lib.rs

//! An implementation of the GNU make jobserver.
//!
//! This crate is an implementation, in Rust, of the GNU `make` jobserver for
//! CLI tools that are interoperating with make or otherwise require some form
//! of parallelism limiting across process boundaries. This was originally
//! written for usage in Cargo to both (a) work when `cargo` is invoked from
//! `make` (using `make`'s jobserver) and (b) work when `cargo` invokes build
//! scripts, exporting a jobserver implementation for `make` processes to
//! transitively use.
//!
//! The jobserver implementation can be found in [detail online][docs] but
//! basically boils down to a cross-process semaphore. On Unix this is
//! implemented with the `pipe` syscall and read/write ends of a pipe and on
//! Windows this is implemented literally with IPC semaphores.
//!
//! The jobserver protocol in `make` also dictates when tokens are acquire to
//! run child work, and clients using this crate should take care to implement
//! such details to ensure correct interoperation with `make` itself.
//!
//! ## Examples
//!
//! Connect to a jobserver that was set up by `make` or a different process:
//!
//! ```no_run
//! use jobserver::Client;
//!
//! // See API documentation for why this is `unsafe`
//! let client = match unsafe { Client::from_env() } {
//!     Some(client) => client,
//!     None => panic!("client not configured"),
//! };
//! ```
//!
//! Acquire and release token from a jobserver:
//!
//! ```no_run
//! use jobserver::Client;
//!
//! let client = unsafe { Client::from_env().unwrap() };
//! let token = client.acquire().unwrap(); // blocks until it is available
//! drop(token); // releases the token when the work is done
//! ```
//!
//! Create a new jobserver and configure a child process to have access:
//!
//! ```
//! use std::process::Command;
//! use jobserver::Client;
//!
//! let client = Client::new(4).expect("failed to create jobserver");
//! let mut cmd = Command::new("make");
//! client.configure(&mut cmd);
//! ```
//!
//! ## Caveats
//!
//! This crate makes no attempt to release tokens back to a jobserver on
//! abnormal exit of a process. If a process which acquires a token is killed
//! with ctrl-c or some similar signal then tokens will not be released and the
//! jobserver may be in a corrupt state.
//!
//! Note that this is typically ok as ctrl-c means that an entire build process
//! is being torn down, but it's worth being aware of at least!
//!
//! ## Windows caveats
//!
//! There appear to be two implementations of `make` on Windows. On MSYS2 one
//! typically comes as `mingw32-make` and the other as `make` itself. I'm not
//! personally too familiar with what's going on here, but for jobserver-related
//! information the `mingw32-make` implementation uses Windows semaphores
//! whereas the `make` program does not. The `make` program appears to use file
//! descriptors and I'm not really sure how it works, so this crate is not
//! compatible with `make` on Windows. It is, however, compatible with
//! `mingw32-make`.
//!
//! [docs]: http://make.mad-scientist.net/papers/jobserver-implementation/

#![deny(missing_docs, missing_debug_implementations)]
#![doc(html_root_url = "https://docs.rs/jobserver/0.1")]

#[macro_use]
extern crate log;

use std::env;
use std::io;
use std::process::Command;
use std::sync::Arc;
use std::sync::mpsc::{self, Sender};

/// A client of a jobserver
///
/// This structure is the main type exposed by this library, and is where
/// interaction to a jobserver is configured through. Clients are either created
/// from scratch in which case the internal semphore is initialied on the spot,
/// or a client is created from the environment to connect to a jobserver
/// already created.
///
/// Some usage examples can be found in the crate documentation for using a
/// client.
///
/// Note that a `Client` implements the `Clone` trait, and all instances of a
/// `Client` refer to the same jobserver instance.
#[derive(Clone, Debug)]
pub struct Client {
    inner: Arc<imp::Client>,
}

