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// Copyright (c) 2022, Mysten Labs, Inc.
// SPDX-License-Identifier: Apache-2.0

use std::net::{TcpListener, TcpStream};

/// Return an ephemeral, available port. On unix systems, the port returned will be in the
/// TIME_WAIT state ensuring that the OS won't hand out this port for some grace period.
/// Callers should be able to bind to this port given they use SO_REUSEADDR.
pub fn get_available_port(host: &str) -> u16 {
    const MAX_PORT_RETRIES: u32 = 1000;

    for _ in 0..MAX_PORT_RETRIES {
        if let Ok(port) = get_ephemeral_port(host) {
            return port;
        }
    }

    panic!("Error: could not find an available port");
}

fn get_ephemeral_port(host: &str) -> ::std::io::Result<u16> {
    // Request a random available port from the OS
    let listener = TcpListener::bind((host, 0))?;
    let addr = listener.local_addr()?;

    // Create and accept a connection (which we'll promptly drop) in order to force the port
    // into the TIME_WAIT state, ensuring that the port will be reserved from some limited
    // amount of time (roughly 60s on some Linux systems)
    let _sender = TcpStream::connect(addr)?;
    let _incoming = listener.accept()?;

    Ok(addr.port())
}