Add back preadv2 optimization for try_acquire on Linux

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

src/unix.rs

@@ -285,6 +285,33 @@ 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.
+        //
+        // TODO: musl libc supports preadv2 since 1.2.5, but `libc` crate
+        // hasn't yet added it.
+        #[cfg(target_os = "linux")]
+        {
+            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 +395,82 @@ impl Client {
     }
 }
 
+// This should be available for all linux targets,
+// though only [`non_blocking_read`] currently uses it so adding gnu cfg.
+#[cfg(target_os = "linux")]
+mod linux {
+    use super::*;
+
+    use libc::{iovec, off_t, ssize_t, syscall, SYS_preadv2};
+
+    fn cvt_ssize(t: ssize_t) -> io::Result<ssize_t> {
+        if t == -1 {
+            Err(io::Error::last_os_error())
+        } else {
+            Ok(t)
+        }
+    }
+
+    unsafe fn preadv2(
+        fd: c_int,
+        iov: *const iovec,
+        iovcnt: c_int,
+        offset: off_t,
+        flags: c_int,
+    ) -> ssize_t {
+        #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
+        let res = syscall(SYS_preadv2, fd, iov, iovcnt, offset, 0, flags);
+
+        #[cfg(all(target_arch = "x86_64", target_pointer_width = "32"))]
+        let res = syscall(SYS_preadv2, fd, iov, iovcnt, offset, 0, flags);
+
+        #[cfg(not(target_arch = "x86_64"))]
+        let res = syscall(
+            SYS_preadv2,
+            fd,
+            iov,
+            iovcnt,
+            offset as libc::c_long,
+            ((offset as u64) >> 32) as libc::c_long,
+            flags,
+        );
+
+        res.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(unsafe {
+            preadv2(
+                fd,
+                &iovec {
+                    iov_base: buf.as_ptr() as *mut _,
+                    iov_len: buf.len(),
+                },
+                1,
+                -1,
+                libc::RWF_NOWAIT,
+            )
+        }) {
+            Ok(cnt) => Ok(cnt.try_into().unwrap()),
+            Err(err) if err.raw_os_error() == Some(libc::EOPNOTSUPP) => {
+                IS_NONBLOCKING_READ_UNSUPPORTED.store(true, Ordering::Relaxed);
+                Err(io::ErrorKind::Unsupported.into())
+            }
+            Err(err) if err.kind() == io::ErrorKind::Unsupported => {
+                IS_NONBLOCKING_READ_UNSUPPORTED.store(true, Ordering::Relaxed);
+                Err(err)
+            }
+            Err(err) => Err(err),
+        }
+    }
+}
+
 #[derive(Debug)]
 pub struct Helper {
     thread: JoinHandle<()>,