/// An acquired token from a jobserver.
///
/// This token will be released back to the jobserver when it is dropped and
/// otherwise represents the ability to spawn off another thread of work.
#[derive(Debug)]
pub struct Acquired {
    client: Arc<imp::Client>,
    data: imp::Acquired,
}

impl Client {
    /// Creates a new jobserver initialized with the given parallelism limit.
    ///
    /// A client to the jobserver created will be returned. This client will
    /// allow at most `limit` tokens to be acquired from it in parallel. More
    /// calls to `acquire` will cause the calling thread to block.
    ///
    /// Note that the created `Client` is not automatically inherited into
    /// spawned child processes from this program. Manual usage of the
    /// `configure` function is required for a child process to have access to a
    /// job server.
    ///
    /// # Examples
    ///
    /// ```
    /// use jobserver::Client;
    ///
    /// let client = Client::new(4).expect("failed to create jobserver");
    /// ```
    ///
    /// # Errors
    ///
    /// Returns an error if any I/O error happens when attempting to create the
    /// jobserver client.
    pub fn new(limit: usize) -> io::Result<Client> {
        Ok(Client {
            inner: Arc::new(imp::Client::new(limit)?),
        })
    }

    /// Attempts to connect to the jobserver specified in this process's
    /// environment.
    ///
    /// When the a `make` executable calls a child process it will configure the
    /// environment of the child to ensure that it has handles to the jobserver
    /// it's passing down. This function will attempt to look for these details
    /// and connect to the jobserver.
    ///
    /// Note that the created `Client` is not automatically inherited into
    /// spawned child processes from this program. Manual usage of the
    /// `configure` function is required for a child process to have access to a
    /// job server.
    ///
    /// # Return value
    ///
    /// If a jobserver was found in the environment and it looks correct then
    /// `Some` of the connected client will be returned. If no jobserver was
    /// found then `None` will be returned.
    ///
    /// Note that on Unix the `Client` returned **takes ownership of the file
    /// descriptors specified in the environment**. Jobservers on Unix are
    /// implemented with `pipe` file descriptors, and they're inherited from
    /// parent processes. This `Client` returned takes ownership of the file
    /// descriptors for this process and will close the file descriptors after
    /// this value is dropped.
    ///
    /// Additionally on Unix this function will configure the file descriptors
    /// with `CLOEXEC` so they're not automatically inherited by spawned
    /// children.
    ///
    /// # Unsafety
    ///
    /// This function is `unsafe` to call on Unix specifically as it
    /// transitively requires usage of the `from_raw_fd` function, which is
    /// itself unsafe in some circumstances.
    ///
    /// It's recommended to call this function very early in the lifetime of a
    /// program before any other file descriptors are opened. That way you can
    /// make sure to take ownership properly of the file descriptors passed
    /// down, if any.
    ///
    /// It's generally unsafe to call this function twice in a program if the
    /// previous invocation returned `Some`.
    ///
    /// Note, though, that on Windows it should be safe to call this function
    /// any number of times.
    pub unsafe fn from_env() -> Option<Client> {
        let var = match env::var("CARGO_MAKEFLAGS")
                            .or(env::var("MAKEFLAGS"))
                            .or(env::var("MFLAGS")) {
            Ok(s) => s,
            Err(_) => return None,
        };
        let mut arg = "--jobserver-fds=";
        let pos = match var.find(arg) {
            Some(i) => i,
            None => {
                arg = "--jobserver-auth=";
                match var.find(arg) {
                    Some(i) => i,
                    None => return None,
                }
            }
        };

        let s = var[pos + arg.len()..].split(' ').next().unwrap();
        imp::Client::open(s).map(|c| {
            Client { inner: Arc::new(c) }
        })
    }

    /// Acquires a token from this jobserver client.
    ///
    /// This function will block the calling thread until a new token can be
    /// acquired from the jobserver.
    ///
    /// # Return value
    ///
    /// On successful acquisition of a token an instance of `Acquired` is
    /// returned. This structure, when dropped, will release the token back to
    /// the jobserver. It's recommended to avoid leaking this value.
    ///
    /// # Errors
    ///
    /// If an I/O error happens while acquiring a token then this function will
    /// return immediately with the error. If an error is returned then a token
    /// was not acquired.
    pub fn acquire(&self) -> io::Result<Acquired> {
        let data = try!(self.inner.acquire());
        Ok(Acquired {
            client: self.inner.clone(),
            data: data,
        })
    }

