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// Copyright 2020-2021 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.

use std::net::{Ipv4Addr, Ipv6Addr, SocketAddrV4, SocketAddrV6, TcpListener, ToSocketAddrs};

pub type Port = u16;

/// Asks the OS for a free port
fn ask_free_tcp_port() -> Option<Port> {
    let ipv4 = SocketAddrV4::new(Ipv4Addr::LOCALHOST, 0);
    let ipv6 = SocketAddrV6::new(Ipv6Addr::LOCALHOST, 0, 0, 0);
    test_bind_tcp(ipv6).or_else(|| test_bind_tcp(ipv4))
}

/// Check if a port is free on TCP
pub fn is_free_tcp_port(port: u16) -> Option<Port> {
    let ipv4 = SocketAddrV4::new(Ipv4Addr::LOCALHOST, port);
    let ipv6 = SocketAddrV6::new(Ipv6Addr::LOCALHOST, port, 0, 0);
    if test_bind_tcp(ipv6).is_some() && test_bind_tcp(ipv4).is_some() {
        return Some(port);
    }
    None
}

/// Picks an available tcp port
pub fn pick_unused_port() -> Option<Port> {
    for _ in 0..10 {
        if let Some(port) = ask_free_tcp_port() {
            return Some(port);
        }
    }
    None
}

// Try to bind to a socket using TCP
fn test_bind_tcp<A: ToSocketAddrs>(addr: A) -> Option<Port> {
    Some(TcpListener::bind(addr).ok()?.local_addr().ok()?.port())
}