Enum BufferCollectionRequest 

pub enum BufferCollectionRequest {
    Sync {
        responder: BufferCollectionSyncResponder,
    },
    Release {
        control_handle: BufferCollectionControlHandle,
    },
    SetName {
        payload: NodeSetNameRequest,
        control_handle: BufferCollectionControlHandle,
    },
    SetDebugClientInfo {
        payload: NodeSetDebugClientInfoRequest,
        control_handle: BufferCollectionControlHandle,
    },
    SetDebugTimeoutLogDeadline {
        payload: NodeSetDebugTimeoutLogDeadlineRequest,
        control_handle: BufferCollectionControlHandle,
    },
    SetVerboseLogging {
        control_handle: BufferCollectionControlHandle,
    },
    GetNodeRef {
        responder: BufferCollectionGetNodeRefResponder,
    },
    IsAlternateFor {
        payload: NodeIsAlternateForRequest,
        responder: BufferCollectionIsAlternateForResponder,
    },
    GetBufferCollectionId {
        responder: BufferCollectionGetBufferCollectionIdResponder,
    },
    SetWeak {
        control_handle: BufferCollectionControlHandle,
    },
    SetWeakOk {
        payload: NodeSetWeakOkRequest,
        control_handle: BufferCollectionControlHandle,
    },
    AttachNodeTracking {
        payload: NodeAttachNodeTrackingRequest,
        control_handle: BufferCollectionControlHandle,
    },
    SetConstraints {
        payload: BufferCollectionSetConstraintsRequest,
        control_handle: BufferCollectionControlHandle,
    },
    WaitForAllBuffersAllocated {
        responder: BufferCollectionWaitForAllBuffersAllocatedResponder,
    },
    CheckAllBuffersAllocated {
        responder: BufferCollectionCheckAllBuffersAllocatedResponder,
    },
    AttachToken {
        payload: BufferCollectionAttachTokenRequest,
        control_handle: BufferCollectionControlHandle,
    },
    AttachLifetimeTracking {
        payload: BufferCollectionAttachLifetimeTrackingRequest,
        control_handle: BufferCollectionControlHandle,
    },
    #[non_exhaustive]
    _UnknownMethod {
        ordinal: u64,
        control_handle: BufferCollectionControlHandle,
        method_type: MethodType,
    },
}

[fuchsia.sysmem2/BufferCollection] is a connection directly from a participant to sysmem re. a buffer collection; often the buffer collection is shared with other participants which have their own BufferCollection client end(s) associated with the same buffer collection. In other words, an instance of the BufferCollection interface is a view of a buffer collection, not the buffer collection itself.

The BufferCollection connection exists to facilitate async indication of when the buffer collection has been populated with buffers.

Also, the channel’s closure by the sysmem server is an indication to the client that the client should close all VMO handles that were obtained from the BufferCollection ASAP.

Some buffer collections can use enough memory that it can be worth avoiding allocation overlap (in time) using [fuchsia.sysmem2/BufferCollection.AttachLifetimeTracking] so that the initiator can tell when enough buffers of the buffer collection have been fully deallocated prior to the initiator allocating a new buffer collection.

Epitaphs are not used in this protocol.

Variants

Sync

Ensure that previous messages have been received server side. This is particularly useful after previous messages that created new tokens, because a token must be known to the sysmem server before sending the token to another participant.

Calling [fuchsia.sysmem2/BufferCollectionToken.Sync] on a token that isn’t/wasn’t a valid token risks the Sync stalling forever. See [fuchsia.sysmem2/Allocator.ValidateBufferCollectionToken] for one way to mitigate the possibility of a hostile/fake [fuchsia.sysmem2/BufferCollectionToken] at the cost of one round trip. Another way is to pass the token to [fuchsia.sysmem2/Allocator/BindSharedCollection], which also validates the token as part of exchanging it for a [fuchsia.sysmem2/BufferCollection] channel, and [fuchsia.sysmem2/BufferCollection.Sync] can then be used without risk of stalling.

After creating one or more fuchsia.sysmem2/BufferCollectionToken and then starting and completing a Sync, it’s then safe to send the BufferCollectionToken client ends to other participants knowing the server will recognize the tokens when they’re sent by the other participants to sysmem in a [fuchsia.sysmem2/Allocator.BindSharedCollection] message. This is an efficient way to create tokens while avoiding unnecessary round trips.

Other options include waiting for each [fuchsia.sysmem2/BufferCollectionToken.Duplicate] to complete individually (using separate call to Sync after each), or calling [fuchsia.sysmem2/BufferCollection.Sync] after a token has been converted to a BufferCollection via [fuchsia.sysmem2/Allocator.BindSharedCollection], or using [fuchsia.sysmem2/BufferCollectionToken.DuplicateSync] which includes the sync step and can create multiple tokens at once.

