//! 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. Starting from
//! GNU `make` version 4.4, named pipe becomes the default way in communication
//! on Unix. This crate also supports that feature in the sense of inheriting
//! and forwarding the correct environment.
//!
//! The jobserver protocol in `make` also dictates when tokens are acquired 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]:
https://make.mad-scientist.net/papers/jobserver-implementation/
#![deny(missing_docs, missing_debug_implementations)]
#![doc(html_root_url = "
https://docs.rs/jobserver/0.1")]
use std::env;
use std::ffi::OsString;
use std::io;
use std::process::Command;
use std::sync::{Arc, Condvar, Mutex, MutexGuard};
mod error;
#[cfg(unix)]
#[path = "unix.rs"]
mod imp;
#[cfg(windows)]
#[path = "windows.rs"]
mod imp;
#[cfg(not(any(unix, windows)))]
#[path = "wasm.rs"]
mod imp;
/// 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,
disabled: bool,
}
impl Acquired {
/// This drops the [`Acquired`] token without releasing the associated token.
///
/// This is not generally useful, but can be helpful if you do not have the
/// ability to store an Acquired token but need to not yet release it.
///
/// You'll typically want to follow this up with a call to
/// [`Client::release_raw`] or similar to actually release the token later on.
pub fn drop_without_releasing(mut self) {
self.disabled = true;
}
}
#[derive(Default, Debug)]
struct HelperState {
lock: Mutex<HelperInner>,
cvar: Condvar,
}
#[derive(Default, Debug)]
struct HelperInner {
requests: usize,
producer_done: bool,
consumer_done: bool,
}
use error::FromEnvErrorInner;
pub use error::{FromEnvError, FromEnvErrorKind};
/// Return type for [`Client::from_env_ext`] function.
#[derive(Debug)]
pub struct FromEnv {
/// Result of trying to get jobserver client from env.
pub client: Result<Client, FromEnvError>,
/// Name and value of the environment variable.
/// `None` if no relevant environment variable is found.
pub var: Option<(&'static str, OsString)>,
}
impl FromEnv {
fn new_ok(client: Client, var_name: &'static str, var_value: OsString) -> FromEnv {
FromEnv {
client: Ok(client),
var: Some((var_name, var_value)),
}
}
fn new_err(kind: FromEnvErrorInner, var_name: &'static str, var_value: OsString) -> FromEnv {
FromEnv {
client: Err(FromEnvError { inner: kind }),
var: Some((var_name, var_value)),
}
}
}
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 [`Client::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
/// [`Client::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
/// [`Client::configure`] function is required for a child process to have
/// access to a job server.
///
/// # Return value
///
/// [`FromEnv`] contains result and relevant environment variable.
/// If a jobserver was found in the environment and it looks correct then
/// result with the connected client will be returned. In other cases
/// result will contain `Err(FromEnvErr)`.
///
/// Additionally on Unix this function will configure the file descriptors
/// with `CLOEXEC` so they're not automatically inherited by spawned
/// children.
///
/// On unix if `check_pipe` enabled this function will check if provided
/// files are actually pipes.
///
/// # Safety
///
/// 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 is ok to call this function any number of times.
pub unsafe fn from_env_ext(check_pipe: bool) -> FromEnv {
let (env, var_os) = match ["CARGO_MAKEFLAGS", "MAKEFLAGS", "MFLAGS"]
.iter()
.map(|&env| env::var_os(env).map(|var| (env, var)))
.find_map(|p| p)
{
Some((env, var_os)) => (env, var_os),
None => return FromEnv::new_err(FromEnvErrorInner::NoEnvVar, "", Default::default()),
};
let var = match var_os.to_str() {
Some(var) => var,
None => {
let err = FromEnvErrorInner::CannotParse("not valid UTF-8".to_string());
return FromEnv::new_err(err, env, var_os);
}
};
let s = match find_jobserver_auth(var) {
Some(s) => s,
None => return FromEnv::new_err(FromEnvErrorInner::NoJobserver, env, var_os),
};
match imp::Client::open(s, check_pipe) {
Ok(c) => FromEnv::new_ok(Client { inner: Arc::new(c) }, env, var_os),
Err(err) => FromEnv::new_err(err, env, var_os),
}
}
/// Attempts to connect to the jobserver specified in this process's
/// environment.
