {
                state: fnet_neighbor::EntryState::Stale,
                ..valid_neighbor_entry()
            },
        ]
        .into_iter()
        .map(Into::into)
        .map(fnet_neighbor::EntryIteratorItem::Existing)
        .chain(std::iter::once(fnet_neighbor::EntryIteratorItem::Idle(fnet_neighbor::IdleEvent)))
        .collect();
        let batches = vec![events];
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(batches));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);

        let ((), (_worker, _remaining)) = futures::join!(server_fut, worker_fut);
    }

    #[fuchsia::test]
    async fn neighbors_worker_create_success() {
        use fnet_neighbor_ext::testutil::EventSpec::*;
        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            Existing(1),
            Existing(2),
            Existing(3),
            Idle,
            Added(4),
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![
                events.clone(),
            ]));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);

        let ((), (worker, event_stream)) = futures::join!(server_fut, worker_fut);

        let remaining_events: Vec<_> = event_stream.collect().await;
        assert_matches!(
            &remaining_events[..],
            [
                Ok(fnet_neighbor_ext::Event::Added(_)),
                Err(fnet_neighbor_ext::EntryIteratorError::Fidl(
                    fidl::Error::ClientChannelClosed { .. }
                ))
            ]
        );

        for event in events {
            match event {
                fnet_neighbor::EntryIteratorItem::Existing(fidl_entry) => {
                    let entry: fnet_neighbor_ext::Entry = fidl_entry.try_into().unwrap();
                    assert_eq!(worker.neighbor_table.get(&(&entry).into()), Some(&entry));
                }
                _ => {}
            }
        }
    }

    #[test_case(
        EventSpec::Added(2),
        |e| matches!(e, HandleWatchEventError::ConflictingNeighborAdded { .. });
        "conflicting added"
    )]
    #[test_case(
        EventSpec::Removed(4),
        |e| matches!(e, HandleWatchEventError::UnknownNeighborRemoved(_));
        "unknown removed"
    )]
    #[test_case(
        EventSpec::Changed(4),
        |e| matches!(e, HandleWatchEventError::UnknownNeighborChanged(_));
        "unknown changed"
    )]
    #[test_case(
        EventSpec::Existing(4),
        |e| matches!(e, HandleWatchEventError::UnexpectedEventReceived(_));
        "existing after initial collection"
    )]
    #[test_case(
        EventSpec::Idle,
        |e| matches!(e, HandleWatchEventError::UnexpectedEventReceived(_));
        "idle after initial collection"
    )]
    #[fuchsia::test]
    async fn neighbors_worker_handle_watch_event_failure(
        spec: EventSpec,
        error_matcher: fn(&HandleWatchEventError) -> bool,
    ) {
        use fnet_neighbor_ext::testutil::EventSpec::*;
        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            Existing(1),
            Existing(2),
            Existing(3),
            Idle,
            spec,
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![
                events.clone(),
            ]));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);

        let ((), (mut worker, event_stream)) = futures::join!(server_fut, worker_fut);

        let client_table = ClientTable::<NetlinkRoute, FakeSender<_>>::default();

        let remaining_events: Vec<_> = event_stream.collect().await;
        assert_eq!(remaining_events.len(), 2);
        match &remaining_events[0] {
            Ok(event) => {
                assert_matches!(
                    worker.handle_neighbor_watcher_event(event.clone(), &client_table),
                    Err(error) if error_matcher(&error)
                );
            }
            _ => panic!("expected bad event in stream"),
        }
        match &remaining_events[1] {
            Err(fnet_neighbor_ext::EntryIteratorError::Fidl(
                fidl::Error::ClientChannelClosed { .. },
            )) => {}
            _ => panic!("expected PEER_CLOSED error at end of stream"),
        }
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_added_event() {
        use fnet_neighbor_ext::testutil::EventSpec::*;
        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            Existing(1),
            Existing(2),
            Existing(3),
            Idle,
            Added(4),
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![
                events.clone(),
            ]));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);

        let ((), (mut worker, event_stream)) = futures::join!(server_fut, worker_fut);

        let client_table = ClientTable::<NetlinkRoute, FakeSender<_>>::default();

        let remaining_events: Vec<_> = event_stream.collect().await;
        assert_eq!(remaining_events.len(), 2);
        match &remaining_events[0] {
            Ok(e @ fnet_neighbor_ext::Event::Added(entry)) => {
                let key = NeighborKey::from(entry);
                assert_eq!(worker.neighbor_table.get(&key), None);
                assert_matches!(
                    worker.handle_neighbor_watcher_event(e.clone(), &client_table),
                    Ok(_)
                );
                assert_eq!(worker.neighbor_table.get(&key), Some(entry));
            }
            _ => panic!("expected Added event in stream"),
        }
        match &remaining_events[1] {
            Err(fnet_neighbor_ext::EntryIteratorError::Fidl(
                fidl::Error::ClientChannelClosed { .. },
            )) => {}
            _ => panic!("expected PEER_CLOSED error at end of stream"),
        }
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_removed_event() {
        use fnet_neighbor_ext::testutil::EventSpec::*;
        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            Existing(1),
            Existing(2),
            Existing(3),
            Idle,
            Removed(2),
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![
                events.clone(),
            ]));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);

        let ((), (mut worker, event_stream)) = futures::join!(server_fut, worker_fut);

        let client_table = ClientTable::<NetlinkRoute, FakeSender<_>>::default();

        let remaining_events: Vec<_> = event_stream.collect().await;
        assert_eq!(remaining_events.len(), 2);
        match &remaining_events[0] {
            Ok(e @ fnet_neighbor_ext::Event::Removed(entry)) => {
                let key = NeighborKey::from(entry);
                assert_eq!(worker.neighbor_table.get(&key), Some(entry));
                assert_matches!(
                    worker.handle_neighbor_watcher_event(e.clone(), &client_table),
                    Ok(_)
                );
                assert_eq!(worker.neighbor_table.get(&key), None);
            }
            _ => panic!("expected Removed event in stream"),
        }
        match &remaining_events[1] {
            Err(fnet_neighbor_ext::EntryIteratorError::Fidl(
                fidl::Error::ClientChannelClosed { .. },
            )) => {}
            _ => panic!("expected PEER_CLOSED error at end of stream"),
        }
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_changed_event() {
        use fnet_neighbor_ext::testutil::EventSpec::*;
        let mut events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            Existing(1),
            Existing(2),
            Existing(3),
            Idle,
            Changed(2),
        ]);
        match &mut events[1] {
            fnet_neighbor::EntryIteratorItem::Existing(entry) => {
                entry.updated_at = Some(1234);
            }
            _ => panic!("expected Existing event in stream"),
        }
        match &mut events[4] {
            fnet_neighbor::EntryIteratorItem::Changed(entry) => {
                entry.updated_at = Some(5678);
            }
            _ => panic!("expected Changed event in stream"),
        }

        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![
                events.clone(),
            ]));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);

        let ((), (mut worker, event_stream)) = futures::join!(server_fut, worker_fut);

        let client_table = ClientTable::<NetlinkRoute, FakeSender<_>>::default();

        let remaining_events: Vec<_> = event_stream.collect().await;
        assert_eq!(remaining_events.len(), 2);
        match &remaining_events[0] {
            Ok(e @ fnet_neighbor_ext::Event::Changed(entry)) => {
                let key = NeighborKey::from(entry);
                assert_matches!(
                    worker.neighbor_table.get(&key),
                    Some(fnet_neighbor_ext::Entry { updated_at: 1234, .. })
                );
                assert_matches!(
                    worker.handle_neighbor_watcher_event(e.clone(), &client_table),
                    Ok(_)
                );
                assert_matches!(
                    worker.neighbor_table.get(&key),
                    Some(fnet_neighbor_ext::Entry { updated_at: 5678, .. })
                );
            }
            _ => panic!("expected Changed event in stream"),
        }
        match &remaining_events[1] {
            Err(fnet_neighbor_ext::EntryIteratorError::Fidl(
                fidl::Error::ClientChannelClosed { .. },
            )) => {}
            _ => panic!("expected PEER_CLOSED error at end of stream"),
        }
    }

    impl LookupIfInterfaceExists for HashSet<u64> {
        fn exists(&self, idx: NonZeroU64) -> bool {
            self.contains(&idx.get())
        }
    }