Fields

responder: BufferCollectionSyncResponder

Release

On a [fuchsia.sysmem2/BufferCollectionToken] channel:

Normally a participant will convert a BufferCollectionToken into a [fuchsia.sysmem2/BufferCollection], but a participant can instead send Release via the token (and then close the channel immediately or shortly later in response to server closing the server end), which avoids causing buffer collection failure. Without a prior Release, closing the BufferCollectionToken client end will cause buffer collection failure.

On a [fuchsia.sysmem2/BufferCollection] channel:

By default the server handles unexpected closure of a [fuchsia.sysmem2/BufferCollection] client end (without Release first) by failing the buffer collection. Partly this is to expedite closing VMO handles to reclaim memory when any participant fails. If a participant would like to cleanly close a BufferCollection without causing buffer collection failure, the participant can send Release before closing the BufferCollection client end. The Release can occur before or after SetConstraints. If before SetConstraints, the buffer collection won’t require constraints from this node in order to allocate. If after SetConstraints, the constraints are retained and aggregated, despite the lack of BufferCollection connection at the time of constraints aggregation.

On a [fuchsia.sysmem2/BufferCollectionTokenGroup] channel:

By default, unexpected closure of a BufferCollectionTokenGroup client end (without Release first) will trigger failure of the buffer collection. To close a BufferCollectionTokenGroup channel without failing the buffer collection, ensure that AllChildrenPresent() has been sent, and send Release before closing the BufferCollectionTokenGroup client end.

If Release occurs before [fuchsia.sysmem2/BufferCollectionTokenGroup.AllChildrenPresent], the buffer collection will fail (triggered by reception of Releasewithout priorAllChildrenPresent). This is intentionally not analogous to how [fuchsia.sysmem2/BufferCollection.Release] without [fuchsia.sysmem2/BufferCollection.SetConstraints] first doesn't cause buffer collection failure. For a BufferCollectionTokenGroup, clean close requires AllChildrenPresent(if not already sent), thenRelease`, then close client end.

If Release occurs after AllChildrenPresent, the children and all their constraints remain intact (just as they would if the BufferCollectionTokenGroup channel had remained open), and the client end close doesn’t trigger buffer collection failure.

On all [fuchsia.sysmem2/Node] channels (any of the above):

For brevity, the per-channel-protocol paragraphs above ignore the separate failure domain created by [fuchsia.sysmem2/BufferCollectionToken.SetDispensable] or [fuchsia.sysmem2/BufferCollection.AttachToken]. When a client end unexpectedly closes (without Release first) and that client end is under a failure domain, instead of failing the whole buffer collection, the failure domain is failed, but the buffer collection itself is isolated from failure of the failure domain. Such failure domains can be nested, in which case only the inner-most failure domain in which the Node resides fails.

Fields

control_handle: BufferCollectionControlHandle

SetName

Set a name for VMOs in this buffer collection.

If the name doesn’t fit in ZX_MAX_NAME_LEN, the name of the vmo itself will be truncated to fit. The name of the vmo will be suffixed with the buffer index within the collection (if the suffix fits within ZX_MAX_NAME_LEN). The name specified here (without truncation) will be listed in the inspect data.

The name only affects VMOs allocated after the name is set; this call does not rename existing VMOs. If multiple clients set different names then the larger priority value will win. Setting a new name with the same priority as a prior name doesn’t change the name.

All table fields are currently required.

• request priority The name is only set if this is the first SetName or if priority is greater than any previous priority value in prior SetName calls across all Node(s) of this buffer collection.
• request name The name for VMOs created under this buffer collection.

Fields

payload: NodeSetNameRequest

control_handle: BufferCollectionControlHandle

SetDebugClientInfo

Set information about the current client that can be used by sysmem to help diagnose leaking memory and allocation stalls waiting for a participant to send [fuchsia.sysmem2/BufferCollection.SetConstraints].

This sets the debug client info on this [fuchsia.sysmem2/Node] and all Node(s) derived from this Node, unless overriden by [fuchsia.sysmem2/Allocator.SetDebugClientInfo] or a later [fuchsia.sysmem2/Node.SetDebugClientInfo].

Sending [fuchsia.sysmem2/Allocator.SetDebugClientInfo] once per Allocator is the most efficient way to ensure that all fuchsia.sysmem2/Node will have at least some debug client info set, and is also more efficient than separately sending the same debug client info via [fuchsia.sysmem2/Node.SetDebugClientInfo] for each created [fuchsia.sysmem2/Node].

