Optimize Build::compile_objects: Only spawns one thread

Cargo.toml

@@ -19,6 +19,7 @@ edition = "2018"
 
 [dependencies]
 jobserver = { version = "0.1.16", optional = true }
+os_pipe = "1"
 
 [features]
 parallel = ["jobserver"]

src/lib.rs

@@ -1218,8 +1218,7 @@ impl Build {
     }
 
     #[cfg(feature = "parallel")]
-    fn compile_objects<'me>(&'me self, objs: &[Object]) -> Result<(), Error> {
-        use std::sync::atomic::{AtomicBool, Ordering::SeqCst};
+    fn compile_objects(&self, objs: &[Object]) -> Result<(), Error> {
         use std::sync::Once;
 
         // Limit our parallelism globally with a jobserver. Start off by
@@ -1242,56 +1241,28 @@ impl Build {
         //   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.
+        // * Next we use spawn the process to actually compile objects in
+        //   parallel after we've acquired a token to perform some work
         //
         // 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 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)));
-        }
+        // we wait on all the processes and propagate the results of compilation.
+        let print = PrintThread::new()?;
 
-        for mut thread in threads {
-            if let Some(thread) = thread.0.take() {
-                thread.join().expect("thread should not panic")?;
-            }
+        let children = objs
+            .iter()
+            .map(|obj| {
+                let (mut cmd, program) = self.create_compile_object_cmd(obj)?;
+                let token = server.acquire()?;
+
+                let child = spawn(&mut cmd, &program, print.pipe_writer_cloned()?.unwrap())?;
+
+                Ok((cmd, program, KillOnDrop(child), token))
+            })
+            .collect::<Result<Vec<_>, Error>>()?;
+
+        for (cmd, program, mut child, _token) in children {
+            wait_on_child(&cmd, &program, &mut child.0)?;
         }
 
         // Reacquire our process's token before we proceed, which we released
@@ -1302,16 +1273,6 @@ impl Build {
 
         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 {
@@ -1357,26 +1318,30 @@ impl Build {
             return client;
         }
 
-        struct JoinOnDrop(Option<thread::JoinHandle<Result<(), Error>>>);
+        struct KillOnDrop(Child);
 
-        impl Drop for JoinOnDrop {
+        impl Drop for KillOnDrop {
             fn drop(&mut self) {
-                if let Some(thread) = self.0.take() {
-                    drop(thread.join());
-                }
+                let child = &mut self.0;
+
+                child.kill().ok();
             }
         }
     }
 
     #[cfg(not(feature = "parallel"))]
     fn compile_objects(&self, objs: &[Object]) -> Result<(), Error> {
+        let print = PrintThread::new()?;
+
         for obj in objs {
-            self.compile_object(obj)?;
+            let (mut cmd, name) = self.create_compile_object_cmd(obj)?;
+            run_inner(&mut cmd, &name, print.pipe_writer_cloned()?.unwrap())?;
         }
+
         Ok(())
     }
 
-    fn compile_object(&self, obj: &Object) -> Result<(), Error> {
+    fn create_compile_object_cmd(&self, obj: &Object) -> Result<(Command, String), Error> {
         let asm_ext = AsmFileExt::from_path(&obj.src);
         let is_asm = asm_ext.is_some();
         let target = self.get_target()?;
@@ -1425,8 +1390,7 @@ impl Build {
             self.fix_env_for_apple_os(&mut cmd)?;
         }
 
-        run(&mut cmd, &name)?;
-        Ok(())
+        Ok((cmd, name))
     }
 
     /// This will return a result instead of panicing; see expand() for the complete description.
@@ -3463,21 +3427,19 @@ impl Tool {
     }
 }
 
-fn run(cmd: &mut Command, program: &str) -> Result<(), Error> {
-    let (mut child, print) = spawn(cmd, program)?;
+fn wait_on_child(cmd: &Command, program: &str, child: &mut Child) -> Result<(), Error> {
     let status = match child.wait() {
         Ok(s) => s,
-        Err(_) => {
+        Err(e) => {
             return Err(Error::new(
                 ErrorKind::ToolExecError,
                 &format!(
-                    "Failed to wait on spawned child process, command {:?} with args {:?}.",
-                    cmd, program
+                    "Failed to wait on spawned child process, command {:?} with args {:?}: {}.",
+                    cmd, program, e
                 ),
             ));
         }
     };
-    print.join().unwrap();
     println!("{}", status);
 
     if status.success() {
@@ -3493,63 +3455,62 @@ fn run(cmd: &mut Command, program: &str) -> Result<(), Error> {
     }
 }
 
+fn run_inner(
+    cmd: &mut Command,
+    program: &str,
+    pipe_writer: os_pipe::PipeWriter,
+) -> Result<(), Error> {
+    let mut child = spawn(cmd, program, pipe_writer)?;
+    wait_on_child(cmd, program, &mut child)
+}
+
+fn run(cmd: &mut Command, program: &str) -> Result<(), Error> {
+    let mut print = PrintThread::new()?;
+    run_inner(cmd, program, print.pipe_writer().take().unwrap())?;
+
+    Ok(())
+}
+
 fn run_output(cmd: &mut Command, program: &str) -> Result<Vec<u8>, Error> {
     cmd.stdout(Stdio::piped());
-    let (mut child, print) = spawn(cmd, program)?;
+
+    let mut print = PrintThread::new()?;
+    let mut child = spawn(cmd, program, print.pipe_writer().take().unwrap())?;
+
     let mut stdout = vec![];
     child
         .stdout
         .take()
         .unwrap()
         .read_to_end(&mut stdout)
         .unwrap();
-    let status = match child.wait() {
-        Ok(s) => s,
-        Err(_) => {
-            return Err(Error::new(
-                ErrorKind::ToolExecError,
-                &format!(
-                    "Failed to wait on spawned child process, command {:?} with args {:?}.",
-                    cmd, program
-                ),
-            ));
-        }
-    };
-    print.join().unwrap();
-    println!("{}", status);
 