    #[test_case(HashSet::new(), RequestError::InterfaceNotFound; "interface does not exist")]
    #[test_case(
        hashset!{1}, RequestError::NeighborNotFound;
        "interface exists"
    )]
    #[fuchsia::test]
    async fn neighbors_worker_handle_get_neighbor_not_found(
        interface_lookup: HashSet<u64>,
        expected_error: RequestError,
    ) {
        let (mut sender_sink, client, _async_work_drain_task) =
            new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, completer_rcv) = oneshot::channel();
        let request = Request {
            args: NeighborRequestArgs::Get(GetNeighborArgs::Get {
                ip: fidl_ip!("192.168.0.1"),
                interface: NonZeroU64::new(1).unwrap(),
            }),
            sequence_number: 1,
            client,
            completer,
        };

        let events: Vec<_> = [
            fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(2).unwrap(),
                neighbor: fidl_ip!("192.168.0.1"),
                ..valid_neighbor_entry()
            },
            fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(1).unwrap(),
                neighbor: fidl_ip!("fe80::2"),
                ..valid_neighbor_entry()
            },
        ]
        .into_iter()
        .map(Into::into)
        .map(fnet_neighbor::EntryIteratorItem::Existing)
        .chain(std::iter::once(fnet_neighbor::EntryIteratorItem::Idle(fnet_neighbor::IdleEvent)))
        .collect();

        let batches = vec![events];
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(batches));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        assert_matches!(
            worker.handle_request(request, &interface_lookup).await,
            None // No pending work expected.
        );

        let result = completer_rcv.await.expect("completer channel should not be closed");
        assert_matches!(result, Err(e) if e == expected_error);
        assert_eq!(&sender_sink.take_messages()[..], &[]);
    }

    #[test_case(
        GetNeighborArgs::Dump{ ip_version: None, interface: None },
        &[1, 2, 3, 4];
        "dump all"
    )]
    #[test_case(
        GetNeighborArgs::Dump{ ip_version: Some(IpVersion::V4), interface: None },
        &[1, 3];
        "dump ipv4 only"
    )]
    #[test_case(
        GetNeighborArgs::Dump{ ip_version: Some(IpVersion::V6), interface: None },
        &[2, 4];
        "dump ipv6 only"
    )]
    #[test_case(
        GetNeighborArgs::Dump{
            ip_version: Some(IpVersion::V6),
            interface: Some(NonZeroU64::new(4).unwrap())
        },
        &[4];
        "dump interface 4 ipv6"
    )]
    #[test_case(
        GetNeighborArgs::Dump{
            ip_version: Some(IpVersion::V4),
            interface: Some(NonZeroU64::new(4).unwrap())
        },
        &[];
        "dump interface 4 ipv4"
    )]
    #[test_case(
        GetNeighborArgs::Get{ ip: fidl_ip!("192.168.0.1"), interface: NonZeroU64::new(1).unwrap() },
        &[1];
        "get ipv4"
    )]
    #[test_case(
        GetNeighborArgs::Get{ ip: fidl_ip!("fe80::2"), interface: NonZeroU64::new(2).unwrap() },
        &[2];
        "get ipv6"
    )]
    #[fuchsia::test]
    async fn neighbors_worker_handle_get_request(
        get_args: GetNeighborArgs,
        expected_ifindexes: &[u32],
    ) {
        let (mut sender_sink, client, _async_work_drain_task) =
            new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, completer_rcv) = oneshot::channel();
        let request = Request {
            args: NeighborRequestArgs::Get(get_args),
            sequence_number: 1,
            client,
            completer,
        };

        let events: Vec<_> = [
            fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(1).unwrap(),
                neighbor: fidl_ip!("192.168.0.1"),
                ..valid_neighbor_entry()
            },
            fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(2).unwrap(),
                neighbor: fidl_ip!("fe80::2"),
                ..valid_neighbor_entry()
            },
            fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(3).unwrap(),
                neighbor: fidl_ip!("192.168.0.3"),
                ..valid_neighbor_entry()
            },
            fnet_neighbor_ext::Entry {
                interface: NonZeroU64::new(4).unwrap(),
                neighbor: fidl_ip!("fe80::4"),
                ..valid_neighbor_entry()
            },
        ]
        .into_iter()
        .map(Into::into)
        .map(fnet_neighbor::EntryIteratorItem::Existing)
        .chain(std::iter::once(fnet_neighbor::EntryIteratorItem::Idle(fnet_neighbor::IdleEvent)))
        .collect();

        let batches = vec![events];
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(batches));

        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        assert_matches!(
            worker.handle_request(request, &BTreeMap::new()).await,
            None // No pending work expected.
        );

        completer_rcv
            .await
            .expect("completer channel should not be closed")
            .expect("request handling result should be OK");

        let mut ifindexes_seen = Vec::new();
        for sent_message in sender_sink.take_messages() {
            match sent_message.message.payload {
                NetlinkPayload::InnerMessage(RouteNetlinkMessage::NewNeighbour(
                    NeighbourMessage { header: NeighbourHeader { ifindex, .. }, .. },
                )) => {
                    ifindexes_seen.push(ifindex);
                }
                _ => panic!("unexpected message sent"),
            }
        }
        ifindexes_seen.sort();
        assert_eq!(&ifindexes_seen[..], expected_ifindexes);
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_del_request_controller_error() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, completer_rcv) = oneshot::channel();
        let request = Request {
            args: NeighborRequestArgs::Del(DelNeighborArgs {
                ip: fidl_ip!("192.168.0.1"),
                interface: NonZeroU64::new(1).unwrap(),
            }),
            sequence_number: 1,
            client,
            completer,
        };

        let events = {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            fnet_neighbor_ext::testutil::generate_events_from_spec(&[Idle])
        };
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![events]));
        let (controller, mut controller_request_stream) =
            fidl::endpoints::create_proxy_and_stream::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let interfaces = BTreeMap::new();
        let handle_request_fut = worker.handle_request(request, &interfaces);
        let controller_fut = async {
            match controller_request_stream
                .next()
                .await
                .expect("controller stream should not be closed")
                .expect("failed to receive controller request")
            {
                fnet_neighbor::ControllerRequest::RemoveEntry {
                    interface: _,
                    neighbor: _,
                    responder,
                } => {
                    responder
                        .send(Err(fnet_neighbor::ControllerError::InterfaceNotFound))
                        .expect("failed to send error response");
                }
                _ => panic!("unexpected controller request"),
            }
        };

        let (handle_result, ()) = futures::join!(handle_request_fut, controller_fut);
        assert_matches!(handle_result, None);

        let result = completer_rcv.await.expect("completer channel should not be closed");
        assert_matches!(result, Err(RequestError::InterfaceNotFound));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_del_request_success() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, _completer_rcv) = oneshot::channel();
        let args =
            DelNeighborArgs { ip: fidl_ip!("192.168.0.1"), interface: NonZeroU64::new(1).unwrap() };
        let request =
            Request { args: NeighborRequestArgs::Del(args), sequence_number: 1, client, completer };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let batches = vec![events];
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(batches));
        let (controller, mut controller_request_stream) =
            fidl::endpoints::create_proxy_and_stream::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let interfaces = BTreeMap::new();
        let handle_request_fut = worker.handle_request(request, &interfaces);
        let controller_fut = async {
            match controller_request_stream
                .next()
                .await
                .expect("controller stream should not be closed")
                .expect("failed to receive controller request")
            {
                fnet_neighbor::ControllerRequest::RemoveEntry {
                    interface: _,
                    neighbor: _,
                    responder,
                } => {
                    responder.send(Ok(())).expect("failed to send success response");
                }
                _ => panic!("unexpected controller request"),
            }
        };

        let (handle_result, ()) = futures::join!(handle_request_fut, controller_fut);
        let pending = handle_result.expect("expected pending request");
        assert_eq!(pending.request_args, PendingNeighborRequestArgs::Del(args));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_pending_del_request() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, mut completer_rcv) = oneshot::channel();
        let args =
            DelNeighborArgs { ip: fidl_ip!("192.168.0.1"), interface: NonZeroU64::new(1).unwrap() };
        let pending = PendingNeighborRequest {
            request_args: PendingNeighborRequestArgs::Del(args),
            client,
            completer,
        };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let batches = vec![events];
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(batches));
        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let key = NeighborKey { interface: args.interface, neighbor: args.ip };
        let _ = worker.neighbor_table.insert(
            key,
            fnet_neighbor_ext::Entry {
                interface: args.interface,
                neighbor: args.ip,
                ..valid_neighbor_entry()
            },
        );

        // Still present, should remain pending.
        let pending = worker.handle_pending_request(pending).expect("expected pending");
        assert_matches!(completer_rcv.try_recv(), Ok(None)); // Completer still blocked.

