Rejigger parallel compilation support

This commit reimplements parallel compilation support with a few goals
in mind:

* Primarily the `jobserver` crate is now used to limit parallelism
  across build scripts
* The `rayon` crate is no longer used to ensure this is a pretty
  lightweight dependency crate.

It's hoped that after this bakes for a bit we may be able to turn this
on by default!

Cargo.toml

@@ -17,10 +17,11 @@ categories = ["development-tools::build-utils"]
 exclude = ["/.travis.yml", "/appveyor.yml"]
 
 [dependencies]
-rayon = { version = "1.0", optional = true }
+num_cpus = { version = "1.10", optional = true }
+jobserver = { version = "0.1.16", optional = true }
 
 [features]
-parallel = ["rayon"]
+parallel = ["num_cpus", "jobserver"]
 
 [dev-dependencies]
 tempdir = "0.3"

src/lib.rs

@@ -58,9 +58,6 @@
 #![allow(deprecated)]
 #![deny(missing_docs)]
 
-#[cfg(feature = "parallel")]
-extern crate rayon;
-
 use std::collections::HashMap;
 use std::env;
 use std::ffi::{OsStr, OsString};
@@ -944,22 +941,132 @@ impl Build {
     }
 
     #[cfg(feature = "parallel")]
-    fn compile_objects(&self, objs: &[Object]) -> Result<(), Error> {
-        use self::rayon::prelude::*;
+    fn compile_objects<'me>(&'me self, objs: &[Object]) -> Result<(), Error> {
+        use std::sync::atomic::{AtomicBool, Ordering::SeqCst};
+        use std::sync::Once;
+
+        // When compiling objects in parallel we do a few dirty tricks to speed
+        // things up:
+        //
+        // * First is that we use the `jobserver` crate to limit the parallelism
+        //   of this build script. The `jobserver` crate will use a jobserver
+        //   configured by Cargo for build scripts to ensure that parallelism is
+        //   coordinated across C compilations and Rust compilations. Before we
+        //   compile anything we make sure to wait until we acquire a token.
+        //
+        //   Note that this jobserver is cached globally so we only used one per
+        //   process and only worry about creating it once.
+        //
+        // * Next we use a raw `thread::spawn` per thread to actually compile
+        //   objects in parallel. We only actually spawn a thread after we've
+        //   acquired a token to perform some work
+        //
+        // * Finally though we want to keep the dependencies of this crate
+        //   pretty light, so we avoid using a safe abstraction like `rayon` and
+        //   instead rely on some bits of `unsafe` code. We know that this stack
+        //   frame persists while everything is compiling so we use all the
+        //   stack-allocated objects without cloning/reallocating. We use a
+        //   transmute to `State` with a `'static` lifetime to persist
+        //   everything we need across the boundary, and the join-on-drop
+        //   semantics of `JoinOnDrop` should ensure that our stack frame is
+        //   alive while threads are alive.
+        //
+        // With all that in mind we compile all objects in a loop here, after we
+        // acquire the appropriate tokens, Once all objects have been compiled
+        // we join on all the threads and propagate the results of compilation.
+        //
+        // Note that as a slight optimization we try to break out as soon as
+        // possible as soon as any compilation fails to ensure that errors get
+        // out to the user as fast as possible.
+        let server = jobserver();
+        let error = AtomicBool::new(false);
+        let mut threads = Vec::new();
+        for obj in objs {
+            if error.load(SeqCst) {
+                break;
+            }
+            let token = server.acquire()?;
+            let state = State {
+                build: self,
+                obj,
+                error: &error,
+            };
+            let state = unsafe { std::mem::transmute::<State, State<'static>>(state) };
+            let thread = thread::spawn(|| {
+                let state: State<'me> = state; // erase the `'static` lifetime
+                let result = state.build.compile_object(state.obj);
+                if result.is_err() {
+                    state.error.store(true, SeqCst);
+                }
+                drop(token); // make sure our jobserver token is released after the compile
+                return result;
+            });
+            threads.push(JoinOnDrop(Some(thread)));
+        }
 
-        if let Some(amt) = self.getenv("NUM_JOBS") {
-            if let Ok(amt) = amt.parse() {
-                let _ = rayon::ThreadPoolBuilder::new()
-                    .num_threads(amt)
-                    .build_global();
+        for mut thread in threads {
+            if let Some(thread) = thread.0.take() {
+                thread.join().expect("thread should not panic")?;
             }
         }
 
-        // Check for any errors and return the first one found.
-        objs.par_iter()
-            .with_max_len(1)
-            .map(|obj| self.compile_object(obj))
-            .collect()
+        return Ok(());
+
+        /// Shared state from the parent thread to the child thread. This
+        /// package of pointers is temporarily transmuted to a `'static`
+        /// lifetime to cross the thread boundary and then once the thread is
+        /// running we erase the `'static` to go back to an anonymous lifetime.
+        struct State<'a> {
+            build: &'a Build,
+            obj: &'a Object,
+            error: &'a AtomicBool,
+        }
+
+        /// Returns a suitable `jobserver::Client` used to coordinate
+        /// parallelism between build scripts.
+        fn jobserver() -> &'static jobserver::Client {
+            static INIT: Once = Once::new();
+            static mut JOBSERVER: Option<jobserver::Client> = None;
+
+            fn _assert_sync<T: Sync>() {}
+            _assert_sync::<jobserver::Client>();
+
+            unsafe {
+                INIT.call_once(|| {
+                    let server = default_jobserver();
+                    JOBSERVER = Some(server);
+                });
+                JOBSERVER.as_ref().unwrap()
+            }
+        }
+
+        unsafe fn default_jobserver() -> jobserver::Client {
+            // Try to use the environmental jobserver which Cargo typically
+            // initializes for us...
+            if let Some(client) = jobserver::Client::from_env() {
+                return client;
+            }
+
+            // ... but if that fails for whatever reason fall back to the number
+            // of cpus on the system or the `NUM_JOBS` env var.
+            let mut parallelism = num_cpus::get();
+            if let Ok(amt) = env::var("NUM_JOBS") {
+                if let Ok(amt) = amt.parse() {
+                    parallelism = amt;
+                }
+            }
+            jobserver::Client::new(parallelism).expect("failed to create jobserver")
+        }
+
+        struct JoinOnDrop(Option<thread::JoinHandle<Result<(), Error>>>);
+
+        impl Drop for JoinOnDrop {
+            fn drop(&mut self) {
+                if let Some(thread) = self.0.take() {
+                    drop(thread.join());
+                }
+            }
+        }
     }
 
     #[cfg(not(feature = "parallel"))]