Also used when verbose logging is enabled (see SetVerboseLogging) to indicate which client is closing their channel first, leading to subtree failure (which can be normal if the purpose of the subtree is over, but if happening earlier than expected, the client-channel-specific name can help diagnose where the failure is first coming from, from sysmem’s point of view).

All table fields are currently required.

• request name This can be an arbitrary string, but the current process name (see fsl::GetCurrentProcessName) is a good default.
• request id This can be an arbitrary id, but the current process ID (see fsl::GetCurrentProcessKoid) is a good default.

Fields

payload: NodeSetDebugClientInfoRequest

control_handle: BufferCollectionControlHandle

SetDebugTimeoutLogDeadline

Sysmem logs a warning if sysmem hasn’t seen [fuchsia.sysmem2/BufferCollection.SetConstraints] from all clients within 5 seconds after creation of a new collection.

Clients can call this method to change when the log is printed. If multiple client set the deadline, it’s unspecified which deadline will take effect.

In most cases the default works well.

All table fields are currently required.

• request deadline The time at which sysmem will start trying to log the warning, unless all constraints are with sysmem by then.

Fields

payload: NodeSetDebugTimeoutLogDeadlineRequest

control_handle: BufferCollectionControlHandle

SetVerboseLogging

This enables verbose logging for the buffer collection.

Verbose logging includes constraints set via [fuchsia.sysmem2/BufferCollection.SetConstraints] from each client along with info set via [fuchsia.sysmem2/Node.SetDebugClientInfo] (or [fuchsia.sysmem2/Allocator.SetDebugClientInfo]) and the structure of the tree of Node(s).

Normally sysmem prints only a single line complaint when aggregation fails, with just the specific detailed reason that aggregation failed, with little surrounding context. While this is often enough to diagnose a problem if only a small change was made and everything was working before the small change, it’s often not particularly helpful for getting a new buffer collection to work for the first time. Especially with more complex trees of nodes, involving things like [fuchsia.sysmem2/BufferCollection.AttachToken], [fuchsia.sysmem2/BufferCollectionToken.SetDispensable], [fuchsia.sysmem2/BufferCollectionTokenGroup] nodes, and associated subtrees of nodes, verbose logging may help in diagnosing what the tree looks like and why it’s failing a logical allocation, or why a tree or subtree is failing sooner than expected.

The intent of the extra logging is to be acceptable from a performance point of view, under the assumption that verbose logging is only enabled on a low number of buffer collections. If we’re not tracking down a bug, we shouldn’t send this message.

Fields

control_handle: BufferCollectionControlHandle

GetNodeRef

This gets a handle that can be used as a parameter to [fuchsia.sysmem2/Node.IsAlternateFor] called on any [fuchsia.sysmem2/Node]. This handle is only for use as proof that the client obtained this handle from this Node.

Because this is a get not a set, no [fuchsia.sysmem2/Node.Sync] is needed between the GetNodeRef and the call to IsAlternateFor, despite the two calls typically being on different channels.

See also [fuchsia.sysmem2/Node.IsAlternateFor].

All table fields are currently required.

• response node_ref This handle can be sent via IsAlternateFor on a different Node channel, to prove that the client obtained the handle from this Node.

Fields

responder: BufferCollectionGetNodeRefResponder

IsAlternateFor

Check whether the calling [fuchsia.sysmem2/Node] is in a subtree rooted at a different child token of a common parent [fuchsia.sysmem2/BufferCollectionTokenGroup], in relation to the passed-in node_ref.

This call is for assisting with admission control de-duplication, and with debugging.

The node_ref must be obtained using [fuchsia.sysmem2/Node.GetNodeRef].

The node_ref can be a duplicated handle; it’s not necessary to call GetNodeRef for every call to [fuchsia.sysmem2/Node.IsAlternateFor].

If a calling token may not actually be a valid token at all due to a potentially hostile/untrusted provider of the token, call [fuchsia.sysmem2/Allocator.ValidateBufferCollectionToken] first instead of potentially getting stuck indefinitely if IsAlternateFor never responds due to a calling token not being a real token (not really talking to sysmem). Another option is to call [fuchsia.sysmem2/Allocator.BindSharedCollection] with this token first which also validates the token along with converting it to a [fuchsia.sysmem2/BufferCollection], then call IsAlternateFor.

All table fields are currently required.