-    if status.success() {
-        Ok(stdout)
-    } else {
-        Err(Error::new(
-            ErrorKind::ToolExecError,
-            &format!(
-                "Command {:?} with args {:?} did not execute successfully (status code {}).",
-                cmd, program, status
-            ),
-        ))
-    }
+    wait_on_child(cmd, program, &mut child)?;
+
+    Ok(stdout)
 }
 
-fn spawn(cmd: &mut Command, program: &str) -> Result<(Child, JoinHandle<()>), Error> {
-    println!("running: {:?}", cmd);
+fn spawn(
+    cmd: &mut Command,
+    program: &str,
+    pipe_writer: os_pipe::PipeWriter,
+) -> Result<Child, Error> {
+    struct ResetStderr<'cmd>(&'cmd mut Command);
 
-    // Capture the standard error coming from these programs, and write it out
-    // with cargo:warning= prefixes. Note that this is a bit wonky to avoid
-    // requiring the output to be UTF-8, we instead just ship bytes from one
-    // location to another.
-    match cmd.stderr(Stdio::piped()).spawn() {
-        Ok(mut child) => {
-            let stderr = BufReader::new(child.stderr.take().unwrap());
-            let print = thread::spawn(move || {
-                for line in stderr.split(b'\n').filter_map(|l| l.ok()) {
-                    print!("cargo:warning=");
-                    std::io::stdout().write_all(&line).unwrap();
-                    println!("");
-                }
-            });
-            Ok((child, print))
+    impl Drop for ResetStderr<'_> {
+        fn drop(&mut self) {
+            // Reset stderr to default to release pipe_writer so that print thread will
+            // not block forever.
+            self.0.stderr(Stdio::inherit());
         }
+    }
+
+    println!("running: {:?}", cmd);
+
+    let cmd = ResetStderr(cmd);
+
+    match cmd.0.stderr(pipe_writer).spawn() {
+        Ok(child) => Ok(child),
         Err(ref e) if e.kind() == io::ErrorKind::NotFound => {
             let extra = if cfg!(windows) {
                 " (see https://github.com/rust-lang/cc-rs#compile-time-requirements \
@@ -3562,11 +3523,11 @@ fn spawn(cmd: &mut Command, program: &str) -> Result<(Child, JoinHandle<()>), Er
                 &format!("Failed to find tool. Is `{}` installed?{}", program, extra),
             ))
         }
-        Err(ref e) => Err(Error::new(
+        Err(e) => Err(Error::new(
             ErrorKind::ToolExecError,
             &format!(
                 "Command {:?} with args {:?} failed to start: {:?}",
-                cmd, program, e
+                cmd.0, program, e
             ),
         )),
     }
@@ -3767,3 +3728,58 @@ impl AsmFileExt {
         None
     }
 }
+
+struct PrintThread {
+    handle: Option<JoinHandle<()>>,
+    pipe_writer: Option<os_pipe::PipeWriter>,
+}
+
+impl PrintThread {
+    fn new() -> Result<Self, Error> {
+        let (pipe_reader, pipe_writer) = os_pipe::pipe()?;
+
+        // Capture the standard error coming from compilation, and write it out
+        // with cargo:warning= prefixes. Note that this is a bit wonky to avoid
+        // requiring the output to be UTF-8, we instead just ship bytes from one
+        // location to another.
+        let print = thread::spawn(move || {
+            let mut stderr = BufReader::with_capacity(4096, pipe_reader);
+            let mut line = String::with_capacity(20);
+            let mut stdout = io::stdout();
+
+            // read_line returns 0 on Eof
+            while stderr.read_line(&mut line).unwrap() != 0 {
+                writeln!(&mut stdout, "cargo:warning={}", line).ok();
+
+                // read_line does not clear the buffer
+                line.clear();
+            }
+        });
+
+        Ok(Self {
+            handle: Some(print),
+            pipe_writer: Some(pipe_writer),
+        })
+    }
+
+    fn pipe_writer(&mut self) -> &mut Option<os_pipe::PipeWriter> {
+        &mut self.pipe_writer
+    }
+
+    fn pipe_writer_cloned(&self) -> Result<Option<os_pipe::PipeWriter>, Error> {
+        self.pipe_writer
+            .as_ref()
+            .map(os_pipe::PipeWriter::try_clone)
+            .transpose()
+            .map_err(From::from)
+    }
+}
+
+impl Drop for PrintThread {
+    fn drop(&mut self) {
+        // Drop pipe_writer first to avoid deadlock
+        self.pipe_writer.take();
+
+        self.handle.take().unwrap().join().unwrap();
+    }
+}