File: Try doing a non-blocking read before punting to the threadpool

...on Linux.

If the data is already available in cache this will avoid cross-thread
interaction and remove a copy.  It should help with latency too as reads
that can be satisfied now won't need to wait in queue until other fs
operations are complete.

tokio/Cargo.toml

@@ -43,7 +43,7 @@ full = [
   "time",
 ]
 
-fs = []
+fs = ["libc"]
 io-util = ["memchr", "bytes"]
 # stdin, stdout, stderr
 io-std = []
@@ -104,11 +104,11 @@ parking_lot = { version = "0.11.0", optional = true }
 tracing = { version = "0.1.21", default-features = false, features = ["std"], optional = true } # Not in full
 
 [target.'cfg(unix)'.dependencies]
-libc = { version = "0.2.42", optional = true }
+libc = { git = "https://github.com/rust-lang/libc", rev = "a61fd8c79c21b387cdf6e4bcabc8b04afe594960", optional = true }
 signal-hook-registry = { version = "1.1.1", optional = true }
 
 [target.'cfg(unix)'.dev-dependencies]
-libc = { version = "0.2.42" }
+libc = { git = "https://github.com/rust-lang/libc", rev = "a61fd8c79c21b387cdf6e4bcabc8b04afe594960" }
 nix = { version = "0.19.0" }
 
 [target.'cfg(windows)'.dependencies.winapi]

tokio/src/fs/file.rs

@@ -499,6 +499,10 @@ impl AsyncRead for File {
                         return Ready(Ok(()));
                     }
 
+                    if let Some(x) = read_nowait::try_nonblocking_read(me.std.as_ref(), dst) {
+                        return Ready(x);
+                    }
+
                     buf.ensure_capacity_for(dst);
                     let std = me.std.clone();
 
@@ -756,3 +760,78 @@ impl Inner {
         }
     }
 }
+
+#[cfg(all(target_os = "linux", not(test)))]
+mod read_nowait {
+    use crate::io::ReadBuf;
+    use libc::{c_int, c_void, iovec, off_t, preadv2};
+    use std::{
+        os::unix::prelude::AsRawFd,
+        sync::atomic::{AtomicBool, Ordering},
+    };
+
+    static NONBLOCKING_READ_SUPPORTED: AtomicBool = AtomicBool::new(true);
+
+    pub(crate) fn try_nonblocking_read(
+        file: &crate::fs::sys::File,
+        dst: &mut ReadBuf<'_>,
+    ) -> Option<std::io::Result<()>> {
+        if !NONBLOCKING_READ_SUPPORTED.load(Ordering::Relaxed) {
+            return None;
+        }
+        let out = preadv2_safe(file, dst, -1, libc::RWF_NOWAIT);
+        if let Err(err) = &out {
+            match err.raw_os_error() {
+                Some(libc::ENOSYS) => {
+                    NONBLOCKING_READ_SUPPORTED.store(false, Ordering::Relaxed);
+                    return None;
+                }
+                Some(libc::ENOTSUP) | Some(libc::EAGAIN) => return None,
+                _ => {}
+            }
+        }
+        Some(out)
+    }
+
+    fn preadv2_safe(
+        file: &crate::fs::sys::File,
+        dst: &mut ReadBuf<'_>,
+        offset: off_t,
+        flags: c_int,
+    ) -> std::io::Result<()> {
+        unsafe {
+            /* We're manually have to defend against buffer overflows here.  The slice API makes
+             * this fairly streightforward. */
+            let unfilled = dst.unfilled_mut();
+            let iov = iovec {
+                iov_base: unfilled.as_mut_ptr() as *mut c_void,
+                iov_len: unfilled.len(),
+            };
+            /* We take a File object rather than an fd as reading from a sensitive fd may confuse
+             * other unsafe code that assumes that only they have access to that fd. */
+            let bytes_read = preadv2(file.as_raw_fd(), &iov as *const iovec, 1, offset, flags);
+            if bytes_read < 0 {
+                Err(std::io::Error::last_os_error())
+            } else {
+                /* preadv2 returns the number of bytes read, e.g. the number of bytes that have
+                 * written into `unfilled`. So it's safe to assume that the data is now
+                 * initialised */
+                dst.assume_init(dst.filled().len() + bytes_read as usize);
+                dst.advance(bytes_read as usize);
+                Ok(())
+            }
+        }
+    }
+}
+
+#[cfg(any(not(target_os = "linux"), test))]
+mod read_nowait {
+    use crate::io::ReadBuf;
+
+    pub(crate) fn try_nonblocking_read(
+        _file: &crate::fs::sys::File,
+        _dst: &mut ReadBuf<'_>,
+    ) -> Option<std::io::Result<()>> {
+        None
+    }
+}