        // Remove from table, should complete.
        let _ = worker.neighbor_table.remove(&key);
        assert_matches!(worker.handle_pending_request(pending), None);

        let result = completer_rcv.try_recv().expect("completer channel should not be closed");
        assert_matches!(result, Some(Ok(())));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_create_static_request_controller_error() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, completer_rcv) = oneshot::channel();
        let args = NewNeighborArgs::CreateStatic {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
            mac: fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] },
        };
        let request =
            Request { args: NeighborRequestArgs::New(args), sequence_number: 1, client, completer };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let batches = vec![events];
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(batches));
        let (controller, mut controller_request_stream) =
            fidl::endpoints::create_proxy_and_stream::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let interfaces = BTreeMap::new();
        let handle_request_fut = worker.handle_request(request, &interfaces);
        let controller_fut = async {
            match controller_request_stream
                .next()
                .await
                .expect("controller stream should not be closed")
                .expect("failed to receive controller request")
            {
                fnet_neighbor::ControllerRequest::AddEntry {
                    interface: _,
                    neighbor: _,
                    mac: _,
                    responder,
                } => {
                    responder
                        .send(Err(fnet_neighbor::ControllerError::InterfaceNotFound))
                        .expect("failed to send error response");
                }
                _ => panic!("unexpected controller request"),
            }
        };

        let (handle_result, ()) = futures::join!(handle_request_fut, controller_fut);
        assert_matches!(handle_result, None);

        let result = completer_rcv.await.expect("completer channel should not be closed");
        assert_matches!(result, Err(RequestError::InterfaceNotFound));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_create_static_request_success() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, _completer_rcv) = oneshot::channel();
        let args = NewNeighborArgs::CreateStatic {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
            mac: fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] },
        };
        let request =
            Request { args: NeighborRequestArgs::New(args), sequence_number: 1, client, completer };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![events]));
        let (controller, mut controller_request_stream) =
            fidl::endpoints::create_proxy_and_stream::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let interfaces = BTreeMap::new();
        let handle_request_fut = worker.handle_request(request, &interfaces);
        let controller_fut = async {
            match controller_request_stream
                .next()
                .await
                .expect("controller stream should not be closed")
                .expect("failed to receive controller request")
            {
                fnet_neighbor::ControllerRequest::AddEntry {
                    interface: _,
                    neighbor: _,
                    mac: _,
                    responder,
                } => {
                    responder.send(Ok(())).expect("failed to send success response");
                }
                _ => panic!("unexpected controller request"),
            }
        };

        let (handle_result, ()) = futures::join!(handle_request_fut, controller_fut);
        let pending = handle_result.expect("expected pending request");
        assert_eq!(pending.request_args, PendingNeighborRequestArgs::New(args));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_probe_request_controller_error() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, completer_rcv) = oneshot::channel();
        let args = NewNeighborArgs::ProbeExisting {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
        };
        let request =
            Request { args: NeighborRequestArgs::New(args), sequence_number: 1, client, completer };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![events]));
        let (controller, mut controller_request_stream) =
            fidl::endpoints::create_proxy_and_stream::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let interfaces = BTreeMap::new();
        let handle_request_fut = worker.handle_request(request, &interfaces);
        let controller_fut = async {
            match controller_request_stream
                .next()
                .await
                .expect("controller stream should not be closed")
                .expect("failed to receive controller request")
            {
                fnet_neighbor::ControllerRequest::ProbeEntry {
                    interface: _,
                    neighbor: _,
                    responder,
                } => {
                    responder
                        .send(Err(fnet_neighbor::ControllerError::InterfaceNotFound))
                        .expect("failed to send error response");
                }
                _ => panic!("unexpected controller request"),
            }
        };

        let (handle_result, ()) = futures::join!(handle_request_fut, controller_fut);
        assert_matches!(handle_result, None);

        let result = completer_rcv.await.expect("completer channel should not be closed");
        assert_matches!(result, Err(RequestError::InterfaceNotFound));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_probe_request_success() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, _completer_rcv) = oneshot::channel();
        let args = NewNeighborArgs::ProbeExisting {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
        };
        let request =
            Request { args: NeighborRequestArgs::New(args), sequence_number: 1, client, completer };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![events]));
        let (controller, mut controller_request_stream) =
            fidl::endpoints::create_proxy_and_stream::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let interfaces = BTreeMap::new();
        let handle_request_fut = worker.handle_request(request, &interfaces);
        let controller_fut = async {
            match controller_request_stream
                .next()
                .await
                .expect("controller stream should not be closed")
                .expect("failed to receive controller request")
            {
                fnet_neighbor::ControllerRequest::ProbeEntry {
                    interface: _,
                    neighbor: _,
                    responder,
                } => {
                    responder.send(Ok(())).expect("failed to send success response");
                }
                _ => panic!("unexpected controller request"),
            }
        };

        let (handle_result, ()) = futures::join!(handle_request_fut, controller_fut);
        let pending = handle_result.expect("expected pending request");
        assert_eq!(pending.request_args, PendingNeighborRequestArgs::New(args));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_pending_create_static_request() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, mut completer_rcv) = oneshot::channel();
        let neighbor = valid_neighbor_entry();
        let args = NewNeighborArgs::CreateStatic {
            ip: neighbor.neighbor,
            interface: neighbor.interface,
            mac: neighbor.mac.unwrap(),
        };
        let pending = PendingNeighborRequest {
            request_args: PendingNeighborRequestArgs::New(args),
            client,
            completer,
        };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![events]));
        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let key = NeighborKey { interface: neighbor.interface, neighbor: neighbor.neighbor };

        // Not present, should remain pending.
        let pending = worker.handle_pending_request(pending).expect("expected pending");
        assert_matches!(completer_rcv.try_recv(), Ok(None));

        // Insert with wrong MAC, should remain pending.
        let _ = worker.neighbor_table.insert(
            key,
            fnet_neighbor_ext::Entry {
                mac: Some(fnet::MacAddress { octets: [0, 0, 0, 0, 0, 0] }),
                state: fnet_neighbor::EntryState::Static,
                ..neighbor
            },
        );
        let pending = worker.handle_pending_request(pending).expect("expected pending");
        assert_matches!(completer_rcv.try_recv(), Ok(None));

        // Insert with wrong state, should remain pending.
        let _ = worker.neighbor_table.insert(
            key,
            fnet_neighbor_ext::Entry {
                mac: Some(fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] }),
                state: fnet_neighbor::EntryState::Reachable,
                ..neighbor
            },
        );
        let pending = worker.handle_pending_request(pending).expect("expected pending");
        assert_matches!(completer_rcv.try_recv(), Ok(None));

        // Insert correct entry, should complete.
        let _ = worker.neighbor_table.insert(
            key,
            fnet_neighbor_ext::Entry {
                mac: Some(fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] }),
                state: fnet_neighbor::EntryState::Static,
                ..neighbor
            },
        );
        assert_matches!(worker.handle_pending_request(pending), None);

        let result = completer_rcv.try_recv().expect("completer channel should not be closed");
        assert_matches!(result, Some(Ok(())));
    }

    #[fuchsia::test]
    async fn neighbors_worker_handle_pending_probe_request() {
        let (_sender_sink, client, _async_work_drain_task) = new_fake_client(CLIENT_ID_1, vec![]);
        let (completer, mut completer_rcv) = oneshot::channel();
        let neighbor = valid_neighbor_entry();
        let args =
            NewNeighborArgs::ProbeExisting { ip: neighbor.neighbor, interface: neighbor.interface };
        let pending = PendingNeighborRequest {
            request_args: PendingNeighborRequestArgs::New(args),
            client,
            completer,
        };

        let events = fnet_neighbor_ext::testutil::generate_events_from_spec(&[
            fnet_neighbor_ext::testutil::EventSpec::Idle,
        ]);
        let (view, server_fut) =
            fnet_neighbor_ext::testutil::create_fake_view(futures::stream::iter(vec![events]));
        let (controller, _controller_server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let worker_fut = NeighborsWorker::create(&view, controller);
        let ((), (mut worker, _event_stream)) = futures::join!(server_fut, worker_fut);

        let key = NeighborKey { interface: neighbor.interface, neighbor: neighbor.neighbor };

