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

1 files changed, 56 insertions, 0 deletions

diff --git a/src/libstd/io/stdio.rs b/src/libstd/io/stdio.rs
index a064c552c84..fd5a1291785 100644
--- a/src/libstd/io/stdio.rs
+++ b/src/libstd/io/stdio.rs
@@ -88,6 +88,11 @@ impl Read for StdinRaw {
     }
 
     #[inline]
+    fn can_read_vectored(&self) -> bool {
+        self.0.can_read_vectored()
+    }
+
+    #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
     }
@@ -101,6 +106,11 @@ impl Write for StdoutRaw {
         self.0.write_vectored(bufs)
     }
 
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        self.0.can_write_vectored()
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         self.0.flush()
     }
@@ -114,6 +124,11 @@ impl Write for StderrRaw {
         self.0.write_vectored(bufs)
     }
 
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        self.0.can_write_vectored()
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         self.0.flush()
     }
@@ -140,6 +155,14 @@ impl<W: io::Write> io::Write for Maybe<W> {
         }
     }
 
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        match self {
+            Maybe::Real(w) => w.can_write_vectored(),
+            Maybe::Fake => true,
+        }
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         match *self {
             Maybe::Real(ref mut w) => handle_ebadf(w.flush(), ()),
@@ -162,6 +185,14 @@ impl<R: io::Read> io::Read for Maybe<R> {
             Maybe::Fake => Ok(0),
         }
     }
+
+    #[inline]
+    fn can_read_vectored(&self) -> bool {
+        match self {
+            Maybe::Real(w) => w.can_read_vectored(),
+            Maybe::Fake => true,
+        }
+    }
 }
 
 fn handle_ebadf<T>(r: io::Result<T>, default: T) -> io::Result<T> {
@@ -352,6 +383,10 @@ impl Read for Stdin {
         self.lock().read_vectored(bufs)
     }
     #[inline]
+    fn can_read_vectored(&self) -> bool {
+        self.lock().can_read_vectored()
+    }
+    #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
     }
@@ -377,6 +412,11 @@ impl Read for StdinLock<'_> {
     }
 
     #[inline]
+    fn can_read_vectored(&self) -> bool {
+        self.inner.can_read_vectored()
+    }
+
+    #[inline]
     unsafe fn initializer(&self) -> Initializer {
         Initializer::nop()
     }
@@ -543,6 +583,10 @@ impl Write for Stdout {
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
         self.lock().write_vectored(bufs)
     }
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        self.lock().can_write_vectored()
+    }
     fn flush(&mut self) -> io::Result<()> {
         self.lock().flush()
     }
@@ -561,6 +605,10 @@ impl Write for StdoutLock<'_> {
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
         self.inner.borrow_mut().write_vectored(bufs)
     }
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        self.inner.borrow_mut().can_write_vectored()
+    }
     fn flush(&mut self) -> io::Result<()> {
         self.inner.borrow_mut().flush()
     }
@@ -709,6 +757,10 @@ impl Write for Stderr {
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
         self.lock().write_vectored(bufs)
     }
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        self.lock().can_write_vectored()
+    }
     fn flush(&mut self) -> io::Result<()> {
         self.lock().flush()
     }
@@ -727,6 +779,10 @@ impl Write for StderrLock<'_> {
     fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
         self.inner.borrow_mut().write_vectored(bufs)
     }
+    #[inline]
+    fn can_write_vectored(&self) -> bool {
+        self.inner.borrow_mut().can_write_vectored()
+    }
     fn flush(&mut self) -> io::Result<()> {
         self.inner.borrow_mut().flush()
     }