Auto merge of rust-lang#112330 - the8472:use-buf-reader-buffer, r=thomcc

Extend io::copy buffer reuse to BufReader too

previously it was only able to use BufWriter. This was due to a limitation in the BufReader generics that prevented specialization. This change works around the issue by using `BufReader where Self: Read` instead of `BufReader<I> where I: Read`. This limits our options, e.g. we can't access the inner reader, but it happens to work out if we rely on some implementation details.

Copying 1MiB from `/dev/zero` to `/dev/null` through a 256KiB BufReader yields following improvements

```
OLD:
    io::copy::tests::bench_copy_buf_reader  51.44µs/iter +/- 703.00ns
NEW:
    io::copy::tests::bench_copy_buf_reader  18.55µs/iter +/- 237.00ns
```

Previously this would read 256KiB into the reader but then copy 8KiB chunks to the writer through an additional intermediate buffer inside `io::copy`. Since those devices don't do much work most of the speedup should come from fewer syscalls and avoided memcopies.

The b3sum crate [notes that the default buffer size in io::copy is too small](https://github.com/BLAKE3-team/BLAKE3/blob/4108923f5284e0f8c3cf97b59041c2b6b2f601d3/b3sum/src/main.rs#L235-L239). With this optimization they could achieve the desired performance by wrapping the reader in a `BufReader` instead of handrolling it.

Currently the optimization doesn't apply to things like `StdinLock`, but this can be addressed with an additional `AsMutBufReader` specialization trait.

library/std/src/io/buffered/bufreader.rs

@@ -222,7 +222,7 @@ impl<R: ?Sized> BufReader<R> {
 
     /// Invalidates all data in the internal buffer.
     #[inline]
-    fn discard_buffer(&mut self) {
+    pub(in crate::io) fn discard_buffer(&mut self) {
         self.buf.discard_buffer()
     }
 }

library/std/src/io/copy.rs

@@ -1,6 +1,9 @@
-use super::{BorrowedBuf, BufWriter, ErrorKind, Read, Result, Write, DEFAULT_BUF_SIZE};
+use super::{BorrowedBuf, BufReader, BufWriter, ErrorKind, Read, Result, Write, DEFAULT_BUF_SIZE};
 use crate::mem::MaybeUninit;
 
+#[cfg(test)]
+mod tests;
+
 /// Copies the entire contents of a reader into a writer.
 ///
 /// This function will continuously read data from `reader` and then
@@ -71,32 +74,113 @@ where
     R: Read,
     W: Write,
 {
-    BufferedCopySpec::copy_to(reader, writer)
+    let read_buf = BufferedReaderSpec::buffer_size(reader);
+    let write_buf = BufferedWriterSpec::buffer_size(writer);
+
+    if read_buf >= DEFAULT_BUF_SIZE && read_buf >= write_buf {
+        return BufferedReaderSpec::copy_to(reader, writer);
+    }
+
+    BufferedWriterSpec::copy_from(writer, reader)
+}
+
+/// Specialization of the read-write loop that reuses the internal
+/// buffer of a BufReader. If there's no buffer then the writer side
+/// should be used intead.
+trait BufferedReaderSpec {
+    fn buffer_size(&self) -> usize;
+
+    fn copy_to(&mut self, to: &mut (impl Write + ?Sized)) -> Result<u64>;
+}
+
+impl<T> BufferedReaderSpec for T
+where
+    Self: Read,
+    T: ?Sized,
+{
+    #[inline]
+    default fn buffer_size(&self) -> usize {
+        0
+    }
+
+    default fn copy_to(&mut self, _to: &mut (impl Write + ?Sized)) -> Result<u64> {
+        unimplemented!("only called from specializations");
+    }
+}
+
+impl<I> BufferedReaderSpec for BufReader<I>
+where
+    Self: Read,
+    I: ?Sized,
+{
+    fn buffer_size(&self) -> usize {
+        self.capacity()
+    }
+
+    fn copy_to(&mut self, to: &mut (impl Write + ?Sized)) -> Result<u64> {
+        let mut len = 0;
+
+        loop {
+            // Hack: this relies on `impl Read for BufReader` always calling fill_buf
+            // if the buffer is empty, even for empty slices.
+            // It can't be called directly here since specialization prevents us
+            // from adding I: Read
+            match self.read(&mut []) {
+                Ok(_) => {}
+                Err(e) if e.kind() == ErrorKind::Interrupted => continue,
+                Err(e) => return Err(e),
+            }
+            let buf = self.buffer();
+            if self.buffer().len() == 0 {
+                return Ok(len);
+            }
+
+            // In case the writer side is a BufWriter then its write_all
+            // implements an optimization that passes through large
+            // buffers to the underlying writer. That code path is #[cold]
+            // but we're still avoiding redundant memcopies when doing
+            // a copy between buffered inputs and outputs.
+            to.write_all(buf)?;
+            len += buf.len() as u64;
+            self.discard_buffer();
+        }
+    }
 }
 