    /// Configures a child process to have access to this client's jobserver as
    /// well.
    ///
    /// This function is required to be called to ensure that a jobserver is
    /// properly inherited to a child process. If this function is *not* called
    /// then this `Client` will not be accessible in the child process. In other
    /// words, if not called, then `Client::from_env` will return `None` in the
    /// child process (or the equivalent of `Child::from_env` that `make` uses).
    ///
    /// ## Platform-specific behavior
    ///
    /// On Unix and Windows this will clobber the `CARGO_MAKEFLAGS` environment
    /// variables for the child process, and on Unix this will also allow the
    /// two file descriptors for this client to be inherited to the child.
    pub fn configure(&self, cmd: &mut Command) {
        let arg = self.inner.string_arg();
        // Older implementations of make use `--jobserver-fds` and newer
        // implementations use `--jobserver-auth`, pass both to try to catch
        // both implementations.
        let value = format!("--jobserver-fds={0} --jobserver-auth={0}", arg);
        cmd.env("CARGO_MAKEFLAGS", &value);
        self.inner.configure(cmd);
    }

    /// Converts this `Client` into a helper thread to deal with a blocking
    /// `acquire` function a little more easily.
    ///
    /// The fact that the `acquire` function on `Client` blocks isn't always
    /// the easiest to work with. Typically you're using a jobserver to
    /// manage running other events in parallel! This means that you need to
    /// either (a) wait for an existing job to finish or (b) wait for a
    /// new token to become available.
    ///
    /// Unfortunately the blocking in `acquire` happens at the implementation
    /// layer of jobservers. On Unix this requires a blocking call to `read`
    /// and on Windows this requires one of the `WaitFor*` functions. Both
    /// of these situations aren't the easiest to deal with:
    ///
    /// * On Unix there's basically only one way to wake up a `read` early, and
    ///   that's through a signal. This is what the `make` implementation
    ///   itself uses, relying on `SIGCHLD` to wake up a blocking acquisition
    ///   of a new job token. Unfortunately nonblocking I/O is not an option
    ///   here, so it means that "waiting for one of two events" means that
    ///   the latter event must generate a signal! This is not always the case
    ///   on unix for all jobservers.
    ///
    /// * On Windows you'd have to basically use the `WaitForMultipleObjects`
    ///   which means that you've got to canonicalize all your event sources
    ///   into a `HANDLE` which also isn't the easiest thing to do
    ///   unfortunately.
    ///
    /// This function essentially attempts to ease these limitations by
    /// converting this `Client` into a helper thread spawned into this
    /// process. The application can then request that the helper thread
    /// acquires tokens and the provided closure will be invoked for each token
    /// acquired.
    ///
    /// The intention is that this function can be used to translate the event
    /// of a token acquisition into an arbitrary user-defined event.
    ///
    /// # Arguments
    ///
    /// This function will consume the `Client` provided to be transferred to
    /// the helper thread that is spawned. Additionally a closure `f` is
    /// provided to be invoked whenever a token is acquired.
    ///
    /// This closure is only invoked after calls to
    /// `HelperThread::request_token` have been made and a token itself has
    /// been acquired. If an error happens while acquiring the token then
    /// an error will be yielded to the closure as well.
    ///
    /// # Return Value
    ///
    /// This function will return an instance of the `HelperThread` structure
    /// which is used to manage the helper thread associated with this client.
    /// Through the `HelperThread` you'll request that tokens are acquired.
    /// When acquired, the closure provided here is invoked.
    ///
    /// When the `HelperThread` structure is returned it will be gracefully
    /// torn down, and the calling thread will be blocked until the thread is
    /// torn down (which should be prompt).
    ///
    /// # Errors
    ///
    /// This function may fail due to creation of the helper thread or
    /// auxiliary I/O objects to manage the helper thread. In any of these
    /// situations the error is propagated upwards.
    ///
    /// # Platform-specific behavior
    ///
    /// On Windows this function behaves pretty normally as expected, but on
    /// Unix the implementation is... a little heinous. As mentioned above
    /// we're forced into blocking I/O for token acquisition, namely a blocking
    /// call to `read`. We must be able to unblock this, however, to tear down
    /// the helper thread gracefully!
    ///
    /// Essentially what happens is that we'll send a signal to the helper
    /// thread spawned and rely on `EINTR` being returned to wake up the helper
    /// thread. This involves installing a global `SIGUSR1` handler that does
    /// nothing along with sending signals to that thread. This may cause
    /// odd behavior in some applications, so it's recommended to review and
    /// test thoroughly before using this.
    pub fn into_helper_thread<F>(self, f: F) -> io::Result<HelperThread>
        where F: FnMut(io::Result<Acquired>) + Send + 'static,
    {
        let (tx, rx) = mpsc::channel();
        Ok(HelperThread {
            inner: Some(imp::spawn_helper(self, rx, Box::new(f))?),
            tx: Some(tx),
        })
    }
}