• response is_alternate
  • true: The first parent node in common between the calling node and the node_ref Node is a BufferCollectionTokenGroup. This means that the calling Node and the node_ref Node will not have both their constraints apply - rather sysmem will choose one or the other of the constraints - never both. This is because only one child of a BufferCollectionTokenGroup is selected during logical allocation, with only that one child’s subtree contributing to constraints aggregation.
  • false: The first parent node in common between the calling Node and the node_ref Node is not a BufferCollectionTokenGroup. Currently, this means the first parent node in common is a BufferCollectionToken or BufferCollection (regardless of not Releaseed). This means that the calling Node and the node_ref Node may have both their constraints apply during constraints aggregation of the logical allocation, if both Node(s) are selected by any parent BufferCollectionTokenGroup(s) involved. In this case, there is no BufferCollectionTokenGroup that will directly prevent the two Node(s) from both being selected and their constraints both aggregated, but even when false, one or both Node(s) may still be eliminated from consideration if one or both Node(s) has a direct or indirect parent BufferCollectionTokenGroup which selects a child subtree other than the subtree containing the calling Node or node_ref Node.

• error [fuchsia.sysmem2/Error.NOT_FOUND] The node_ref wasn’t associated with the same buffer collection as the calling Node. Another reason for this error is if the node_ref is an [zx.Handle.EVENT] handle with sufficient rights, but isn’t actually a real node_ref obtained from GetNodeRef.
• error [fuchsia.sysmem2/Error.PROTOCOL_DEVIATION] The caller passed a node_ref that isn’t a [zx.Handle:EVENT] handle , or doesn’t have the needed rights expected on a real node_ref.
• No other failing status codes are returned by this call. However, sysmem may add additional codes in future, so the client should have sensible default handling for any failing status code.

Fields

payload: NodeIsAlternateForRequest

responder: BufferCollectionIsAlternateForResponder

GetBufferCollectionId

Get the buffer collection ID. This ID is also available from [fuchsia.sysmem2/Allocator.GetVmoInfo] (along with the buffer_index within the collection).

This call is mainly useful in situations where we can’t convey a [fuchsia.sysmem2/BufferCollectionToken] or [fuchsia.sysmem2/BufferCollection] directly, but can only convey a VMO handle, which can be joined back up with a BufferCollection client end that was created via a different path. Prefer to convey a BufferCollectionToken or BufferCollection directly when feasible.

Trusting a buffer_collection_id value from a source other than sysmem is analogous to trusting a koid value from a source other than zircon. Both should be avoided unless really necessary, and both require caution. In some situations it may be reasonable to refer to a pre-established BufferCollection by buffer_collection_id via a protocol for efficiency reasons, but an incoming value purporting to be a buffer_collection_id is not sufficient alone to justify granting the sender of the buffer_collection_id any capability. The sender must first prove to a receiver that the sender has/had a VMO or has/had a BufferCollectionToken to the same collection by sending a handle that sysmem confirms is a valid sysmem handle and which sysmem maps to the buffer_collection_id value. The receiver should take care to avoid assuming that a sender had a BufferCollectionToken in cases where the sender has only proven that the sender had a VMO.

• response buffer_collection_id This ID is unique per buffer collection per boot. Each buffer is uniquely identified by the buffer_collection_id and buffer_index together.

Fields

responder: BufferCollectionGetBufferCollectionIdResponder

SetWeak

Sets the current [fuchsia.sysmem2/Node] and all child Node(s) created after this message to weak, which means that a client’s Node client end (or a child created after this message) is not alone sufficient to keep allocated VMOs alive.

All VMOs obtained from weak Node(s) are weak sysmem VMOs. See also close_weak_asap.

This message is only permitted before the Node becomes ready for allocation (else the server closes the channel with ZX_ERR_BAD_STATE):

• BufferCollectionToken: any time
• BufferCollection: before SetConstraints
• BufferCollectionTokenGroup: before AllChildrenPresent

Currently, no conversion from strong Node to weak Node after ready for allocation is provided, but a client can simulate that by creating an additional Node before allocation and setting that additional Node to weak, and then potentially at some point later sending Release and closing the client end of the client’s strong Node, but keeping the client’s weak Node.

Zero strong Node(s) and zero strong VMO handles will result in buffer collection failure (all Node client end(s) will see ZX_CHANNEL_PEER_CLOSED and all close_weak_asap client_end(s) will see ZX_EVENTPAIR_PEER_CLOSED), but sysmem (intentionally) won’t notice this situation until all Node(s) are ready for allocation. For initial allocation to succeed, at least one strong Node is required to exist at allocation time, but after that client receives VMO handles, that client can BufferCollection.Release and close the client end without causing this type of failure.

This implies [fuchsia.sysmem2/Node.SetWeakOk] as well, but does not imply SetWeakOk with for_children_also true, which can be sent separately as appropriate.

