std: Implement `LineWriter::write_vectored`

This commit implements the `write_vectored` method of the `LineWriter`
type. First discovered in bytecodealliance/wasmtime#629 the
`write_vectored` method of `Stdout` bottoms out here but only ends up
writing the first buffer due to the default implementation of
`write_vectored`.

Like `BufWriter`, however, `LineWriter` can have a non-default
implementation of `write_vectored` which tries to preserve the
vectored-ness as much as possible. Namely we can have a vectored write
for everything before the newline and everything after the newline if
all the stars align well.

Also like `BufWriter`, though, special care is taken to ensure that
whenever bytes are written we're sure to signal success since that
represents a "commit" of writing bytes.

src/libstd/io/buffered.rs

@@ -989,6 +989,68 @@ impl<W: Write> Write for LineWriter<W> {
         }
     }
 
+    // Vectored writes are very similar to the writes above, but adjusted for
+    // the list of buffers that we have to write.
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        if self.need_flush {
+            self.flush()?;
+        }
+
+        // Find the last newline, and failing that write the whole buffer
+        let last_newline = bufs
+            .iter()
+            .enumerate()
+            .rev()
+            .filter_map(|(i, buf)| {
+                let pos = memchr::memrchr(b'\n', buf)?;
+                Some((i, pos))
+            })
+            .next();
+        let (i, j) = match last_newline {
+            Some(pair) => pair,
+            None => return self.inner.write_vectored(bufs),
+        };
+        let (prefix, suffix) = bufs.split_at(i);
+        let (buf, suffix) = suffix.split_at(1);
+        let buf = &buf[0];
+
+        // Write everything up to the last newline, flushing afterwards. Note
+        // that only if we finished our entire `write_vectored` do we try the
+        // subsequent
+        // `write`
+        let mut n = 0;
+        let prefix_amt = prefix.iter().map(|i| i.len()).sum();
+        if prefix_amt > 0 {
+            n += self.inner.write_vectored(prefix)?;
+            self.need_flush = true;
+        }
+        if n == prefix_amt {
+            match self.inner.write(&buf[..=j]) {
+                Ok(m) => n += m,
+                Err(e) if n == 0 => return Err(e),
+                Err(_) => return Ok(n),
+            }
+            self.need_flush = true;
+        }
+        if self.flush().is_err() || n != j + 1 + prefix_amt {
+            return Ok(n);
+        }
+
+        // ... and now write out everything remaining
+        match self.inner.write(&buf[j + 1..]) {
+            Ok(i) => n += i,
+            Err(_) => return Ok(n),
+        }
+
+        if suffix.iter().map(|s| s.len()).sum::<usize>() == 0 {
+            return Ok(n)
+        }
+        match self.inner.write_vectored(suffix) {
+            Ok(i) => Ok(n + i),
+            Err(_) => Ok(n),
+        }
+    }
+
     fn flush(&mut self) -> io::Result<()> {
         self.inner.flush()?;
         self.need_flush = false;
@@ -1015,7 +1077,7 @@ where
 #[cfg(test)]
 mod tests {
     use crate::io::prelude::*;
-    use crate::io::{self, BufReader, BufWriter, LineWriter, SeekFrom};
+    use crate::io::{self, BufReader, BufWriter, LineWriter, SeekFrom, IoSlice};
     use crate::sync::atomic::{AtomicUsize, Ordering};
     use crate::thread;
 
@@ -1483,4 +1545,112 @@ mod tests {
 
         assert_eq!(l.write(b"a").unwrap_err().kind(), io::ErrorKind::Other)
     }
+
+    #[test]
+    fn line_vectored() {
+        let mut a = LineWriter::new(Vec::new());
+        assert_eq!(
+            a.write_vectored(&[
+                IoSlice::new(&[]),
+                IoSlice::new(b"\n"),
+                IoSlice::new(&[]),
+                IoSlice::new(b"a"),
+            ])
+            .unwrap(),
+            2,
+        );
+        assert_eq!(a.get_ref(), b"\n");
+
+        assert_eq!(
+            a.write_vectored(&[
+                IoSlice::new(&[]),
+                IoSlice::new(b"b"),
+                IoSlice::new(&[]),
+                IoSlice::new(b"a"),
+                IoSlice::new(&[]),
+                IoSlice::new(b"c"),
+            ])
+            .unwrap(),
+            3,
+        );
+        assert_eq!(a.get_ref(), b"\n");
+        a.flush().unwrap();
+        assert_eq!(a.get_ref(), b"\nabac");
+        assert_eq!(a.write_vectored(&[]).unwrap(), 0);
+        assert_eq!(
+            a.write_vectored(&[
+                IoSlice::new(&[]),
+                IoSlice::new(&[]),
+                IoSlice::new(&[]),
+                IoSlice::new(&[]),
+            ])
+            .unwrap(),
+            0,
+        );
+        assert_eq!(a.write_vectored(&[IoSlice::new(b"a\nb"),]).unwrap(), 3);
+        assert_eq!(a.get_ref(), b"\nabaca\n");
+    }
+
+    #[test]
+    fn line_vectored_partial_and_errors() {
+        enum Call {
+            Write { inputs: Vec<&'static [u8]>, output: io::Result<usize> },
+            Flush { output: io::Result<()> },
+        }
+        struct Writer {
+            calls: Vec<Call>,
+        }
+
+        impl Write for Writer {
+            fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+                self.write_vectored(&[IoSlice::new(buf)])
+            }
+
+            fn write_vectored(&mut self, buf: &[IoSlice<'_>]) -> io::Result<usize> {
+                match self.calls.pop().unwrap() {
+                    Call::Write { inputs, output } => {
+                        assert_eq!(inputs, buf.iter().map(|b| &**b).collect::<Vec<_>>());
+                        output
+                    }
+                    _ => panic!("unexpected call to write"),
+                }
+            }
+
+            fn flush(&mut self) -> io::Result<()> {
+                match self.calls.pop().unwrap() {
+                    Call::Flush { output } => output,
+                    _ => panic!("unexpected call to flush"),
+                }
+            }
+        }
+
+        impl Drop for Writer {
+            fn drop(&mut self) {
+                if !thread::panicking() {
+                    assert_eq!(self.calls.len(), 0);
+                }
+            }
+        }
+
+        // partial writes keep going
+        let mut a = LineWriter::new(Writer { calls: Vec::new() });
+        a.write_vectored(&[IoSlice::new(&[]), IoSlice::new(b"abc")]).unwrap();
+        a.get_mut().calls.push(Call::Flush { output: Ok(()) });
+        a.get_mut().calls.push(Call::Write { inputs: vec![b"bcx\n"], output: Ok(4) });
+        a.get_mut().calls.push(Call::Write { inputs: vec![b"abcx\n"], output: Ok(1) });
+        a.write_vectored(&[IoSlice::new(b"x"), IoSlice::new(b"\n")]).unwrap();
+        a.get_mut().calls.push(Call::Flush { output: Ok(()) });
+        a.flush().unwrap();
+
+        // erroneous writes stop and don't write more
+        a.get_mut().calls.push(Call::Write { inputs: vec![b"x\n"], output: Err(err()) });
+        assert_eq!(a.write_vectored(&[IoSlice::new(b"x"), IoSlice::new(b"\na")]).unwrap(), 2);
+        a.get_mut().calls.push(Call::Flush { output: Ok(()) });
+        a.get_mut().calls.push(Call::Write { inputs: vec![b"x\n"], output: Ok(2) });
+        a.flush().unwrap();
+
+        fn err() -> io::Error {
+            io::Error::new(io::ErrorKind::Other, "x")
+        }
+    }
 }