Add back preadv2 optimization for try_acquire on Linux

src/unix.rs

@@ -285,6 +285,30 @@ impl Client {
     pub fn try_acquire(&self) -> io::Result<Option<Acquired>> {
         let mut buf = [0];
 
+        // On Linux, we can use preadv2 to do non-blocking read,
+        // even if `O_NONBLOCK` is not set.
+        #[cfg(any(target_os = "linux", target_os = "android"))]
+        {
+            let read = self.read().as_raw_fd();
+            loop {
+                match linux::non_blocking_read(read, &mut buf) {
+                    Ok(1) => return Ok(Some(Acquired { byte: buf[0] })),
+                    Ok(_) => {
+                        return Err(io::Error::new(
+                            io::ErrorKind::UnexpectedEof,
+                            "early EOF on jobserver pipe",
+                        ))
+                    }
+
+                    Err(e) if e.kind() == io::ErrorKind::WouldBlock => return Ok(None),
+                    Err(e) if e.kind() == io::ErrorKind::Interrupted => continue,
+                    Err(e) if e.kind() == io::ErrorKind::Unsupported => break,
+
+                    Err(err) => return Err(err),
+                }
+            }
+        }
+
         let (mut fifo, is_non_blocking) = match self {
             Self::Fifo {
                 file,
@@ -368,6 +392,73 @@ impl Client {
     }
 }
 
+#[cfg(any(target_os = "linux", target_os = "android"))]
+mod linux {
+    use super::*;
+
+    use libc::{iovec, off_t, ssize_t, syscall, SYS_preadv2};
+
+    // TODO: Replace this with libc::RWF_NOWAIT once they have it for musl
+    //  targets
+    const RWF_NOWAIT: c_int = 0x00000008;
+
+    fn cvt_ssize(t: ssize_t) -> io::Result<ssize_t> {
+        if t == -1 {
+            Err(io::Error::last_os_error())
+        } else {
+            Ok(t)
+        }
+    }
+
+    fn preadv2(fd: c_int, iov: &iovec) -> ssize_t {
+        let iovcnt: c_int = 1;
+        let offset: off_t = -1;
+
+        if cfg!(all(target_arch = "x86_64", target_pointer_width = "64")) {
+            unsafe { syscall(SYS_preadv2, fd, iov, iovcnt, offset, 0 as off_t, RWF_NOWAIT) }
+        } else if cfg!(all(target_arch = "x86_64", target_pointer_width = "32")) {
+            unsafe { syscall(SYS_preadv2, fd, iov, iovcnt, offset, RWF_NOWAIT) }
+        } else {
+            unsafe {
+                syscall(
+                    SYS_preadv2,
+                    fd,
+                    iov,
+                    iovcnt,
+                    offset as libc::c_long,
+                    ((offset as u64) >> 32) as libc::c_long,
+                    RWF_NOWAIT,
+                )
+            }
+        }
+        .try_into()
+        .unwrap()
+    }
+
+    pub fn non_blocking_read(fd: c_int, buf: &mut [u8]) -> io::Result<usize> {
+        static IS_NONBLOCKING_READ_UNSUPPORTED: AtomicBool = AtomicBool::new(false);
+
+        if IS_NONBLOCKING_READ_UNSUPPORTED.load(Ordering::Relaxed) {
+            return Err(io::ErrorKind::Unsupported.into());
+        }
+
+        match cvt_ssize(preadv2(
+            fd,
+            &iovec {
+                iov_base: buf.as_ptr() as *mut _,
+                iov_len: buf.len(),
+            },
+        )) {
+            Ok(cnt) => Ok(cnt.try_into().unwrap()),
+            Err(err) if matches!(err.raw_os_error(), Some(libc::EOPNOTSUPP | libc::ENOSYS)) => {
+                IS_NONBLOCKING_READ_UNSUPPORTED.store(true, Ordering::Relaxed);
+                Err(io::ErrorKind::Unsupported.into())
+            }
+            Err(err) => Err(err),
+        }
+    }
+}
+
 #[derive(Debug)]
 pub struct Helper {
     thread: JoinHandle<()>,