///
/// Wraps [`Client::from_env_ext`] and discards error details.
///
/// # Safety
///
/// 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 is ok to call this function any number of times.
pub unsafe fn from_env() -> Option<Client> {
Self::from_env_ext(false).client.ok()
}
/// 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 = self.inner.acquire()?;
Ok(Acquired {
client: self.inner.clone(),
data,
disabled: false,
})
}
/// Acquires a token from this jobserver client in a non-blocking way.
///
/// # 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.
///
/// If non-blocking acquire is not supported, the return error will have its `kind()`
/// set to [`io::ErrorKind::Unsupported`].
pub fn try_acquire(&self) -> io::Result<Option<Acquired>> {
let ret = self.inner.try_acquire()?;
Ok(ret.map(|data| Acquired {
client: self.inner.clone(),
data,
disabled: false,
}))
}
/// Returns amount of tokens in the read-side pipe.
///
/// # Return value
///
/// Number of bytes available to be read from the jobserver pipe
///
/// # Errors
///
/// Underlying errors from the ioctl will be passed up.
pub fn available(&self) -> io::Result<usize> {
self.inner.available()
}
/// 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 [`Client::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.
///
/// On platforms other than Unix and Windows this panics.
pub fn configure(&self, cmd: &mut Command) {
cmd.env("CARGO_MAKEFLAGS", &self.mflags_env());
self.inner.configure(cmd);
}
/// 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 [`Client::from_env`] that
/// `make` uses).
///
/// ## Platform-specific behavior
///
/// On Unix and Windows this will clobber the `CARGO_MAKEFLAGS`,
/// `MAKEFLAGS` and `MFLAGS` 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.
///
/// On platforms other than Unix and Windows this panics.
pub fn configure_make(&self, cmd: &mut Command) {
let value = self.mflags_env();
cmd.env("CARGO_MAKEFLAGS", &value);
cmd.env("MAKEFLAGS", &value);
cmd.env("MFLAGS", &value);
self.inner.configure(cmd);
}
fn mflags_env(&self) -> String {
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.
format!("-j --jobserver-fds={0} --jobserver-auth={0}", arg)
}
/// Converts this [`Client`] into a helper thread to deal with a blocking
/// [`Client::acquire`] function a little more easily.
///
/// The fact that the [`Client::acquire`] 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 [`Client::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 state = Arc::new(HelperState::default());
Ok(HelperThread {
inner: Some(imp::spawn_helper(self, state.clone(), Box::new(f))?),
state,
})
}
/// Blocks the current thread until a token is acquired.
///
/// This is the same as [`Client::acquire`], except that it doesn't return
/// an RAII helper. If successful the process will need to guarantee that
/// [`Client::release_raw`] is called in the future.
pub fn acquire_raw(&self) -> io::Result<()> {
self.inner.acquire()?;
Ok(())
}
/// Releases a jobserver token back to the original jobserver.
///
/// This is intended to be paired with [`Client::acquire_raw`] if it was
/// called, but in some situations it could also be called to relinquish a
/// process's implicit token temporarily which is then re-acquired later.
pub fn release_raw(&self) -> io::Result<()> {
self.inner.release(None)?;
Ok(())
}
}
impl Drop for Acquired {
fn drop(&mut self) {
if !self.disabled {
drop(self.client.release(Some(&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>,
state: Arc<HelperState>,
}
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 [`Client::into_helper_thread`].
pub fn request_token(&self) {
// Indicate that there's one more request for a token and then wake up
// the helper thread if it's sleeping.
self.state.lock().requests += 1;
self.state.cvar.notify_one();
}
}
impl Drop for HelperThread {
fn drop(&mut self) {
// Flag that the producer half is done so the helper thread should exit
// quickly if it's waiting. Wake it up if it's actually waiting
self.state.lock().producer_done = true;
self.state.cvar.notify_one();
// ... and afterwards perform any thread cleanup logic
self.inner.take().unwrap().join();
}
}
impl HelperState {
fn lock(&self) -> MutexGuard<'_, HelperInner> {
self.lock.lock().unwrap_or_else(|e| e.into_inner())
}
/// Executes `f` for each request for a token, where `f` is expected to
/// block and then provide the original closure with a token once it's
/// acquired.
