Fix make resource failure when cc parallel is enabled (#985)

Cargo.toml

@@ -19,13 +19,16 @@ exclude = ["/.github", "tests", "src/bin"]
 edition = "2018"
 rust-version = "1.53"
 
+[dependencies]
+jobserver = { version = "0.1.20", default-features = false, optional = true }
+
 [target.'cfg(unix)'.dependencies]
 # Don't turn on the feature "std" for this, see https://github.com/rust-lang/cargo/issues/4866
 # which is still an issue with `resolver = "1"`.
 libc = { version = "0.2.62", default-features = false, optional = true }
 
 [features]
-parallel = ["libc"]
+parallel = ["libc", "jobserver"]
 
 [dev-dependencies]
 tempfile = "3"

src/lib.rs

@@ -312,6 +312,9 @@ enum ErrorKind {
     ToolNotFound,
     /// One of the function arguments failed validation.
     InvalidArgument,
+    #[cfg(feature = "parallel")]
+    /// jobserver helpthread failure
+    JobserverHelpThreadError,
 }
 
 /// Represents an internal error that occurred, with an explanation.
@@ -1510,13 +1513,7 @@ impl Build {
         let spawn_future = async {
             for obj in objs {
                 let (mut cmd, program) = self.create_compile_object_cmd(obj)?;
-                let token = loop {
-                    if let Some(token) = tokens.try_acquire()? {
-                        break token;
-                    } else {
-                        YieldOnce::default().await
-                    }
-                };
+                let token = tokens.acquire().await?;
                 let mut child = spawn(&mut cmd, &program, &self.cargo_output)?;
                 let mut stderr_forwarder = StderrForwarder::new(&mut child);
                 stderr_forwarder.set_non_blocking()?;