Fields

control_handle: BufferCollectionControlHandle

SetWeakOk

This indicates to sysmem that the client is prepared to pay attention to close_weak_asap.

If sent, this message must be before [fuchsia.sysmem2/BufferCollection.WaitForAllBuffersAllocated].

All participants using a weak [fuchsia.sysmem2/BufferCollection] must send this message before WaitForAllBuffersAllocated, or a parent Node must have sent [fuchsia.sysmem2/Node.SetWeakOk] with for_child_nodes_also true, else the WaitForAllBuffersAllocated will trigger buffer collection failure.

This message is necessary because weak sysmem VMOs have not always been a thing, so older clients are not aware of the need to pay attention to close_weak_asap ZX_EVENTPAIR_PEER_CLOSED and close all remaining sysmem weak VMO handles asap. By having this message and requiring participants to indicate their acceptance of this aspect of the overall protocol, we avoid situations where an older client is delivered a weak VMO without any way for sysmem to get that VMO to close quickly later (and on a per-buffer basis).

A participant that doesn’t handle close_weak_asap and also doesn’t retrieve any VMO handles via WaitForAllBuffersAllocated doesn’t need to send SetWeakOk (and doesn’t need to have a parent Node send SetWeakOk with for_child_nodes_also true either). However, if that same participant has a child/delegate which does retrieve VMOs, that child/delegate will need to send SetWeakOk before WaitForAllBuffersAllocated.

• request for_child_nodes_also If present and true, this means direct child nodes of this node created after this message plus all descendants of those nodes will behave as if SetWeakOk was sent on those nodes. Any child node of this node that was created before this message is not included. This setting is “sticky” in the sense that a subsequent SetWeakOk without this bool set to true does not reset the server-side bool. If this creates a problem for a participant, a workaround is to SetWeakOk with for_child_nodes_also true on child tokens instead, as appropriate. A participant should only set for_child_nodes_also true if the participant can really promise to obey close_weak_asap both for its own weak VMO handles, and for all weak VMO handles held by participants holding the corresponding child Node(s). When for_child_nodes_also is set, descendent Node(s) which are using sysmem(1) can be weak, despite the clients of those sysmem1 Node(s) not having any direct way to SetWeakOk or any direct way to find out about close_weak_asap. This only applies to descendents of this Node which are using sysmem(1), not to this Node when converted directly from a sysmem2 token to a sysmem(1) token, which will fail allocation unless an ancestor of this Node specified for_child_nodes_also true.

Fields

payload: NodeSetWeakOkRequest

control_handle: BufferCollectionControlHandle

AttachNodeTracking

The server_end will be closed after this Node and any child nodes have have released their buffer counts, making those counts available for reservation by a different Node via [fuchsia.sysmem2/BufferCollection.AttachToken].

The Node buffer counts may not be released until the entire tree of Node(s) is closed or failed, because [fuchsia.sysmem2/BufferCollection.Release] followed by channel close does not immediately un-reserve the Node buffer counts. Instead, the Node buffer counts remain reserved until the orphaned node is later cleaned up.

If the Node exceeds a fairly large number of attached eventpair server ends, a log message will indicate this and the Node (and the appropriate) sub-tree will fail.

The server_end will remain open when [fuchsia.sysmem2/Allocator.BindSharedCollection] converts a [fuchsia.sysmem2/BufferCollectionToken] into a [fuchsia.sysmem2/BufferCollection].

This message can also be used with a [fuchsia.sysmem2/BufferCollectionTokenGroup].

Fields

payload: NodeAttachNodeTrackingRequest

control_handle: BufferCollectionControlHandle

SetConstraints

Provide [fuchsia.sysmem2/BufferCollectionConstraints] to the buffer collection.

A participant may only call [fuchsia.sysmem2/BufferCollection.SetConstraints] up to once per [fuchsia.sysmem2/BufferCollection].

For buffer allocation to be attempted, all holders of a BufferCollection client end need to call SetConstraints before sysmem will attempt to allocate buffers.

• request constraints These are the constraints on the buffer collection imposed by the sending client/participant. The constraints field is not required to be set. If not set, the client is not setting any actual constraints, but is indicating that the client has no constraints to set. A client that doesn’t set the constraints field won’t receive any VMO handles, but can still find out how many buffers were allocated and can still refer to buffers by their buffer_index.

Fields

payload: BufferCollectionSetConstraintsRequest

control_handle: BufferCollectionControlHandle

WaitForAllBuffersAllocated

Wait until all buffers are allocated.

This FIDL call completes when buffers have been allocated, or completes with some failure detail if allocation has been attempted but failed.