///
/// This is an infinite loop until the helper thread is dropped, at which
/// point everything should get interrupted.
fn for_each_request(&self, mut f: impl FnMut(&HelperState)) {
let mut lock = self.lock();
// We only execute while we could receive requests, but as soon as
// that's `false` we're out of here.
while !lock.producer_done {
// If no one's requested a token then we wait for someone to
// request a token.
if lock.requests == 0 {
lock = self.cvar.wait(lock).unwrap_or_else(|e| e.into_inner());
continue;
}
// Consume the request for a token, and then actually acquire a
// token after unlocking our lock (not that acquisition happens in
// `f`). This ensures that we don't actually hold the lock if we
// wait for a long time for a token.
lock.requests -= 1;
drop(lock);
f(self);
lock = self.lock();
}
lock.consumer_done = true;
self.cvar.notify_one();
}
}
/// Finds and returns the value of `--jobserver-auth=<VALUE>` in the given
/// environment variable.
///
/// Precedence rules:
///
/// * The last instance wins [^1].
/// * `--jobserver-fds=` as a fallback when no `--jobserver-auth=` is present [^2].
///
/// [^1]: See ["GNU `make` manual: Sharing Job Slots with GNU `make`"](
https://www.gnu.org/software/make/manual/make.html#Job-Slots)
/// _"Be aware that the `MAKEFLAGS` variable may contain multiple instances of
/// the `--jobserver-auth=` option. Only the last instance is relevant."_
///
/// [^2]: Refer to [the release announcement](
https://git.savannah.gnu.org/cgit/make.git/tree/NEWS?h=4.2#n31)
/// of GNU Make 4.2, which states that `--jobserver-fds` was initially an
/// internal-only flag and was later renamed to `--jobserver-auth`.
fn find_jobserver_auth(var: &str) -> Option<&str> {
["--jobserver-auth=", "--jobserver-fds="]
.iter()
.find_map(|&arg| var.rsplit_once(arg).map(|(_, s)| s))
.and_then(|s| s.split(' ').next())
}
#[cfg(test)]
mod test {
use super::*;
pub(super) fn run_named_fifo_try_acquire_tests(client: &Client) {
assert!(client.try_acquire().unwrap().is_none());
client.release_raw().unwrap();
let acquired = client.try_acquire().unwrap().unwrap();
assert!(client.try_acquire().unwrap().is_none());
drop(acquired);
client.try_acquire().unwrap().unwrap();
}
#[cfg(windows)]
#[test]
fn test_try_acquire() {
let client = Client::new(0).unwrap();
run_named_fifo_try_acquire_tests(&client);
}
#[test]
fn no_helper_deadlock() {
let x = crate::Client::new(32).unwrap();
let _y = x.clone();
std::mem::drop(x.into_helper_thread(|_| {}).unwrap());
}
#[test]
fn test_find_jobserver_auth() {
let cases = [
("", None),
("-j2", None),
("-j2 --jobserver-auth=3,4", Some("3,4")),
("--jobserver-auth=3,4 -j2", Some("3,4")),
("--jobserver-auth=3,4", Some("3,4")),
("--jobserver-auth=fifo:/myfifo", Some("fifo:/myfifo")),
("--jobserver-auth=", Some("")),
("--jobserver-auth", None),
("--jobserver-fds=3,4", Some("3,4")),
("--jobserver-fds=fifo:/myfifo", Some("fifo:/myfifo")),
("--jobserver-fds=", Some("")),
("--jobserver-fds", None),
(
"--jobserver-auth=auth-a --jobserver-auth=auth-b",
Some("auth-b"),
),
(
"--jobserver-auth=auth-b --jobserver-auth=auth-a",
Some("auth-a"),
),
("--jobserver-fds=fds-a --jobserver-fds=fds-b", Some("fds-b")),
("--jobserver-fds=fds-b --jobserver-fds=fds-a", Some("fds-a")),
(
"--jobserver-auth=auth-a --jobserver-fds=fds-a --jobserver-auth=auth-b",
Some("auth-b"),
),
(
"--jobserver-fds=fds-a --jobserver-auth=auth-a --jobserver-fds=fds-b",
Some("auth-a"),
),
];
for (var, expected) in cases {
let actual = find_jobserver_auth(var);
assert_eq!(
actual, expected,
"expect {expected:?}, got {actual:?}, input `{var:?}`"
);
}
}
}