impl Drop for Acquired {
    fn drop(&mut self) {
        drop(self.client.release(&self.data));
    }
}

/// Structure returned from `Client::into_helper_thread` to manage the lifetime
/// of the helper thread returned, see those associated docs for more info.
#[derive(Debug)]
pub struct HelperThread {
    inner: Option<imp::Helper>,
    tx: Option<Sender<()>>,
}

impl HelperThread {
    /// Request that the helper thread acquires a token, eventually calling the
    /// original closure with a token when it's available.
    ///
    /// For more information, see the docs on that function.
    pub fn request_token(&self) {
        self.tx.as_ref().unwrap().send(()).unwrap();
    }
}

impl Drop for HelperThread {
    fn drop(&mut self) {
        drop(self.tx.take());
        self.inner.take().unwrap().join();
    }
}

#[cfg(unix)]
mod imp {
    extern crate libc;

    use std::fs::File;
    use std::io::{self, Read, Write};
    use std::mem;
    use std::os::unix::prelude::*;
    use std::process::Command;
    use std::ptr;
    use std::sync::atomic::{AtomicBool, AtomicUsize,
                            ATOMIC_BOOL_INIT, ATOMIC_USIZE_INIT,
                            Ordering};
    use std::sync::mpsc::{self, Receiver, RecvTimeoutError};
    use std::sync::{Arc, Once, ONCE_INIT};
    use std::thread::{self, JoinHandle, Builder};
    use std::time::Duration;

    use self::libc::c_int;

    #[derive(Debug)]
    pub struct Client {
        read: File,
        write: File,
    }

    #[derive(Debug)]
    pub struct Acquired {
        byte: u8,
    }

    impl Client {
        pub fn new(limit: usize) -> io::Result<Client> {
            let client = unsafe { Client::mk()? };
            // I don't think the character written here matters, but I could be
            // wrong!
            for _ in 0..limit {
                (&client.write).write(&[b'|'])?;
            }
            info!("created a jobserver: {:?}", client);
            Ok(client)
        }

        unsafe fn mk() -> io::Result<Client> {
            static INVALID: AtomicBool = ATOMIC_BOOL_INIT;
            let mut pipes = [0; 2];