The following must occur before buffers will be allocated:

• All fuchsia.sysmem2/BufferCollectionToken of the buffer collection must be turned in via BindSharedCollection to get a [fuchsia.sysmem2/BufferCollection] (for brevity, this is assuming [fuchsia.sysmem2/BufferCollection.AttachToken] isn’t being used), or have had [fuchsia.sysmem2/BufferCollectionToken.Release] sent to them.
• All fuchsia.sysmem2/BufferCollection of the buffer collection must have had [fuchsia.sysmem2/BufferCollection.SetConstraints] sent to them, or had [fuchsia.sysmem2/BufferCollection.Release] sent to them.

• result buffer_collection_info The VMO handles and other related info.

• error [fuchsia.sysmem2/Error.NO_MEMORY] The request is valid but cannot be fulfilled due to resource exhaustion.
• error [fuchsia.sysmem2/Error.PROTOCOL_DEVIATION] The request is malformed.
• error [fuchsia.sysmem2/Error.CONSTRAINTS_INTERSECTION_EMPTY] The request is valid but cannot be satisfied, perhaps due to hardware limitations. This can happen if participants have incompatible constraints (empty intersection, roughly speaking). See the log for more info. In cases where a participant could potentially be treated as optional, see [BufferCollectionTokenGroup]. When using [fuchsia.sysmem2/BufferCollection.AttachToken], this will be the error code if there aren’t enough buffers in the pre-existing collection to satisfy the constraints set on the attached token and any sub-tree of tokens derived from the attached token.

Fields

responder: BufferCollectionWaitForAllBuffersAllocatedResponder

CheckAllBuffersAllocated

Checks whether all the buffers have been allocated, in a polling fashion.

• If the buffer collection has been allocated, returns success.
• If the buffer collection failed allocation, returns the same [fuchsia.sysmem2/Error] as [fuchsia.sysmem2/BufferCollection/WaitForAllBuffersAllocated] would return.
• error [fuchsia.sysmem2/Error.PENDING] The buffer collection hasn’t attempted allocation yet. This means that WaitForAllBuffersAllocated would not respond quickly.

Fields

responder: BufferCollectionCheckAllBuffersAllocatedResponder

AttachToken

Create a new token to add a new participant to an existing logical buffer collection, if the existing collection’s buffer counts, constraints, and participants allow.

This can be useful in replacing a failed participant, and/or in adding/re-adding a participant after buffers have already been allocated.

When [fuchsia.sysmem2/BufferCollection.AttachToken] is used, the sub tree rooted at the attached [fuchsia.sysmem2/BufferCollectionToken] goes through the normal procedure of setting constraints or closing fuchsia.sysmem2/Node, and then appearing to allocate buffers from clients’ point of view, despite the possibility that all the buffers were actually allocated previously. This process is called “logical allocation”. Most instances of “allocation” in docs for other messages can also be read as “allocation or logical allocation” while remaining valid, but we just say “allocation” in most places for brevity/clarity of explanation, with the details of “logical allocation” left for the docs here on AttachToken.

Failure of an attached Node does not propagate to the parent of the attached Node. More generally, failure of a child Node is blocked from reaching its parent Node if the child is attached, or if the child is dispensable and the failure occurred after logical allocation (see [fuchsia.sysmem2/BufferCollectionToken.SetDispensable]).

A participant may in some scenarios choose to initially use a dispensable token for a given instance of a delegate participant, and then later if the first instance of that delegate participant fails, a new second instance of that delegate participant my be given a token created with AttachToken.

From the point of view of the [fuchsia.sysmem2/BufferCollectionToken] client end, the token acts like any other token. The client can [fuchsia.sysmem2/BufferCollectionToken.Duplicate] the token as needed, and can send the token to a different process/participant. The BufferCollectionToken Node should be converted to a BufferCollection Node as normal by sending [fuchsia.sysmem2/Allocator.BindSharedCollection], or can be closed without causing subtree failure by sending [fuchsia.sysmem2/BufferCollectionToken.Release]. Assuming the former, the [fuchsia.sysmem2/BufferCollection.SetConstraints] message or [fuchsia.sysmem2/BufferCollection.Release] message should be sent to the BufferCollection.

Within the subtree, a success result from [fuchsia.sysmem2/BufferCollection.WaitForAllBuffersAllocated] means the subtree participants’ constraints were satisfiable using the already-existing buffer collection, the already-established [fuchsia.sysmem2/BufferCollectionInfo] including image format constraints, and the already-existing other participants (already added via successful logical allocation) and their specified buffer counts in their constraints. A failure result means the new participants’ constraints cannot be satisfied using the existing buffer collection and its already-added participants. Creating a new collection instead may allow all participants’ constraints to be satisfied, assuming SetDispensable is used in place of AttachToken, or a normal token is used.

