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    //! An implementation of IPC locks, guaranteed to be released if a process dies
    //!
    //! This module implements a locking server/client where the main `cargo fix`
    //! process will start up a server and then all the client processes will
    //! connect to it. The main purpose of this file is to ensure that each crate
    //! (aka file entry point) is only fixed by one process at a time, currently
    //! concurrent fixes can't happen.
    //!
    //! The basic design here is to use a TCP server which is pretty portable across
    //! platforms. For simplicity it just uses threads as well. Clients connect to
    //! the main server, inform the server what its name is, and then wait for the
    //! server to give it the lock (aka write a byte).

    use std::collections::HashMap;
    use std::io::{BufRead, BufReader, Read, Write};
    use std::net::{SocketAddr, TcpListener, TcpStream};
    use std::sync::atomic::{AtomicBool, Ordering};
    use std::sync::{Arc, Mutex};
    use std::thread::{self, JoinHandle};

    use anyhow::{Context, Error};

    pub struct LockServer {
        listener: TcpListener,
        addr: SocketAddr,
        threads: HashMap<String, ServerClient>,
        done: Arc<AtomicBool>,
    }

    pub struct LockServerStarted {
        done: Arc<AtomicBool>,
        addr: SocketAddr,
        thread: Option<JoinHandle<()>>,
    }

    pub struct LockServerClient {
        _socket: TcpStream,
    }

    struct ServerClient {
        thread: Option<JoinHandle<()>>,
        lock: Arc<Mutex<(bool, Vec<TcpStream>)>>,
    }

    impl LockServer {
        pub fn new() -> Result<LockServer, Error> {
            let listener = TcpListener::bind("127.0.0.1:0")
                .with_context(|| "failed to bind TCP listener to manage locking")?;
            let addr = listener.local_addr()?;
            Ok(LockServer {
                listener,
                addr,
                threads: HashMap::new(),
                done: Arc::new(AtomicBool::new(false)),
            })
        }

        pub fn addr(&self) -> &SocketAddr {
            &self.addr
        }

        pub fn start(self) -> Result<LockServerStarted, Error> {
            let addr = self.addr;
            let done = self.done.clone();
            let thread = thread::spawn(|| {
                self.run();
            });
            Ok(LockServerStarted {
                addr,
                thread: Some(thread),
                done,
            })
        }

        fn run(mut self) {
            while let Ok((client, _)) = self.listener.accept() {
                if self.done.load(Ordering::SeqCst) {
                    break;
                }

                // Learn the name of our connected client to figure out if it needs
                // to wait for another process to release the lock.
                let mut client = BufReader::new(client);
                let mut name = String::new();
                if client.read_line(&mut name).is_err() {
                    continue;
                }
                let client = client.into_inner();

                // If this "named mutex" is already registered and the thread is
                // still going, put it on the queue. Otherwise wait on the previous
                // thread and we'll replace it just below.
                if let Some(t) = self.threads.get_mut(&name) {
                    let mut state = t.lock.lock().unwrap();
                    if state.0 {
                        state.1.push(client);
                        continue;
                    }
                    drop(t.thread.take().unwrap().join());
                }

                let lock = Arc::new(Mutex::new((true, vec![client])));
                let lock2 = lock.clone();
                let thread = thread::spawn(move || {
                    loop {
                        let mut client = {
                            let mut state = lock2.lock().unwrap();
                            if state.1.is_empty() {
                                state.0 = false;
                                break;
                            } else {
                                state.1.remove(0)
                            }
                        };
                        // Inform this client that it now has the lock and wait for
                        // it to disconnect by waiting for EOF.
                        if client.write_all(&[1]).is_err() {
                            continue;
                        }
                        let mut dst = Vec::new();
                        drop(client.read_to_end(&mut dst));
                    }
                });

                self.threads.insert(
                    name,
                    ServerClient {
                        thread: Some(thread),
                        lock,
                    },
                );
            }
        }
    }

    impl Drop for LockServer {
        fn drop(&mut self) {
            for (_, mut client) in self.threads.drain() {
                if let Some(thread) = client.thread.take() {
                    drop(thread.join());
                }
            }
        }
    }

    impl Drop for LockServerStarted {
        fn drop(&mut self) {
            self.done.store(true, Ordering::SeqCst);
            // Ignore errors here as this is largely best-effort
            if TcpStream::connect(&self.addr).is_err() {
                return;
            }
            drop(self.thread.take().unwrap().join());
        }
    }

    impl LockServerClient {
        pub fn lock(addr: &SocketAddr, name: impl AsRef<[u8]>) -> Result<LockServerClient, Error> {
            let mut client =
                TcpStream::connect(&addr).with_context(|| "failed to connect to parent lock server")?;
            client
                .write_all(name.as_ref())
                .and_then(|_| client.write_all(b"\n"))
                .with_context(|| "failed to write to lock server")?;
            let mut buf = [0];
            client
                .read_exact(&mut buf)
                .with_context(|| "failed to acquire lock")?;
            Ok(LockServerClient { _socket: client })
        }
    }