            // Attempt atomically-create-with-cloexec if we can.  Note that even
            // when libc has the symbol, `pipe2` might still not be supported on
            // the running kernel -> `ENOSYS`, then we need to use the fallback.
            if cfg!(target_os = "linux") && !INVALID.load(Ordering::SeqCst) {
                if let Some(pipe2) = pipe2() {
                    match cvt(pipe2(pipes.as_mut_ptr(), libc::O_CLOEXEC)) {
                        Ok(_) => return Ok(Client::from_fds(pipes[0], pipes[1])),
                        Err(ref e) if e.raw_os_error() == Some(libc::ENOSYS) => {
                            INVALID.store(true, Ordering::SeqCst);
                        }
                        Err(e) => return Err(e),
                    }
                }
            }

            cvt(libc::pipe(pipes.as_mut_ptr()))?;
            drop(set_cloexec(pipes[0], true));
            drop(set_cloexec(pipes[1], true));
            Ok(Client::from_fds(pipes[0], pipes[1]))
        }

        pub unsafe fn open(s: &str) -> Option<Client> {
            let mut parts = s.splitn(2, ',');
            let read = parts.next().unwrap();
            let write = match parts.next() {
                Some(s) => s,
                None => return None,
            };

            let read = match read.parse() {
                Ok(n) => n,
                Err(_) => return None,
            };
            let write = match write.parse() {
                Ok(n) => n,
                Err(_) => return None,
            };

            // Ok so we've got two integers that look like file descriptors, but
            // for extra sanity checking let's see if they actually look like
            // instances of a pipe before we return the client.
            //
            // If we're called from `make` *without* the leading + on our rule
            // then we'll have `MAKEFLAGS` env vars but won't actually have
            // access to the file descriptors.
            if is_valid_fd(read) && is_valid_fd(write) {
                info!("using env fds {} and {}", read, write);
                drop(set_cloexec(read, true));
                drop(set_cloexec(write, true));
                Some(Client::from_fds(read, write))
            } else {
                info!("one of {} or {} isn't a pipe", read, write);
                None
            }
        }

        unsafe fn from_fds(read: c_int, write: c_int) -> Client {
            Client {
                read: File::from_raw_fd(read),
                write: File::from_raw_fd(write),
            }
        }

        pub fn acquire(&self) -> io::Result<Acquired> {
            // We don't actually know if the file descriptor here is set in
            // blocking or nonblocking mode. AFAIK all released versions of
            // `make` use blocking fds for the jobserver, but the unreleased
            // version of `make` doesn't. In the unreleased version jobserver
            // fds are set to nonblocking and combined with `pselect`
            // internally.
            //
            // Here we try to be compatible with both strategies. We
            // unconditionally expect the file descriptor to be in nonblocking
            // mode and if it happens to be in blocking mode then most of this
            // won't end up actually being necessary!
            //
            // We use `poll` here to block this thread waiting for read
            // readiness, and then afterwards we perform the `read` itself. If
            // the `read` returns that it would block then we start over and try
            // again.
            //
            // Also note that we explicitly don't handle EINTR here. That's used
            // to shut us down, so we otherwise punt all errors upwards.
            unsafe {
                let mut fd: libc::pollfd = mem::zeroed();
                fd.fd = self.read.as_raw_fd();
                fd.events = libc::POLLIN;
                loop {
                    fd.revents = 0;
                    if libc::poll(&mut fd, 1, -1) == -1 {
                        return Err(io::Error::last_os_error())
                    }
                    if fd.revents == 0 {
                        continue
                    }
                    let mut buf = [0];
                    match (&self.read).read(&mut buf) {
                        Ok(1) => return Ok(Acquired { byte: buf[0] }),
                        Ok(_) => {
                            return Err(io::Error::new(io::ErrorKind::Other,
                                                      "early EOF on jobserver pipe"))
                        }
                        Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {}
                        Err(e) => return Err(e),
                    }
                }
            }
        }

        pub fn release(&self, data: &Acquired) -> io::Result<()> {
            // Note that the fd may be nonblocking but we're going to go ahead
            // and assume that the writes here are always nonblocking (we can
            // always quickly release a token). If that turns out to not be the
            // case we'll get an error anyway!
            match (&self.write).write(&[data.byte])? {
                1 => Ok(()),
                _ => Err(io::Error::new(io::ErrorKind::Other,
                                        "failed to write token back to jobserver")),
            }
        }