A token created with AttachToken performs constraints aggregation with all constraints currently in effect on the buffer collection, plus the attached token under consideration plus child tokens under the attached token which are not themselves an attached token or under such a token. Further subtrees under this subtree are considered for logical allocation only after this subtree has completed logical allocation.

Assignment of existing buffers to participants’ [fuchsia.sysmem2/BufferCollectionConstraints.min_buffer_count_for_camping] etc is first-come first-served, but a child can’t logically allocate before all its parents have sent SetConstraints.

See also [fuchsia.sysmem2/BufferCollectionToken.SetDispensable], which in contrast to AttachToken, has the created token Node + child Node(s) (in the created subtree but not in any subtree under this subtree) participate in constraints aggregation along with its parent during the parent’s allocation or logical allocation.

Similar to [fuchsia.sysmem2/BufferCollectionToken.Duplicate], the newly created token needs to be [fuchsia.sysmem2/Node.Sync]ed to sysmem before the new token can be passed to BindSharedCollection. The Sync of the new token can be accomplished with [fuchsia.sysmem2/BufferCollection.Sync] after converting the created BufferCollectionToken to a BufferCollection. Alternately, [fuchsia.sysmem2/BufferCollectionToken.Sync] on the new token also works. Or using [fuchsia.sysmem2/BufferCollectionToken.DuplicateSync] works. As usual, a BufferCollectionToken.Sync can be started after any BufferCollectionToken.Duplicate messages have been sent via the newly created token, to also sync those additional tokens to sysmem using a single round-trip.

All table fields are currently required.

• request rights_attentuation_mask This allows attenuating the VMO rights of the subtree. These values for rights_attenuation_mask result in no attenuation (note that 0 is not on this list):
  • ZX_RIGHT_SAME_RIGHTS (preferred)
  • 0xFFFFFFFF (this is reasonable when an attenuation mask is computed)
• request token_request The server end of the BufferCollectionToken channel. The client retains the client end.

Fields

payload: BufferCollectionAttachTokenRequest

control_handle: BufferCollectionControlHandle

AttachLifetimeTracking

Set up an eventpair to be signalled (ZX_EVENTPAIR_PEER_CLOSED) when buffers have been allocated and only the specified number of buffers (or fewer) remain in the buffer collection.

[fuchsia.sysmem2/BufferCollection.AttachLifetimeTracking] allows a client to wait until an old buffer collection is fully or mostly deallocated before attempting allocation of a new buffer collection. The eventpair is only signalled when the buffers of this collection have been fully deallocated (not just un-referenced by clients, but all the memory consumed by those buffers has been fully reclaimed/recycled), or when allocation or logical allocation fails for the tree or subtree including this [fuchsia.sysmem2/BufferCollection].

The eventpair won’t be signalled until allocation or logical allocation has completed; until then, the collection’s current buffer count is ignored.

If logical allocation fails for an attached subtree (using [fuchsia.sysmem2/BufferCollection.AttachToken]), the server end of the eventpair will close during that failure regardless of the number of buffers potenitally allocated in the overall buffer collection. This is for logical allocation consistency with normal allocation.

The lifetime signalled by this event includes asynchronous cleanup of allocated buffers, and this asynchronous cleanup cannot occur until all holders of VMO handles to the buffers have closed those VMO handles. Therefore, clients should take care not to become blocked forever waiting for ZX_EVENTPAIR_PEER_CLOSED to be signalled if any of the participants using the logical buffer collection (including the waiter itself) are less trusted, less reliable, or potentially blocked by the wait itself. Waiting asynchronously is recommended. Setting a deadline for the client wait may be prudent, depending on details of how the collection and/or its VMOs are used or shared. Failure to allocate a new/replacement buffer collection is better than getting stuck forever.

The sysmem server itself intentionally does not perform any waiting on already-failed collections’ VMOs to finish cleaning up before attempting a new allocation, and the sysmem server intentionally doesn’t retry allocation if a new allocation fails due to out of memory, even if that failure is potentially due to continued existence of an old collection’s VMOs. This AttachLifetimeTracking message is how an initiator can mitigate too much overlap of old VMO lifetimes with new VMO lifetimes, as long as the waiting client is careful to not create a deadlock.

Continued existence of old collections that are still cleaning up is not the only reason that a new allocation may fail due to insufficient memory, even if the new allocation is allocating physically contiguous buffers. Overall system memory pressure can also be the cause of failure to allocate a new collection. See also [fuchsia.memorypressure/Provider].