        // Not present, should remain pending.
        let pending = worker.handle_pending_request(pending).expect("expected pending");
        assert_matches!(completer_rcv.try_recv(), Ok(None));

        // Insert with wrong state, should remain pending.
        let _ = worker.neighbor_table.insert(
            key,
            fnet_neighbor_ext::Entry { state: fnet_neighbor::EntryState::Reachable, ..neighbor },
        );
        let pending = worker.handle_pending_request(pending).expect("expected pending");
        assert_matches!(completer_rcv.try_recv(), Ok(None));

        // Insert correct entry, should complete.
        let _ = worker.neighbor_table.insert(
            key,
            fnet_neighbor_ext::Entry { state: fnet_neighbor::EntryState::Probe, ..neighbor },
        );
        assert_matches!(worker.handle_pending_request(pending), None);

        let result = completer_rcv.try_recv().expect("completer channel should not be closed");
        assert_matches!(result, Some(Ok(())));
    }

    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Ok(GetNeighborArgs::Get {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
        });
        "get ipv4 success"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet6,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v6!("fe80::1").into()),
        ] => Ok(GetNeighborArgs::Get {
            ip: fidl_ip!("fe80::1"),
            interface: NonZeroU64::new(1).unwrap(),
        });
        "get ipv6 success"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            state: NeighbourState::Reachable,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::InvalidState {
            actual: NeighbourState::Reachable, expected: NeighbourState::None
        });
        "get invalid request state"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            kind: RouteType::Broadcast,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::InvalidKind(RouteType::Broadcast));
        "get invalid request kind"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            flags: NeighbourFlags::Router,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::InvalidFlags(NeighbourFlags::Router));
        "get invalid request flag"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            flags: NeighbourFlags::Proxy,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::UnsupportedFlags(NeighbourFlags::Proxy));
        "get unsupported request flag"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet6,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::AddressFamilyMismatch(AddressFamily::Inet6));
        "get address family mismatch"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::MissingInterface);
        "get interface unspecified"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[] => Err(RequestError::MissingIpAddress);
        "get destination unspecified"
    )]
    #[test_case(
        false,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
            NeighbourAttribute::LinkLocalAddress(vec![0, 1, 2, 3, 4, 5]),
        ] => Err(RequestError::InvalidAttribute);
        "get invalid attribute"
    )]
    #[test_case(
        true,
        NeighbourHeader::default(),
        &[] => Ok(GetNeighborArgs::Dump {
            ip_version: None,
            interface: None,
        });
        "dump all"
    )]
    #[test_case(
        true,
        NeighbourHeader {
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[] => Ok(GetNeighborArgs::Dump {
            ip_version: Some(IpVersion::V4),
            interface: None,
        });
        "dump ipv4 only"
    )]
    #[test_case(
        true,
        NeighbourHeader {
            family: AddressFamily::Inet6,
            ..Default::default()
        },
        &[] => Ok(GetNeighborArgs::Dump {
            ip_version: Some(IpVersion::V6),
            interface: None,
        });
        "dump ipv6 only"
    )]
    #[test_case(
        true,
        NeighbourHeader::default(),
        &[
            NeighbourAttribute::IfIndex(1),
        ] => Ok(GetNeighborArgs::Dump {
            ip_version: None,
            interface: Some(NonZeroU64::new(1).unwrap()),
        });
        "dump interface 1"
    )]
    #[test_case(
        true,
        NeighbourHeader::default(),
        &[
            NeighbourAttribute::IfIndex(0),
        ] => Ok(GetNeighborArgs::Dump {
            ip_version: None,
            interface: None,
        });
        "dump interface 0 treated as all interfaces"
    )]
    #[test_case(
        true,
        NeighbourHeader {
            family: AddressFamily::Local,
            ..Default::default()
        },
        &[] => Err(RequestError::InvalidAddressFamily(AddressFamily::Local));
        "dump invalid address family"
    )]
    #[test_case(
        true,
        NeighbourHeader::default(),
        &[
            NeighbourAttribute::LinkLocalAddress(vec![0, 1, 2, 3, 4, 5]),
        ] => Ok(GetNeighborArgs::Dump {
            ip_version: None,
            interface: None,
        });
        "dump unsupported attribute ignored (non-strict)"
    )]
    #[test_case(
        true,
        NeighbourHeader {
            flags: NeighbourFlags::Proxy,
            ..Default::default()
        },
        &[] => Err(RequestError::UnsupportedFlags(NeighbourFlags::Proxy));
        "dump unsupported request flag"
    )]
    #[fuchsia::test]
    fn get_neighbor_args_try_from_rtnl_neighbor(
        is_dump: bool,
        header: NeighbourHeader,
        attrs: &[NeighbourAttribute],
    ) -> Result<GetNeighborArgs, RequestError> {
        let mut message = NeighbourMessage::default();
        message.header = header;
        message.attributes = attrs.to_vec();
        GetNeighborArgs::try_from_rtnl_neighbor(&message, is_dump)
    }

    #[test_case(
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Ok(DelNeighborArgs {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
        });
        "del ipv4 success"
    )]
    #[test_case(
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet6,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v6!("fe80::1").into()),
        ] => Ok(DelNeighborArgs {
            ip: fidl_ip!("fe80::1"),
            interface: NonZeroU64::new(1).unwrap(),
        });
        "del ipv6 success"
    )]
    #[test_case(
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            flags: NeighbourFlags::Proxy,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::UnsupportedFlags(NeighbourFlags::Proxy));
        "del unsupported request flag"
    )]
    #[test_case(
        NeighbourHeader {
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::MissingInterface);
        "del interface unspecified"
    )]
    #[test_case(
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[] => Err(RequestError::MissingIpAddress);
        "del destination unspecified"
    )]
    #[fuchsia::test]
    fn del_neighbor_args_try_from_rtnl_neighbor(
        header: NeighbourHeader,
        attrs: &[NeighbourAttribute],
    ) -> Result<DelNeighborArgs, RequestError> {
        let mut message = NeighbourMessage::default();
        message.header = header;
        message.attributes = attrs.to_vec();
        DelNeighborArgs::try_from_rtnl_neighbor(&message)
    }

    #[test_case(
        NLM_F_CREATE | NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            state: NeighbourState::Permanent,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
            NeighbourAttribute::LinkLocalAddress(vec![0, 1, 2, 3, 4, 5]),
        ] => Ok(NewNeighborArgs::CreateStatic {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
            mac: fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] },
        });
        "create static ipv4 success"
    )]
    #[test_case(
        NLM_F_CREATE | NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet6,
            state: NeighbourState::Permanent,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v6!("fe80::1").into()),
            NeighbourAttribute::LinkLocalAddress(vec![0, 1, 2, 3, 4, 5]),
        ] => Ok(NewNeighborArgs::CreateStatic {
            ip: fidl_ip!("fe80::1"),
            interface: NonZeroU64::new(1).unwrap(),
            mac: fnet::MacAddress { octets: [0, 1, 2, 3, 4, 5] },
        });
        "create static ipv6 success"
    )]
    #[test_case(
        NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            state: NeighbourState::Probe,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Ok(NewNeighborArgs::ProbeExisting {
            ip: fidl_ip!("192.168.0.1"),
            interface: NonZeroU64::new(1).unwrap(),
        });
        "probe existing ipv4 success"
    )]
    #[test_case(
        NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet6,
            state: NeighbourState::Probe,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v6!("fe80::1").into()),
        ] => Ok(NewNeighborArgs::ProbeExisting {
            ip: fidl_ip!("fe80::1"),
            interface: NonZeroU64::new(1).unwrap(),
        });
        "probe existing ipv6 success"
    )]
    #[test_case(
        NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            flags: NeighbourFlags::Proxy,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::UnsupportedFlags(NeighbourFlags::Proxy));
        "unsupported proxy flag"
    )]
    #[test_case(
        NLM_F_CREATE | NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            state: NeighbourState::Reachable,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
            NeighbourAttribute::LinkLocalAddress(vec![0, 1, 2, 3, 4, 5]),
        ] => Err(RequestError::InvalidState {
            actual: NeighbourState::Reachable,
            expected: NeighbourState::Permanent,
        });
        "create invalid state"
    )]
    #[test_case(
        NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            state: NeighbourState::Permanent,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::InvalidState {
            actual: NeighbourState::Permanent,
            expected: NeighbourState::Probe,
        });
        "probe invalid state"
    )]
    #[test_case(
        NLM_F_CREATE | NLM_F_REPLACE,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            state: NeighbourState::Permanent,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::MissingMacAddress);
        "create missing mac address"
    )]
    #[test_case(
        NLM_F_EXCL,
        NeighbourHeader {
            ifindex: 1,
            family: AddressFamily::Inet,
            ..Default::default()
        },
        &[
            NeighbourAttribute::Destination(std_ip_v4!("192.168.0.1").into()),
        ] => Err(RequestError::UnsupportedOperation);
        "unsupported operation flags"
    )]
    #[fuchsia::test]
    fn new_neighbor_args_try_from_rtnl_neighbor(
        netlink_flags: u16,
        header: NeighbourHeader,
        attrs: &[NeighbourAttribute],
    ) -> Result<NewNeighborArgs, RequestError> {
        let mut message = NeighbourMessage::default();
        message.header = header;
        message.attributes = attrs.to_vec();
        NewNeighborArgs::try_from_rtnl_neighbor(&message, netlink_flags)
    }

