Add Read/Write::can_read/write_vectored

When working with an arbitrary reader or writer, code that uses vectored
operations may end up being slower than code that copies into a single
buffer when the underlying reader or writer doesn't actually support
vectored operations. These new methods allow you to ask the reader or
witer up front if vectored operations are efficiently supported.

Currently, you have to use some heuristics to guess by e.g. checking if
the read or write only accessed the first buffer. Hyper is one concrete
example of a library that has to do this dynamically:
https://github.com/hyperium/hyper/blob/0eaf304644a396895a4ce1f0146e596640bb666a/src/proto/h1/io.rs#L582-L594

7 files changed, 91 insertions, 0 deletions

diff --git a/src/libstd/sys/unix/ext/net.rs b/src/libstd/sys/unix/ext/net.rs
index 4c3cb67c9ee..60ec73d9de2 100644
--- a/src/libstd/sys/unix/ext/net.rs
+++ b/src/libstd/sys/unix/ext/net.rs
@@ -614,6 +614,11 @@ impl io::Read for UnixStream {
     }
 
     #[inline]
+    fn can_read_vectored(&self) -> bool {
+        io::Read::can_read_vectored(&&*self)
+    }
+
+    #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
     }
@@ -630,6 +635,11 @@ impl<'a> io::Read for &'a UnixStream {
     }
 
     #[inline]
+    fn can_read_vectored(&self) -> bool {
+        self.0.can_read_vectored()
+    }
+
+    #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
     }
@@ -645,6 +655,11 @@ impl io::Write for UnixStream {
         io::Write::write_vectored(&mut &*self, bufs)
     }
 
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        io::Write::can_write_vectored(&&*self)
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         io::Write::flush(&mut &*self)
     }
@@ -660,6 +675,11 @@ impl<'a> io::Write for &'a UnixStream {
         self.0.write_vectored(bufs)
     }
 
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        self.0.can_write_vectored()
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }
diff --git a/src/libstd/sys/unix/fd.rs b/src/libstd/sys/unix/fd.rs
index 1bba56e334a..7083785f426 100644
--- a/src/libstd/sys/unix/fd.rs
+++ b/src/libstd/sys/unix/fd.rs
@@ -64,6 +64,11 @@ impl FileDesc {
         Ok(ret as usize)
     }
 
+    #[inline]
+    pub fn can_read_vectored(&self) -> bool {
+        true
+    }
+
     pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
         let mut me = self;
         (&mut me).read_to_end(buf)
@@ -116,6 +121,11 @@ impl FileDesc {
         Ok(ret as usize)
     }
 
+    #[inline]
+    pub fn can_write_vectored(&self) -> bool {
+        true
+    }
+
     pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
         #[cfg(target_os = "android")]
         use super::android::cvt_pwrite64;
diff --git a/src/libstd/sys/unix/fs.rs b/src/libstd/sys/unix/fs.rs
index a233aa47dff..1e8bbd4325b 100644
--- a/src/libstd/sys/unix/fs.rs
+++ b/src/libstd/sys/unix/fs.rs
@@ -828,6 +828,11 @@ impl File {
         self.0.read_vectored(bufs)
     }
 
+    #[inline]
+    pub fn can_read_vectored(&self) -> bool {
+        self.0.can_read_vectored()
+    }
+
     pub fn read_at(&self, buf: &mut [u8], offset: u64) -> io::Result<usize> {
         self.0.read_at(buf, offset)
     }
@@ -840,6 +845,11 @@ impl File {
         self.0.write_vectored(bufs)
     }
 
+    #[inline]
+    pub fn can_write_vectored(&self) -> bool {
+        self.0.can_write_vectored()
+    }
+
     pub fn write_at(&self, buf: &[u8], offset: u64) -> io::Result<usize> {
         self.0.write_at(buf, offset)
     }
diff --git a/src/libstd/sys/unix/l4re.rs b/src/libstd/sys/unix/l4re.rs
index c6e4f5693ed..8510e56af37 100644
--- a/src/libstd/sys/unix/l4re.rs
+++ b/src/libstd/sys/unix/l4re.rs
@@ -55,6 +55,10 @@ pub mod net {
             unimpl!();
         }
 