        pub fn string_arg(&self) -> String {
            format!("{},{} -j", self.read.as_raw_fd(), self.write.as_raw_fd())
        }

        pub fn configure(&self, cmd: &mut Command) {
            // Here we basically just want to say that in the child process
            // we'll configure the read/write file descriptors to *not* be
            // cloexec, so they're inherited across the exec and specified as
            // integers through `string_arg` above.
            let read = self.read.as_raw_fd();
            let write = self.write.as_raw_fd();
            cmd.before_exec(move || {
                set_cloexec(read, false)?;
                set_cloexec(write, false)?;
                Ok(())
            });
        }
    }

    #[derive(Debug)]
    pub struct Helper {
        thread: JoinHandle<()>,
        quitting: Arc<AtomicBool>,
        rx_done: Receiver<()>,
    }

    pub fn spawn_helper(client: ::Client,
                        rx: Receiver<()>,
                        mut f: Box<FnMut(io::Result<::Acquired>) + Send>)
        -> io::Result<Helper>
    {
        static USR1_INIT: Once = ONCE_INIT;
        let mut err = None;
        USR1_INIT.call_once(|| unsafe {
            let mut new: libc::sigaction = mem::zeroed();
            new.sa_sigaction = sigusr1_handler as usize;
            new.sa_flags = libc::SA_SIGINFO as _;
            if libc::sigaction(libc::SIGUSR1, &new, ptr::null_mut()) != 0 {
                err = Some(io::Error::last_os_error());
            }
        });

        if let Some(e) = err.take() {
            return Err(e)
        }

        let quitting = Arc::new(AtomicBool::new(false));
        let quitting2 = quitting.clone();
        let (tx_done, rx_done) = mpsc::channel();
        let thread = Builder::new().spawn(move || {
            'outer:
            for () in rx {
                loop {
                    let res = client.acquire();
                    if let Err(ref e) = res {
                        if e.kind() == io::ErrorKind::Interrupted {
                            if quitting2.load(Ordering::SeqCst) {
                                break 'outer
                            } else {
                                continue
                            }
                        }
                    }
                    f(res);
                    break
                }
            }
            tx_done.send(()).unwrap();
        })?;

        Ok(Helper {
            thread: thread,
            quitting: quitting,
            rx_done: rx_done,
        })
    }

    impl Helper {
        pub fn join(self) {
            self.quitting.store(true, Ordering::SeqCst);
            let dur = Duration::from_millis(10);
            let mut done = false;
            for _ in 0..100 {
                unsafe {
                    // Ignore the return value here of `pthread_kill`,
                    // apparently on OSX if you kill a dead thread it will
                    // return an error, but on other platforms it may not. In
                    // that sense we don't actually know if this will succeed or
                    // not!
                    libc::pthread_kill(self.thread.as_pthread_t(), libc::SIGUSR1);
                    match self.rx_done.recv_timeout(dur) {
                        Ok(()) |
                        Err(RecvTimeoutError::Disconnected) => {
                            done = true;
                            break
                        }
                        Err(RecvTimeoutError::Timeout) => {}
                    }
                }
                thread::yield_now();
            }
            if done {
                drop(self.thread.join());
            }
        }
    }

    fn is_valid_fd(fd: c_int) -> bool {
        unsafe {
            return libc::fcntl(fd, libc::F_GETFD) != -1;
        }
    }

    fn set_cloexec(fd: c_int, set: bool) -> io::Result<()> {
        unsafe {
            let previous = cvt(libc::fcntl(fd, libc::F_GETFD))?;
            let new = if set {
                previous | libc::FD_CLOEXEC
            } else {
                previous & !libc::FD_CLOEXEC
            };
            if new != previous {
                cvt(libc::fcntl(fd, libc::F_SETFD, new))?;
            }
            Ok(())
        }
    }