    #[test_case(
        RequestError::InvalidState {
            actual: NeighbourState::Reachable, expected:NeighbourState::None
        } => Errno::EINVAL;
        "invalid state"
    )]
    #[test_case(RequestError::InvalidKind(RouteType::Broadcast) => Errno::EINVAL; "invalid kind")]
    #[test_case(
        RequestError::InvalidFlags(NeighbourFlags::Router) => Errno::EINVAL;
        "invalid flags"
    )]
    #[test_case(
        RequestError::UnsupportedFlags(NeighbourFlags::Proxy) => Errno::ENOTSUP;
        "unsupported flags"
    )]
    #[test_case(
        RequestError::InvalidAddressFamily(AddressFamily::Local) => Errno::EAFNOSUPPORT;
        "invalid address family"
    )]
    #[test_case(
        RequestError::AddressFamilyMismatch(AddressFamily::Inet6) => Errno::EINVAL;
        "address family mismatch"
    )]
    #[test_case(RequestError::MissingInterface => Errno::EINVAL; "interface unspecified")]
    #[test_case(RequestError::MissingIpAddress => Errno::EINVAL; "destination unspecified")]
    #[test_case(RequestError::InvalidAttribute => Errno::EINVAL; "invalid attribute")]
    #[test_case(RequestError::NeighborNotFound => Errno::ENOENT; "neighbor not found")]
    #[test_case(RequestError::InterfaceNotFound => Errno::ENODEV; "interface not found")]
    #[test_case(RequestError::InvalidIpAddress => Errno::EINVAL; "invalid IP address")]
    #[test_case(RequestError::InvalidMacAddress => Errno::EINVAL; "invalid MAC address")]
    #[test_case(RequestError::InterfaceUnsupported => Errno::ENOTSUP; "unsupported interface")]
    #[fuchsia::test]
    fn request_error_into_errno(error: RequestError) -> Errno {
        error.into()
    }

    enum NeighborsAndInterfaces {}
    impl EventLoopSpec for NeighborsAndInterfaces {
        type NeighborWorker = Required;

        type InterfacesProxy = Required;
        type InterfacesStateProxy = Required;
        type InterfacesHandler = Required;
        type RouteClients = Required;
        type InterfacesWorker = Required;

        type V4RoutesState = Optional;
        type V6RoutesState = Optional;
        type V4RoutesSetProvider = Optional;
        type V6RoutesSetProvider = Optional;
        type V4RouteTableProvider = Optional;
        type V6RouteTableProvider = Optional;

        type RoutesV4Worker = Optional;
        type RoutesV6Worker = Optional;
        type RuleV4Worker = Optional;
        type RuleV6Worker = Optional;
        type NduseroptWorker = Optional;
    }

    const TEST_SEQUENCE_NUMBER: u32 = 1234;

    struct EventLoopSetup {
        event_loop: EventLoopState<
            FakeInterfacesHandler,
            FakeSender<RouteNetlinkMessage>,
            NeighborsAndInterfaces,
        >,
        request_sink: mpsc::Sender<UnifiedRequest<FakeSender<RouteNetlinkMessage>>>,
        neighbors_controller_request_stream: fnet_neighbor::ControllerRequestStream,
        neighbor_event_sink: mpsc::UnboundedSender<Vec<fnet_neighbor::EntryIteratorItem>>,
        interface_event_sink: mpsc::UnboundedSender<fnet_interfaces::Event>,
    }

    async fn build_event_loop(
        scope: &fuchsia_async::Scope,
        neighbor_events: &[EventSpec],
    ) -> EventLoopSetup {
        let included_workers = IncludedWorkers {
            routes_v4: EventLoopComponent::Absent(Optional),
            routes_v6: EventLoopComponent::Absent(Optional),
            interfaces: EventLoopComponent::Present(()),
            rules_v4: EventLoopComponent::Absent(Optional),
            rules_v6: EventLoopComponent::Absent(Optional),
            neighbors: EventLoopComponent::Present(()),
            nduseropt: EventLoopComponent::Absent(Optional),
        };
        let (request_sink, request_stream) = mpsc::channel(1);

        // Configure fake neighbor watch events.

        let (neighbors_controller, neighbors_controller_request_stream) =
            fidl::endpoints::create_proxy_and_stream::<fnet_neighbor::ControllerMarker>();

        let (neighbors_view, neighbor_event_sink) = {
            let events = fnet_neighbor_ext::testutil::generate_events_from_spec(neighbor_events);
            let (event_sender, event_receiver) = mpsc::unbounded();
            event_sender.unbounded_send(events).expect("failed to send events");
            let (neighbors_view, neighbors_fut) =
                fnet_neighbor_ext::testutil::create_fake_view(event_receiver);
            let _join_handle = scope.spawn(neighbors_fut);
            (neighbors_view, event_sender)
        };

        // Configure fake interface watch events.

        let (interfaces_handler, _interfaces_handler_sink) =
            crate::interfaces::testutil::FakeInterfacesHandler::new();
        let (interfaces_proxy, _interfaces) =
            fidl::endpoints::create_proxy::<fnet_root::InterfacesMarker>();
        let (interfaces_state_proxy, interfaces_state) =
            fidl::endpoints::create_proxy::<fnet_interfaces::StateMarker>();
        let route_clients = ClientTable::default();

        let interface_event_sink = {
            let if_stream = interfaces_state.into_stream();
            let if_watcher_stream = if_stream
                .and_then(|req| match req {
                    fnet_interfaces::StateRequest::GetWatcher {
                        options: _,
                        watcher,
                        control_handle: _,
                    } => futures::future::ready(Ok(watcher.into_stream())),
                })
                .try_flatten()
                .map(|res| res.expect("watcher stream error"));
            let (event_sender, event_receiver) = mpsc::unbounded();
            event_sender
                .unbounded_send(fnet_interfaces::Event::Idle(fnet_interfaces::Empty))
                .expect("failed to send event");
            let interfaces_fut =
                if_watcher_stream.zip(event_receiver).for_each(|(req, update)| async move {
                    match req {
                        fnet_interfaces::WatcherRequest::Watch { responder } => {
                            responder.send(&update).expect("send watch response")
                        }
                    }
                });
            let _join_handle = scope.spawn(interfaces_fut);
            event_sender
        };

        // Set up the event loop.

