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use std::{mem::MaybeUninit, sync::Once};
use crate::Error;
#[cfg(unix)]
#[path = "job_token/unix.rs"]
mod sys;
#[cfg(windows)]
#[path = "job_token/windows.rs"]
mod sys;
pub(super) struct JobToken();
impl Drop for JobToken {
fn drop(&mut self) {
match JobTokenServer::new() {
JobTokenServer::Inherited(jobserver) => jobserver.release_token_raw(),
JobTokenServer::InProcess(jobserver) => jobserver.release_token_raw(),
}
}
}
pub(super) enum JobTokenServer {
Inherited(inherited_jobserver::JobServer),
InProcess(inprocess_jobserver::JobServer),
}
impl JobTokenServer {
/// This function returns a static reference to the jobserver because
/// - creating a jobserver from env is a bit fd-unsafe (e.g. the fd might
/// be closed by other jobserver users in the process) and better do it
/// at the start of the program.
/// - in case a jobserver cannot be created from env (e.g. it's not
/// present), we will create a global in-process only jobserver
/// that has to be static so that it will be shared by all cc
/// compilation.
pub(crate) fn new() -> &'static Self {
static INIT: Once = Once::new();
static mut JOBSERVER: MaybeUninit<JobTokenServer> = MaybeUninit::uninit();
unsafe {
INIT.call_once(|| {
let server = inherited_jobserver::JobServer::from_env()
.map(Self::Inherited)
.unwrap_or_else(|| Self::InProcess(inprocess_jobserver::JobServer::new()));
JOBSERVER = MaybeUninit::new(server);
});
// TODO: Poor man's assume_init_ref, as that'd require a MSRV of 1.55.
&*JOBSERVER.as_ptr()
}
}
pub(crate) fn try_acquire(&self) -> Result<Option<JobToken>, Error> {
match self {
Self::Inherited(jobserver) => jobserver.try_acquire(),
Self::InProcess(jobserver) => Ok(jobserver.try_acquire()),
}
}
}
mod inherited_jobserver {
use super::{sys, Error, JobToken};
use std::{
env::var_os,
sync::atomic::{
AtomicBool,
Ordering::{AcqRel, Acquire},
},
};
pub(crate) struct JobServer {
/// Implicit token for this process which is obtained and will be
/// released in parent. Since JobTokens only give back what they got,
/// there should be at most one global implicit token in the wild.
///
/// Since Rust does not execute any `Drop` for global variables,
/// we can't just put it back to jobserver and then re-acquire it at
/// the end of the process.
global_implicit_token: AtomicBool,
inner: sys::JobServerClient,
}
impl JobServer {
pub(super) unsafe fn from_env() -> Option<Self> {
let var = var_os("CARGO_MAKEFLAGS")
.or_else(|| var_os("MAKEFLAGS"))
.or_else(|| var_os("MFLAGS"))?;
let inner = sys::JobServerClient::open(var)?;
Some(Self {
inner,
global_implicit_token: AtomicBool::new(true),
})
}
pub(super) fn try_acquire(&self) -> Result<Option<JobToken>, Error> {
if !self.global_implicit_token.swap(false, AcqRel) {
// Cold path, no global implicit token, obtain one
if self.inner.try_acquire()?.is_none() {
return Ok(None);
}
}
Ok(Some(JobToken()))
}
pub(super) fn release_token_raw(&self) {
// All tokens will be put back into the jobserver immediately
// and they cannot be cached, since Rust does not call `Drop::drop`
// on global variables.
if self
.global_implicit_token
.compare_exchange(false, true, AcqRel, Acquire)
.is_err()
{
// There's already a global implicit token, so this token must
// be released back into jobserver
let _ = self.inner.release();
}
}
}
}
mod inprocess_jobserver {
use super::JobToken;
use std::{
env::var,
sync::atomic::{
AtomicU32,
Ordering::{AcqRel, Acquire},
},
};
pub(crate) struct JobServer(AtomicU32);
impl JobServer {
pub(super) fn new() -> Self {
// Use `NUM_JOBS` if set (it's configured by Cargo) and otherwise
// just fall back to a semi-reasonable number.
//
// Note that we could use `num_cpus` here but it's an extra
// dependency that will almost never be used, so
// it's generally not too worth it.
let mut parallelism = 4;
// TODO: Use std::thread::available_parallelism as an upper bound
// when MSRV is bumped.
if let Ok(amt) = var("NUM_JOBS") {
if let Ok(amt) = amt.parse() {
parallelism = amt;
}
}
Self(AtomicU32::new(parallelism))
}
pub(super) fn try_acquire(&self) -> Option<JobToken> {
let res = self
.0
.fetch_update(AcqRel, Acquire, |tokens| tokens.checked_sub(1));
res.ok().map(|_| JobToken())
}
pub(super) fn release_token_raw(&self) {
self.0.fetch_add(1, AcqRel);
}
}
}