    fn cvt(t: c_int) -> io::Result<c_int> {
        if t == -1 {
            Err(io::Error::last_os_error())
        } else {
            Ok(t)
        }
    }

    unsafe fn pipe2() -> Option<&'static fn(*mut c_int, c_int) -> c_int> {
        static PIPE2: AtomicUsize = ATOMIC_USIZE_INIT;

        if PIPE2.load(Ordering::SeqCst) == 0 {
            let name = "pipe2\0";
            let n = match libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr() as *const _) as usize {
                0 => 1,
                n => n,
            };
            PIPE2.store(n, Ordering::SeqCst);
        }
        if PIPE2.load(Ordering::SeqCst) == 1 {
            None
        } else {
            mem::transmute(&PIPE2)
        }
    }

    extern fn sigusr1_handler(_signum: c_int,
                              _info: *mut libc::siginfo_t,
                              _ptr: *mut libc::c_void) {
        // nothing to do
    }
}

#[cfg(windows)]
mod imp {
    extern crate rand;

    use std::ffi::CString;
    use std::io;
    use std::process::Command;
    use std::ptr;
    use std::sync::Arc;
    use std::sync::mpsc::Receiver;
    use std::thread::{Builder, JoinHandle};

    #[derive(Debug)]
    pub struct Client {
        sem: Handle,
        name: String,
    }

    #[derive(Debug)]
    pub struct Acquired;

    type BOOL = i32;
    type DWORD = u32;
    type HANDLE = *mut u8;
    type LONG = i32;

    const ERROR_ALREADY_EXISTS: DWORD = 183;
    const FALSE: BOOL = 0;
    const INFINITE: DWORD = 0xffffffff;
    const SEMAPHORE_MODIFY_STATE: DWORD = 0x2;
    const SYNCHRONIZE: DWORD = 0x00100000;
    const TRUE: BOOL = 1;
    const WAIT_OBJECT_0: DWORD = 0;

    extern "system" {
        fn CloseHandle(handle: HANDLE) -> BOOL;
        fn SetEvent(hEvent: HANDLE) -> BOOL;
        fn WaitForMultipleObjects(ncount: DWORD,
                                  lpHandles: *const HANDLE,
                                  bWaitAll: BOOL,
                                  dwMilliseconds: DWORD) -> DWORD;
        fn CreateEventA(lpEventAttributes: *mut u8,
                        bManualReset: BOOL,
                        bInitialState: BOOL,
                        lpName: *const i8) -> HANDLE;
        fn ReleaseSemaphore(hSemaphore: HANDLE,
                            lReleaseCount: LONG,
                            lpPreviousCount: *mut LONG) -> BOOL;
        fn CreateSemaphoreA(lpEventAttributes: *mut u8,
                            lInitialCount: LONG,
                            lMaximumCount: LONG,
                            lpName: *const i8) -> HANDLE;
        fn OpenSemaphoreA(dwDesiredAccess: DWORD,
                          bInheritHandle: BOOL,
                          lpName: *const i8) -> HANDLE;
        fn WaitForSingleObject(hHandle: HANDLE,
                               dwMilliseconds: DWORD) -> DWORD;
    }

    impl Client {
        pub fn new(limit: usize) -> io::Result<Client> {
            // Try a bunch of random semaphore names until we get a unique one,
            // but don't try for too long.
            //
            // Note that `limit == 0` is a valid argument above but Windows
            // won't let us create a semaphore with 0 slots available to it. Get
            // `limit == 0` working by creating a semaphore instead with one
            // slot and then immediately acquire it (without ever releaseing it
            // back).
            for _ in 0..100 {
                let mut name = format!("__rust_jobserver_semaphore_{}\0",
                                       rand::random::<u32>());
                unsafe {
                    let create_limit = if limit == 0 {1} else {limit};
                    let r = CreateSemaphoreA(ptr::null_mut(),
                                             create_limit as LONG,
                                             create_limit as LONG,
                                             name.as_ptr() as *const _);
                    if r.is_null() {
                        return Err(io::Error::last_os_error())
                    }
                    let handle = Handle(r);

                    let err = io::Error::last_os_error();
                    if err.raw_os_error() == Some(ERROR_ALREADY_EXISTS as i32) {
                        continue
                    }
                    name.pop(); // chop off the trailing nul
                    let client = Client {
                        sem: handle,
                        name: name,
                    };
                    if create_limit != limit {
                        client.acquire()?;
                    }
                    info!("created jobserver {:?}", client);
                    return Ok(client)
                }
            }