        let (_async_work_sink, async_work_receiver) = mpsc::unbounded();
        let base_inputs: EventLoopInputs<
            FakeInterfacesHandler,
            FakeSender<RouteNetlinkMessage>,
            NeighborsAndInterfaces,
        > = EventLoopInputs {
            neighbors_view: EventLoopComponent::Present(neighbors_view),
            neighbors_controller: EventLoopComponent::Present(neighbors_controller),

            route_clients: EventLoopComponent::Present(route_clients),
            interfaces_handler: EventLoopComponent::Present(interfaces_handler),
            interfaces_proxy: EventLoopComponent::Present(interfaces_proxy),
            interfaces_state_proxy: EventLoopComponent::Present(interfaces_state_proxy),

            async_work_receiver,

            v4_routes_state: EventLoopComponent::Absent(Optional),
            v6_routes_state: EventLoopComponent::Absent(Optional),
            v4_main_route_table: EventLoopComponent::Absent(Optional),
            v6_main_route_table: EventLoopComponent::Absent(Optional),
            v4_route_table_provider: EventLoopComponent::Absent(Optional),
            v6_route_table_provider: EventLoopComponent::Absent(Optional),
            v4_rule_table: EventLoopComponent::Absent(Optional),
            v6_rule_table: EventLoopComponent::Absent(Optional),
            ndp_option_watcher_provider: EventLoopComponent::Absent(Optional),

            unified_request_stream: request_stream,
        };

        let event_loop = base_inputs.initialize(included_workers).await;
        EventLoopSetup {
            event_loop,
            request_sink,
            neighbors_controller_request_stream,
            neighbor_event_sink,
            interface_event_sink,
        }
    }

    #[fuchsia::test]
    async fn event_loop_with_watch_events_and_get_request() {
        let scope = fuchsia_async::Scope::new();
        use fnet_neighbor_ext::testutil::EventSpec::*;
        let EventLoopSetup {
            mut event_loop,
            mut request_sink,
            neighbors_controller_request_stream: _,
            neighbor_event_sink,
            interface_event_sink,
        } = build_event_loop(&scope, &[Existing(1), Existing(2), Existing(3), Idle, Added(4)])
            .await;

        // Wait for `Added` event to be processed.
        event_loop.run_one_step_in_tests().await;

        // Send a dump request and check the response.

        let (mut response_sink, neighbor_client, async_work_drain_task) =
            crate::client::testutil::new_fake_client::<NetlinkRoute>(
                crate::client::testutil::CLIENT_ID_1,
                [],
            );
        let _join_handle = scope.spawn(async_work_drain_task);

        let (completer, waiter) = oneshot::channel();
        let get_request: UnifiedRequest<FakeSender<RouteNetlinkMessage>> =
            UnifiedRequest::NeighborsRequest(Request {
                args: NeighborRequestArgs::Get(GetNeighborArgs::Dump {
                    ip_version: None,
                    interface: None,
                }),
                sequence_number: TEST_SEQUENCE_NUMBER,
                client: neighbor_client.clone(),
                completer,
            });
        request_sink.send(get_request).await.unwrap();

        // Wait for client request to be processed.
        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.await.unwrap(), Ok(()));

        let responses = response_sink.take_messages();
        assert_eq!(responses.len(), 4); // 3 existing + 1 added.
        for response in responses {
            assert_matches!(
                response.message.payload,
                NetlinkPayload::InnerMessage(RouteNetlinkMessage::NewNeighbour(_))
            );
        }

        neighbor_event_sink.close_channel();
        interface_event_sink.close_channel();
        drop(neighbor_client);
        scope.join().await;
    }

    #[fuchsia::test]
    async fn event_loop_with_watch_events_and_delete_request() {
        let scope = fuchsia_async::Scope::new();
        let neighbor_events = {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            vec![Existing(1), Existing(2), Idle]
        };
        let EventLoopSetup {
            mut event_loop,
            mut request_sink,
            neighbors_controller_request_stream,
            neighbor_event_sink,
            interface_event_sink,
        } = build_event_loop(&scope, &neighbor_events).await;

        let fnet_neighbor::EntryIteratorItem::Existing(to_delete) =
            fnet_neighbor_ext::testutil::generate_event_from_spec(&neighbor_events[0])
        else {
            panic!("unexpected event")
        };
        let to_delete: fnet_neighbor_ext::Entry =
            to_delete.try_into().expect("extension conversion failed");

        let (mut response_sink, neighbor_client, async_work_drain_task) =
            crate::client::testutil::new_fake_client::<NetlinkRoute>(
                crate::client::testutil::CLIENT_ID_1,
                [],
            );
        let _join_handle = scope.spawn(async_work_drain_task);

        // Send an RTM_DELNEIGH request for an existing neighbor.

        let (completer, waiter) = oneshot::channel();
        let waiter = waiter.fuse();
        pin_mut!(waiter);

        let del_request: UnifiedRequest<FakeSender<RouteNetlinkMessage>> =
            UnifiedRequest::NeighborsRequest(Request {
                args: NeighborRequestArgs::Del(DelNeighborArgs {
                    ip: to_delete.neighbor,
                    interface: to_delete.interface,
                }),
                sequence_number: TEST_SEQUENCE_NUMBER,
                client: neighbor_client.clone(),
                completer,
            });
        request_sink.send(del_request).await.expect("failed to send delete request");

        // Handle the expected Controller.RemoveEntry request and verify that
        // the RTM_DELNEIGH request is still pending.

        let controller_req_fut = neighbors_controller_request_stream
            .into_future()
            .then(async move |(req, _rest)| {
                match req
                    .expect("Controller request stream unexpectedly ended")
                    .expect("failed to receive `Controller` request")
                {
                    fnet_neighbor::ControllerRequest::RemoveEntry {
                        interface,
                        neighbor,
                        responder,
                        ..
                    } => {
                        assert_eq!(interface, to_delete.interface.get());
                        assert_eq!(neighbor, to_delete.neighbor);
                        responder.send(Ok(())).expect("failed to respond to RemoveEntry");
                    }
                    _ => panic!("unexpected controller request"),
                }
            })
            .fuse();
        let _join_handle = scope.spawn(controller_req_fut);
        event_loop.run_one_step_in_tests().await;

        assert_matches!(waiter.as_mut().now_or_never(), None);
        assert_eq!(response_sink.take_messages().len(), 0);

        // Send an unrelated neighbor watch event and verify that the request is
        // still pending.

        {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            neighbor_event_sink
                .unbounded_send(fnet_neighbor_ext::testutil::generate_events_from_spec(&[Added(3)]))
                .expect("failed to send event");
        }

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.as_mut().now_or_never(), None);
        assert_eq!(response_sink.take_messages().len(), 0);

        // Send a neighbor watch event indicating successful removal and verify
        // that the request is completed.

        {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            neighbor_event_sink
                .unbounded_send(fnet_neighbor_ext::testutil::generate_events_from_spec(&[Removed(
                    1,
                )]))
                .expect("failed to send event");
        }

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.await.expect("waiter channel should not be closed"), Ok(()));
        // The event loop & worker aren't responsible for the final response to
        // the client (either none, or ACK if requested), so there's nothing
        // more to check here.

        neighbor_event_sink.close_channel();
        interface_event_sink.close_channel();
        drop(neighbor_client);
        scope.join().await;
    }

    #[fuchsia::test]
    async fn event_loop_with_watch_events_create_static_neighbor() {
        let scope = fuchsia_async::Scope::new();
        let neighbor_events = vec![fnet_neighbor_ext::testutil::EventSpec::Idle];
        let EventLoopSetup {
            mut event_loop,
            mut request_sink,
            neighbors_controller_request_stream,
            neighbor_event_sink,
            interface_event_sink,
        } = build_event_loop(&scope, &neighbor_events).await;

        let (mut response_sink, neighbor_client, async_work_drain_task) =
            crate::client::testutil::new_fake_client::<NetlinkRoute>(
                crate::client::testutil::CLIENT_ID_1,
                [],
            );
        let _join_handle = scope.spawn(async_work_drain_task);

        // Create expected entry but without sending neighbor event.

        let (event, entry) = {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            let mut added = fnet_neighbor_ext::testutil::generate_event_from_spec(&Added(1));
            let fnet_neighbor::EntryIteratorItem::Added(to_add) = &mut added else {
                panic!("unexpected event")
            };
            to_add.state = Some(fnet_neighbor::EntryState::Static);
            let entry: fnet_neighbor_ext::Entry =
                to_add.clone().try_into().expect("extension conversion failed");
            (added, entry)
        };

        // Send an RTM_NEWNEIGH request for an existing neighbor.

        let (completer, waiter) = oneshot::channel();
        let waiter = waiter.fuse();
        pin_mut!(waiter);

        let create_request: UnifiedRequest<FakeSender<RouteNetlinkMessage>> =
            UnifiedRequest::NeighborsRequest(Request {
                args: NeighborRequestArgs::New(NewNeighborArgs::CreateStatic {
                    ip: entry.neighbor,
                    interface: entry.interface,
                    mac: entry.mac.unwrap(),
                }),
                sequence_number: TEST_SEQUENCE_NUMBER,
                client: neighbor_client.clone(),
                completer,
            });
        request_sink.send(create_request).await.expect("failed to send create request");