src/parallel/job_token.rs

@@ -0,0 +1,255 @@
+use std::{marker::PhantomData, mem::MaybeUninit, sync::Once};
+
+use crate::Error;
+
+pub(crate) struct JobToken(PhantomData<()>);
+
+impl JobToken {
+    fn new() -> Self {
+        Self(PhantomData)
+    }
+}
+
+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(),
+        }
+    }
+}
+
+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.
+    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) enum ActiveJobTokenServer {
+    Inherited(inherited_jobserver::ActiveJobServer<'static>),
+    InProcess(&'static inprocess_jobserver::JobServer),
+}
+
+impl ActiveJobTokenServer {
+    pub(crate) fn new() -> Result<Self, Error> {
+        match JobTokenServer::new() {
+            JobTokenServer::Inherited(inherited_jobserver) => {
+                inherited_jobserver.enter_active().map(Self::Inherited)
+            }
+            JobTokenServer::InProcess(inprocess_jobserver) => {
+                Ok(Self::InProcess(inprocess_jobserver))
+            }
+        }
+    }
+
+    pub(crate) async fn acquire(&self) -> Result<JobToken, Error> {
+        match &self {
+            Self::Inherited(jobserver) => jobserver.acquire().await,
+            Self::InProcess(jobserver) => Ok(jobserver.acquire().await),
+        }
+    }
+}
+
+mod inherited_jobserver {
+    use super::JobToken;
+
+    use crate::{parallel::async_executor::YieldOnce, Error, ErrorKind};
+
+    use std::{
+        io, mem,
+        sync::{mpsc, Mutex, MutexGuard, PoisonError},
+    };
+
+    pub(super) 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.
+        ///
+        /// Use `Mutex` to avoid race between acquire and release.
+        /// If an `AtomicBool` is used, then it's possible for:
+        ///  - `release_token_raw`: Tries to set `global_implicit_token` to true, but it is already
+        ///    set  to `true`, continue to release it to jobserver
+        ///  - `acquire` takes the global implicit token, set `global_implicit_token` to false
+        ///  - `release_token_raw` now writes the token back into the jobserver, while
+        ///    `global_implicit_token` is `false`
+        ///
+        /// If the program exits here, then cc effectively increases parallelism by one, which is
+        /// incorrect, hence we use a `Mutex` here.
+        global_implicit_token: Mutex<bool>,
+        inner: jobserver::Client,
+    }
+
+    impl JobServer {
+        pub(super) unsafe fn from_env() -> Option<Self> {
+            jobserver::Client::from_env().map(|inner| Self {
+                inner,
+                global_implicit_token: Mutex::new(true),
+            })
+        }
+
+        fn get_global_implicit_token(&self) -> MutexGuard<'_, bool> {
+            self.global_implicit_token
+                .lock()
+                .unwrap_or_else(PoisonError::into_inner)
+        }
+
+        /// All tokens except for the global implicit token will be put back into the jobserver
+        /// immediately and they cannot be cached, since Rust does not call `Drop::drop` on
+        /// global variables.
+        pub(super) fn release_token_raw(&self) {
+            let mut global_implicit_token = self.get_global_implicit_token();
+
+            if *global_implicit_token {
+                // There's already a global implicit token, so this token must
+                // be released back into jobserver.
+                //
+                // `release_raw` should not block
+                let _ = self.inner.release_raw();
+            } else {
+                *global_implicit_token = true;
+            }
+        }
+
+        pub(super) fn enter_active(&self) -> Result<ActiveJobServer<'_>, Error> {
+            ActiveJobServer::new(self)
+        }
+    }
+
+    pub(crate) struct ActiveJobServer<'a> {
+        jobserver: &'a JobServer,
+        helper_thread: jobserver::HelperThread,
+        /// When rx is dropped, all the token stored within it will be dropped.
+        rx: mpsc::Receiver<io::Result<jobserver::Acquired>>,
+    }
+
+    impl<'a> ActiveJobServer<'a> {
+        fn new(jobserver: &'a JobServer) -> Result<Self, Error> {
+            let (tx, rx) = mpsc::channel();
+
+            Ok(Self {
+                rx,
+                helper_thread: jobserver.inner.clone().into_helper_thread(move |res| {
+                    let _ = tx.send(res);
+                })?,
+                jobserver,
+            })
+        }
+
+        pub(super) async fn acquire(&self) -> Result<JobToken, Error> {
+            let mut has_requested_token = false;
+
+            loop {
+                // Fast path
+                if mem::replace(&mut *self.jobserver.get_global_implicit_token(), false) {
+                    break Ok(JobToken::new());
+                }
+
+                // Cold path, no global implicit token, obtain one
+                match self.rx.try_recv() {
+                    Ok(res) => {
+                        let acquired = res?;
+                        acquired.drop_without_releasing();
+                        break Ok(JobToken::new());
+                    }
+                    Err(mpsc::TryRecvError::Disconnected) => {
+                        break Err(Error::new(
+                            ErrorKind::JobserverHelpThreadError,
+                            "jobserver help thread has returned before ActiveJobServer is dropped",
+                        ))
+                    }
+                    Err(mpsc::TryRecvError::Empty) => {
+                        if !has_requested_token {
+                            self.helper_thread.request_token();
+                            has_requested_token = true;
+                        }
+                        YieldOnce::default().await
+                    }
+                }
+            }
+        }
+    }
+}
+
+mod inprocess_jobserver {
+    use super::JobToken;
+
+    use crate::parallel::async_executor::YieldOnce;
+
+    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) async fn acquire(&self) -> JobToken {
+            loop {
+                let res = self
+                    .0
+                    .fetch_update(AcqRel, Acquire, |tokens| tokens.checked_sub(1));
+
+                if res.is_ok() {
+                    break JobToken::new();
+                }
+
+                YieldOnce::default().await
+            }
+        }
+
+        pub(super) fn release_token_raw(&self) {
+            self.0.fetch_add(1, AcqRel);
+        }
+    }
+}