            Err(io::Error::new(io::ErrorKind::Other,
                               "failed to find a unique name for a semaphore"))
        }

        pub unsafe fn open(s: &str) -> Option<Client> {
            let name = match CString::new(s) {
                Ok(s) => s,
                Err(_) => return None,
            };

            let sem = OpenSemaphoreA(SYNCHRONIZE | SEMAPHORE_MODIFY_STATE,
                                     FALSE,
                                     name.as_ptr());
            if sem.is_null() {
                info!("failed to open environment semaphore {}", s);
                None
            } else {
                info!("opened environment semaphore {}", s);
                Some(Client {
                    sem: Handle(sem),
                    name: s.to_string(),
                })
            }
        }

        pub fn acquire(&self) -> io::Result<Acquired> {
            unsafe {
                let r = WaitForSingleObject(self.sem.0, INFINITE);
                if r == WAIT_OBJECT_0 {
                    Ok(Acquired)
                } else {
                    Err(io::Error::last_os_error())
                }
            }
        }

        pub fn release(&self, _data: &Acquired) -> io::Result<()> {
            unsafe {
                let r = ReleaseSemaphore(self.sem.0, 1, ptr::null_mut());
                if r != 0 {
                    Ok(())
                } else {
                    Err(io::Error::last_os_error())
                }
            }
        }

        pub fn string_arg(&self) -> String {
            self.name.clone()
        }

        pub fn configure(&self, _cmd: &mut Command) {
            // nothing to do here, we gave the name of our semaphore to the
            // child above
        }
    }

    #[derive(Debug)]
    struct Handle(HANDLE);
    // HANDLE is a raw ptr, but we're send/sync
    unsafe impl Sync for Handle {}
    unsafe impl Send for Handle {}

    impl Drop for Handle {
        fn drop(&mut self) {
            unsafe {
                CloseHandle(self.0);
            }
        }
    }

    #[derive(Debug)]
    pub struct Helper {
        event: Arc<Handle>,
        thread: JoinHandle<()>,
    }

    pub fn spawn_helper(client: ::Client,
                        rx: Receiver<()>,
                        mut f: Box<FnMut(io::Result<::Acquired>) + Send>)
        -> io::Result<Helper>
    {
        let event = unsafe {
            let r = CreateEventA(ptr::null_mut(), TRUE, FALSE, ptr::null());
            if r.is_null() {
                return Err(io::Error::last_os_error())
            } else {
                Handle(r)
            }
        };
        let event = Arc::new(event);
        let event2 = event.clone();
        let thread = Builder::new().spawn(move || {
            let objects = [event2.0, client.inner.sem.0];
            for () in rx {
                let r = unsafe {
                    WaitForMultipleObjects(2, objects.as_ptr(), FALSE, INFINITE)
                };
                if r == WAIT_OBJECT_0 {
                    break
                }
                if r == WAIT_OBJECT_0 + 1 {
                    f(Ok(::Acquired {
                        client: client.inner.clone(),
                        data: Acquired,
                    }))
                } else {
                    f(Err(io::Error::last_os_error()))
                }
            }
        })?;
        Ok(Helper {
            thread: thread,
            event: event,
        })
    }

    impl Helper {
        pub fn join(self) {
            let r = unsafe { SetEvent(self.event.0) };
            if r == 0 {
                panic!("failed to set event: {}", io::Error::last_os_error());
            }
            drop(self.thread.join());
        }
    }
}