        // Handle the expected Controller.AddEntry request and verify that the
        // RTM_NEWNEIGH request is still pending.

        let controller_req_fut = neighbors_controller_request_stream
            .into_future()
            .then(async move |(req, _rest)| {
                match req
                    .expect("Controller request stream unexpectedly ended")
                    .expect("failed to receive `Controller` request")
                {
                    fnet_neighbor::ControllerRequest::AddEntry {
                        interface,
                        neighbor,
                        mac,
                        responder,
                    } => {
                        assert_eq!(interface, entry.interface.get());
                        assert_eq!(neighbor, entry.neighbor);
                        assert_eq!(mac, entry.mac.unwrap());
                        responder.send(Ok(())).expect("failed to respond to AddEntry");
                    }
                    _ => panic!("unexpected controller request"),
                }
            })
            .fuse();
        let _join_handle = scope.spawn(controller_req_fut);

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.as_mut().now_or_never(), None);
        assert_eq!(response_sink.take_messages().len(), 0);

        // Send an unrelated neighbor watch event and verify that the request is
        // still pending.

        {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            neighbor_event_sink
                .unbounded_send(fnet_neighbor_ext::testutil::generate_events_from_spec(&[Added(3)]))
                .expect("failed to send event");
        }

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.as_mut().now_or_never(), None);
        assert_eq!(response_sink.take_messages().len(), 0);

        // Send a neighbor watch event indicating successful creation and verify
        // that the request is completed.

        neighbor_event_sink.unbounded_send(vec![event]).expect("failed to send event");

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.await.expect("waiter channel should not be closed"), Ok(()));
        // The event loop & worker aren't responsible for the final response to
        // the client (either none, or ACK if requested), so there's nothing
        // more to check here.

        neighbor_event_sink.close_channel();
        interface_event_sink.close_channel();
        drop(neighbor_client);
        scope.join().await;
    }

    #[fuchsia::test]
    async fn event_loop_with_watch_events_create_already_existing_succeeds_without_event() {
        let scope = fuchsia_async::Scope::new();
        let neighbor_events = vec![fnet_neighbor_ext::testutil::EventSpec::Idle];
        let EventLoopSetup {
            mut event_loop,
            mut request_sink,
            neighbors_controller_request_stream,
            neighbor_event_sink,
            interface_event_sink,
        } = build_event_loop(&scope, &neighbor_events).await;

        let (mut response_sink, neighbor_client, async_work_drain_task) =
            crate::client::testutil::new_fake_client::<NetlinkRoute>(
                crate::client::testutil::CLIENT_ID_1,
                [],
            );
        let _join_handle = scope.spawn(async_work_drain_task);

        // Report existing static entry.

        let (event, entry) = {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            let mut added = fnet_neighbor_ext::testutil::generate_event_from_spec(&Added(1));
            let fnet_neighbor::EntryIteratorItem::Added(to_add) = &mut added else {
                panic!("unexpected event")
            };
            to_add.state = Some(fnet_neighbor::EntryState::Static);
            let entry: fnet_neighbor_ext::Entry =
                to_add.clone().try_into().expect("extension conversion failed");
            (added, entry)
        };
        neighbor_event_sink.unbounded_send(vec![event]).expect("failed to send event");
        event_loop.run_one_step_in_tests().await;

        // Send an RTM_NEWNEIGH request for an existing neighbor.

        let (completer, waiter) = oneshot::channel();
        let waiter = waiter.fuse();
        pin_mut!(waiter);

        let create_request: UnifiedRequest<FakeSender<RouteNetlinkMessage>> =
            UnifiedRequest::NeighborsRequest(Request {
                args: NeighborRequestArgs::New(NewNeighborArgs::CreateStatic {
                    ip: entry.neighbor,
                    interface: entry.interface,
                    mac: entry.mac.unwrap(),
                }),
                sequence_number: TEST_SEQUENCE_NUMBER,
                client: neighbor_client.clone(),
                completer,
            });
        request_sink.send(create_request).await.expect("failed to send create request");

        // Handle the expected Controller.AddEntry request and verify that the
        // RTM_NEWNEIGH request is not pending.

        let controller_req_fut = neighbors_controller_request_stream
            .into_future()
            .then(async move |(req, _rest)| {
                match req
                    .expect("Controller request stream unexpectedly ended")
                    .expect("failed to receive `Controller` request")
                {
                    fnet_neighbor::ControllerRequest::AddEntry {
                        interface,
                        neighbor,
                        mac,
                        responder,
                    } => {
                        assert_eq!(interface, entry.interface.get());
                        assert_eq!(neighbor, entry.neighbor);
                        assert_eq!(mac, entry.mac.unwrap());
                        responder.send(Ok(())).expect("failed to respond to AddEntry");
                    }
                    _ => panic!("unexpected controller request"),
                }
            })
            .fuse();
        let _join_handle = scope.spawn(controller_req_fut);

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.await.expect("waiter channel should not be closed"), Ok(()));
        assert_eq!(response_sink.take_messages().len(), 0);
        // The event loop & worker aren't responsible for the final response to
        // the client (either none, or ACK if requested), so there's nothing
        // more to check here.

        neighbor_event_sink.close_channel();
        interface_event_sink.close_channel();
        drop(neighbor_client);
        scope.join().await;
    }

    #[fuchsia::test]
    async fn event_loop_with_watch_events_probe_neighbor() {
        let scope = fuchsia_async::Scope::new();
        let neighbor_events = vec![fnet_neighbor_ext::testutil::EventSpec::Idle];
        let EventLoopSetup {
            mut event_loop,
            mut request_sink,
            neighbors_controller_request_stream,
            neighbor_event_sink,
            interface_event_sink,
        } = build_event_loop(&scope, &neighbor_events).await;

        let (mut response_sink, neighbor_client, async_work_drain_task) =
            crate::client::testutil::new_fake_client::<NetlinkRoute>(
                crate::client::testutil::CLIENT_ID_1,
                [],
            );
        let _join_handle = scope.spawn(async_work_drain_task);

        // Report existing entry not in `Probe` state.

        let (event, entry) = {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            let mut added = fnet_neighbor_ext::testutil::generate_event_from_spec(&Added(1));
            let fnet_neighbor::EntryIteratorItem::Added(to_add) = &mut added else {
                panic!("unexpected event")
            };
            to_add.state = Some(fnet_neighbor::EntryState::Reachable);
            let entry: fnet_neighbor_ext::Entry =
                to_add.clone().try_into().expect("extension conversion failed");
            (added, entry)
        };
        neighbor_event_sink.unbounded_send(vec![event]).expect("failed to send event");
        event_loop.run_one_step_in_tests().await;

        // Send an RTM_NEWNEIGH request to probe the existing neighbor.

        let (completer, waiter) = oneshot::channel();
        let waiter = waiter.fuse();
        pin_mut!(waiter);

        let probe_request: UnifiedRequest<FakeSender<RouteNetlinkMessage>> =
            UnifiedRequest::NeighborsRequest(Request {
                args: NeighborRequestArgs::New(NewNeighborArgs::ProbeExisting {
                    ip: entry.neighbor,
                    interface: entry.interface,
                }),
                sequence_number: TEST_SEQUENCE_NUMBER,
                client: neighbor_client.clone(),
                completer,
            });
        request_sink.send(probe_request).await.expect("failed to send create request");

        // Handle the expected Controller.ProbeEntry request and verify that the
        // RTM_NEWNEIGH request is still pending.

        let controller_req_fut = neighbors_controller_request_stream
            .into_future()
            .then(async move |(req, _rest)| {
                match req
                    .expect("Controller request stream unexpectedly ended")
                    .expect("failed to receive `Controller` request")
                {
                    fnet_neighbor::ControllerRequest::ProbeEntry {
                        interface,
                        neighbor,
                        responder,
                    } => {
                        assert_eq!(interface, entry.interface.get());
                        assert_eq!(neighbor, entry.neighbor);
                        responder.send(Ok(())).expect("failed to respond to ProbeEntry");
                    }
                    _ => panic!("unexpected controller request"),
                }
            })
            .fuse();
        let _join_handle = scope.spawn(controller_req_fut);

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.as_mut().now_or_never(), None);
        assert_eq!(response_sink.take_messages().len(), 0);

        // Send an unrelated neighbor watch event and verify that the request is
        // still pending.