+        pub fn can_read_vectored(&self) -> bool {
+            unimpl!();
+        }
+
         pub fn peek(&self, _: &mut [u8]) -> io::Result<usize> {
             unimpl!();
         }
@@ -75,6 +79,10 @@ pub mod net {
             unimpl!();
         }
 
+        pub fn can_write_vectored(&self) -> bool {
+            unimpl!();
+        }
+
         pub fn set_timeout(&self, _: Option<Duration>, _: libc::c_int) -> io::Result<()> {
             unimpl!();
         }
@@ -171,6 +179,10 @@ pub mod net {
             unimpl!();
         }
 
+        pub fn can_read_vectored(&self) -> bool {
+            unimpl!();
+        }
+
         pub fn write(&self, _: &[u8]) -> io::Result<usize> {
             unimpl!();
         }
@@ -179,6 +191,10 @@ pub mod net {
             unimpl!();
         }
 
+        pub fn can_write_vectored(&self) -> bool {
+            unimpl!();
+        }
+
         pub fn peer_addr(&self) -> io::Result<SocketAddr> {
             unimpl!();
         }
diff --git a/src/libstd/sys/unix/net.rs b/src/libstd/sys/unix/net.rs
index d18c22b0573..ed97ddf333f 100644
--- a/src/libstd/sys/unix/net.rs
+++ b/src/libstd/sys/unix/net.rs
@@ -226,6 +226,11 @@ impl Socket {
         self.0.read_vectored(bufs)
     }
 
+    #[inline]
+    pub fn can_read_vectored(&self) -> bool {
+        self.0.can_read_vectored()
+    }
+
     fn recv_from_with_flags(
         &self,
         buf: &mut [u8],
@@ -263,6 +268,11 @@ impl Socket {
         self.0.write_vectored(bufs)
     }
 
+    #[inline]
+    pub fn can_write_vectored(&self) -> bool {
+        self.0.can_write_vectored()
+    }
+
     pub fn set_timeout(&self, dur: Option<Duration>, kind: libc::c_int) -> io::Result<()> {
         let timeout = match dur {
             Some(dur) => {
diff --git a/src/libstd/sys/unix/pipe.rs b/src/libstd/sys/unix/pipe.rs
index 2a861c87801..d8ac36df9ec 100644
--- a/src/libstd/sys/unix/pipe.rs
+++ b/src/libstd/sys/unix/pipe.rs
@@ -64,6 +64,11 @@ impl AnonPipe {
         self.0.read_vectored(bufs)
     }
 
+    #[inline]
+    pub fn can_read_vectored(&self) -> bool {
+        self.0.can_read_vectored()
+    }
+
     pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
         self.0.write(buf)
     }
@@ -72,6 +77,11 @@ impl AnonPipe {
         self.0.write_vectored(bufs)
     }
 
+    #[inline]
+    pub fn can_write_vectored(&self) -> bool {
+        self.0.can_write_vectored()
+    }
+
     pub fn fd(&self) -> &FileDesc {
         &self.0
     }
diff --git a/src/libstd/sys/unix/stdio.rs b/src/libstd/sys/unix/stdio.rs
index b9c56963885..4026dd4e497 100644
--- a/src/libstd/sys/unix/stdio.rs
+++ b/src/libstd/sys/unix/stdio.rs
@@ -20,6 +20,11 @@ impl io::Read for Stdin {
     fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
         ManuallyDrop::new(FileDesc::new(libc::STDIN_FILENO)).read_vectored(bufs)
     }
+
+    #[inline]
+    fn can_read_vectored(&self) -> bool {
+        true
+    }
 }
 
 impl Stdout {
@@ -37,6 +42,11 @@ impl io::Write for Stdout {
         ManuallyDrop::new(FileDesc::new(libc::STDOUT_FILENO)).write_vectored(bufs)
     }
 
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        true
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }
@@ -57,6 +67,11 @@ impl io::Write for Stderr {
         ManuallyDrop::new(FileDesc::new(libc::STDERR_FILENO)).write_vectored(bufs)
     }
 
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        true
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         Ok(())
     }