AttachLifetimeTracking is meant to be compatible with other protocols with a similar AttachLifetimeTracking message; duplicates of the same eventpair handle (server end) can be sent via more than one AttachLifetimeTracking message to different protocols, and the ZX_EVENTPAIR_PEER_CLOSED will be signalled for the client end when all the conditions are met (all holders of duplicates have closed their server end handle(s)). Also, thanks to how eventpair endponts work, the client end can (also) be duplicated without preventing the ZX_EVENTPAIR_PEER_CLOSED signal.

The server intentionally doesn’t “trust” any signals set on the server_end. This mechanism intentionally uses only ZX_EVENTPAIR_PEER_CLOSED set on the client end, which can’t be set “early”, and is only set when all handles to the server end eventpair are closed. No meaning is associated with any of the other signals, and clients should ignore any other signal bits on either end of the eventpair.

The server_end may lack ZX_RIGHT_SIGNAL or ZX_RIGHT_SIGNAL_PEER, but must have ZX_RIGHT_DUPLICATE (and must have ZX_RIGHT_TRANSFER to transfer without causing BufferCollection channel failure).

All table fields are currently required.

• request server_end This eventpair handle will be closed by the sysmem server when buffers have been allocated initially and the number of buffers is then less than or equal to buffers_remaining.
• request buffers_remaining Wait for all but buffers_remaining (or fewer) buffers to be fully deallocated. A number greater than zero can be useful in situations where a known number of buffers are intentionally not closed so that the data can continue to be used, such as for keeping the last available video frame displayed in the UI even if the video stream was using protected output buffers. It’s outside the scope of the BufferCollection interface (at least for now) to determine how many buffers may be held without closing, but it’ll typically be in the range 0-2.

Fields

payload: BufferCollectionAttachLifetimeTrackingRequest

control_handle: BufferCollectionControlHandle

#[non_exhaustive]

_UnknownMethod

An interaction was received which does not match any known method.

Fields

Non-exhaustive enum variants could have additional fields added in future. Therefore, non-exhaustive enum variants cannot be constructed in external crates and cannot be matched against.

ordinal: u64

Ordinal of the method that was called.

control_handle: BufferCollectionControlHandle

method_type: MethodType

Implementations

impl BufferCollectionRequest

pub fn into_sync(self) -> Option<BufferCollectionSyncResponder>

pub fn into_release(self) -> Option<BufferCollectionControlHandle>

pub fn into_set_name( self, ) -> Option<(NodeSetNameRequest, BufferCollectionControlHandle)>

pub fn into_set_debug_client_info( self, ) -> Option<(NodeSetDebugClientInfoRequest, BufferCollectionControlHandle)>

pub fn into_set_debug_timeout_log_deadline( self, ) -> Option<(NodeSetDebugTimeoutLogDeadlineRequest, BufferCollectionControlHandle)>

pub fn into_set_verbose_logging(self) -> Option<BufferCollectionControlHandle>

pub fn into_get_node_ref(self) -> Option<BufferCollectionGetNodeRefResponder>

pub fn into_is_alternate_for( self, ) -> Option<(NodeIsAlternateForRequest, BufferCollectionIsAlternateForResponder)>

pub fn into_get_buffer_collection_id( self, ) -> Option<BufferCollectionGetBufferCollectionIdResponder>

pub fn into_set_weak(self) -> Option<BufferCollectionControlHandle>

pub fn into_set_weak_ok( self, ) -> Option<(NodeSetWeakOkRequest, BufferCollectionControlHandle)>

pub fn into_attach_node_tracking( self, ) -> Option<(NodeAttachNodeTrackingRequest, BufferCollectionControlHandle)>

pub fn into_set_constraints( self, ) -> Option<(BufferCollectionSetConstraintsRequest, BufferCollectionControlHandle)>

pub fn into_wait_for_all_buffers_allocated( self, ) -> Option<BufferCollectionWaitForAllBuffersAllocatedResponder>

pub fn into_check_all_buffers_allocated( self, ) -> Option<BufferCollectionCheckAllBuffersAllocatedResponder>

pub fn into_attach_token( self, ) -> Option<(BufferCollectionAttachTokenRequest, BufferCollectionControlHandle)>

pub fn into_attach_lifetime_tracking( self, ) -> Option<(BufferCollectionAttachLifetimeTrackingRequest, BufferCollectionControlHandle)>

pub fn method_name(&self) -> &'static str

Name of the method defined in FIDL

Trait Implementations

impl Debug for BufferCollectionRequest

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.