        {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            neighbor_event_sink
                .unbounded_send(fnet_neighbor_ext::testutil::generate_events_from_spec(&[Added(3)]))
                .expect("failed to send event");
        }

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.as_mut().now_or_never(), None);
        assert_eq!(response_sink.take_messages().len(), 0);

        // Send a neighbor watch event indicating successful transition to
        // `Probe` and verify that the request is completed.

        {
            let mut changed = entry.clone();
            changed.state = fnet_neighbor::EntryState::Probe;
            let changed_event = fnet_neighbor::EntryIteratorItem::Changed(changed.into());
            neighbor_event_sink.unbounded_send(vec![changed_event]).expect("failed to send event");
        }

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.await.expect("waiter channel should not be closed"), Ok(()));
        // The event loop & worker aren't responsible for the final response to
        // the client (either none, or ACK if requested), so there's nothing
        // more to check here.

        neighbor_event_sink.close_channel();
        interface_event_sink.close_channel();
        drop(neighbor_client);
        scope.join().await;
    }

    #[fuchsia::test]
    async fn event_loop_with_watch_events_probe_already_probing_succeeds_without_event() {
        let scope = fuchsia_async::Scope::new();
        let neighbor_events = vec![fnet_neighbor_ext::testutil::EventSpec::Idle];
        let EventLoopSetup {
            mut event_loop,
            mut request_sink,
            neighbors_controller_request_stream,
            neighbor_event_sink,
            interface_event_sink,
        } = build_event_loop(&scope, &neighbor_events).await;

        let (mut response_sink, neighbor_client, async_work_drain_task) =
            crate::client::testutil::new_fake_client::<NetlinkRoute>(
                crate::client::testutil::CLIENT_ID_1,
                [],
            );
        let _join_handle = scope.spawn(async_work_drain_task);

        // Report existing `Probe` entry.

        let (event, entry) = {
            use fnet_neighbor_ext::testutil::EventSpec::*;
            let mut added = fnet_neighbor_ext::testutil::generate_event_from_spec(&Added(1));
            let fnet_neighbor::EntryIteratorItem::Added(to_add) = &mut added else {
                panic!("unexpected event")
            };
            to_add.state = Some(fnet_neighbor::EntryState::Probe);
            let entry: fnet_neighbor_ext::Entry =
                to_add.clone().try_into().expect("extension conversion failed");
            (added, entry)
        };
        neighbor_event_sink.unbounded_send(vec![event]).expect("failed to send event");
        event_loop.run_one_step_in_tests().await;

        // Send an RTM_NEWNEIGH request for the neighbor in `Probe` state.

        let (completer, waiter) = oneshot::channel();
        let waiter = waiter.fuse();
        pin_mut!(waiter);

        let probe_request: UnifiedRequest<FakeSender<RouteNetlinkMessage>> =
            UnifiedRequest::NeighborsRequest(Request {
                args: NeighborRequestArgs::New(NewNeighborArgs::ProbeExisting {
                    ip: entry.neighbor,
                    interface: entry.interface,
                }),
                sequence_number: TEST_SEQUENCE_NUMBER,
                client: neighbor_client.clone(),
                completer,
            });
        request_sink.send(probe_request).await.expect("failed to send create request");

        // Handle the expected Controller.ProbeEntry request and verify that the
        // RTM_NEWNEIGH request is not pending.

        let controller_req_fut = neighbors_controller_request_stream
            .into_future()
            .then(async move |(req, _rest)| {
                match req
                    .expect("Controller request stream unexpectedly ended")
                    .expect("failed to receive `Controller` request")
                {
                    fnet_neighbor::ControllerRequest::ProbeEntry {
                        interface,
                        neighbor,
                        responder,
                    } => {
                        assert_eq!(interface, entry.interface.get());
                        assert_eq!(neighbor, entry.neighbor);
                        responder.send(Ok(())).expect("failed to respond to ProbeEntry");
                    }
                    _ => panic!("unexpected controller request"),
                }
            })
            .fuse();
        let _join_handle = scope.spawn(controller_req_fut);

        event_loop.run_one_step_in_tests().await;
        assert_matches!(waiter.await.expect("waiter channel should not be closed"), Ok(()));
        assert_eq!(response_sink.take_messages().len(), 0);
        // The event loop & worker aren't responsible for the final response to
        // the client (either none, or ACK if requested), so there's nothing
        // more to check here.

        neighbor_event_sink.close_channel();
        interface_event_sink.close_channel();
        drop(neighbor_client);
        scope.join().await;
    }

    #[fuchsia::test]
    async fn neighbors_worker_sends_multicast_updates() {
        let (mut sender_sink_member, client_member, _async_work_drain_task_1) =
            new_fake_client::<NetlinkRoute>(
                CLIENT_ID_1,
                [ModernGroup(rtnetlink_groups_RTNLGRP_NEIGH)],
            );
        let (mut sender_sink_non_member, client_non_member, _async_work_drain_task_2) =
            new_fake_client::<NetlinkRoute>(CLIENT_ID_2, []);

        let client_table = ClientTable::default();
        client_table.add_client(client_member);
        client_table.add_client(client_non_member);

        let (neighbors_controller, _server_end) =
            fidl::endpoints::create_proxy::<fnet_neighbor::ControllerMarker>();
        let mut worker = NeighborsWorker { neighbor_table: HashMap::new(), neighbors_controller };

        let entry = valid_neighbor_entry();

        // Verify that an `Added` event results in an `RTM_NEWNEIGH` multicast
        // message.

        worker
            .handle_neighbor_watcher_event(
                fnet_neighbor_ext::Event::Added(entry.clone()),
                &client_table,
            )
            .expect("handle added");

        let messages = sender_sink_member.take_messages();
        assert_eq!(messages.len(), 1);
        let SentMessage { message, group } = &messages[0];
        assert_eq!(group, &Some(ModernGroup(rtnetlink_groups_RTNLGRP_NEIGH)));
        assert_matches!(
            &message.payload,
            NetlinkPayload::InnerMessage(RouteNetlinkMessage::NewNeighbour(msg)) => {
                assert_eq!(msg.header.ifindex as u64, entry.interface.get());
            }
        );
        assert_eq!(message.header.sequence_number, UNSPECIFIED_SEQUENCE_NUMBER);
        assert_eq!(sender_sink_non_member.take_messages().len(), 0);

        // Verify that a `Changed` event results in an `RTM_NEWNEIGH` multicast
        // message.

        let mut new_entry = entry.clone();
        new_entry.state = fnet_neighbor::EntryState::Stale;
        worker
            .handle_neighbor_watcher_event(
                fnet_neighbor_ext::Event::Changed(new_entry.clone()),
                &client_table,
            )
            .expect("handle changed");

        let messages = sender_sink_member.take_messages();
        assert_eq!(messages.len(), 1);
        let SentMessage { message, group } = &messages[0];
        assert_eq!(group, &Some(ModernGroup(rtnetlink_groups_RTNLGRP_NEIGH)));
        assert_matches!(
            &message.payload,
            NetlinkPayload::InnerMessage(RouteNetlinkMessage::NewNeighbour(msg)) => {
                assert_eq!(msg.header.state, NeighbourState::Stale);
            }
        );
        assert_eq!(sender_sink_non_member.take_messages().len(), 0);

        // Verify that a `Removed` event results in a `RTM_DELNEIGH` multicast
        // message.

        worker
            .handle_neighbor_watcher_event(
                fnet_neighbor_ext::Event::Removed(new_entry.clone()),
                &client_table,
            )
            .expect("handle removed");

        let messages = sender_sink_member.take_messages();
        assert_eq!(messages.len(), 1);
        let SentMessage { message, group } = &messages[0];
        assert_eq!(group, &Some(ModernGroup(rtnetlink_groups_RTNLGRP_NEIGH)));
        assert_matches!(
            &message.payload,
            NetlinkPayload::InnerMessage(RouteNetlinkMessage::DelNeighbour(msg)) => {
                assert_eq!(msg.header.ifindex as u64, entry.interface.get());
            }
        );
        assert_eq!(sender_sink_non_member.take_messages().len(), 0);
    }
}