 /// Specialization of the read-write loop that either uses a stack buffer
 /// or reuses the internal buffer of a BufWriter
-trait BufferedCopySpec: Write {
-    fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64>;
+trait BufferedWriterSpec: Write {
+    fn buffer_size(&self) -> usize;
+
+    fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64>;
 }
 
-impl<W: Write + ?Sized> BufferedCopySpec for W {
-    default fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
-        stack_buffer_copy(reader, writer)
+impl<W: Write + ?Sized> BufferedWriterSpec for W {
+    #[inline]
+    default fn buffer_size(&self) -> usize {
+        0
+    }
+
+    default fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64> {
+        stack_buffer_copy(reader, self)
     }
 }
 
-impl<I: ?Sized + Write> BufferedCopySpec for BufWriter<I> {
-    fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
-        if writer.capacity() < DEFAULT_BUF_SIZE {
-            return stack_buffer_copy(reader, writer);
+impl<I: Write + ?Sized> BufferedWriterSpec for BufWriter<I> {
+    fn buffer_size(&self) -> usize {
+        self.capacity()
+    }
+
+    fn copy_from<R: Read + ?Sized>(&mut self, reader: &mut R) -> Result<u64> {
+        if self.capacity() < DEFAULT_BUF_SIZE {
+            return stack_buffer_copy(reader, self);
         }
 
         let mut len = 0;
         let mut init = 0;
 
         loop {
-            let buf = writer.buffer_mut();
+            let buf = self.buffer_mut();
             let mut read_buf: BorrowedBuf<'_> = buf.spare_capacity_mut().into();
 
             unsafe {
@@ -127,7 +211,7 @@ impl<I: ?Sized + Write> BufferedCopySpec for BufWriter<I> {
                     Err(e) => return Err(e),
                 }
             } else {
-                writer.flush_buf()?;
+                self.flush_buf()?;
                 init = 0;
             }
         }

library/std/src/io/copy/tests.rs

@@ -0,0 +1,108 @@
+use crate::cmp::{max, min};
+use crate::io::*;
+
+#[test]
+fn copy_copies() {
+    let mut r = repeat(0).take(4);
+    let mut w = sink();
+    assert_eq!(copy(&mut r, &mut w).unwrap(), 4);
+
+    let mut r = repeat(0).take(1 << 17);
+    assert_eq!(copy(&mut r as &mut dyn Read, &mut w as &mut dyn Write).unwrap(), 1 << 17);
+}
+
+struct ShortReader {
+    cap: usize,
+    read_size: usize,
+    observed_buffer: usize,
+}
+
+impl Read for ShortReader {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        let bytes = min(self.cap, self.read_size);
+        self.cap -= bytes;
+        self.observed_buffer = max(self.observed_buffer, buf.len());
+        Ok(bytes)
+    }
+}
+
+struct WriteObserver {
+    observed_buffer: usize,
+}
+
+impl Write for WriteObserver {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        self.observed_buffer = max(self.observed_buffer, buf.len());
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> Result<()> {
+        Ok(())
+    }
+}
+
+#[test]
+fn copy_specializes_bufwriter() {
+    let cap = 117 * 1024;
+    let buf_sz = 16 * 1024;
+    let mut r = ShortReader { cap, observed_buffer: 0, read_size: 1337 };
+    let mut w = BufWriter::with_capacity(buf_sz, WriteObserver { observed_buffer: 0 });
+    assert_eq!(
+        copy(&mut r, &mut w).unwrap(),
+        cap as u64,
+        "expected the whole capacity to be copied"
+    );
+    assert_eq!(r.observed_buffer, buf_sz, "expected a large buffer to be provided to the reader");
+    assert!(w.get_mut().observed_buffer > DEFAULT_BUF_SIZE, "expected coalesced writes");
+}
+
+#[test]
+fn copy_specializes_bufreader() {
+    let mut source = vec![0; 768 * 1024];
+    source[1] = 42;
+    let mut buffered = BufReader::with_capacity(256 * 1024, Cursor::new(&mut source));
+
+    let mut sink = Vec::new();
+    assert_eq!(crate::io::copy(&mut buffered, &mut sink).unwrap(), source.len() as u64);
+    assert_eq!(source.as_slice(), sink.as_slice());
+
+    let buf_sz = 71 * 1024;
+    assert!(buf_sz > DEFAULT_BUF_SIZE, "test precondition");
+
+    let mut buffered = BufReader::with_capacity(buf_sz, Cursor::new(&mut source));
+    let mut sink = WriteObserver { observed_buffer: 0 };
+    assert_eq!(crate::io::copy(&mut buffered, &mut sink).unwrap(), source.len() as u64);
+    assert_eq!(
+        sink.observed_buffer, buf_sz,
+        "expected a large buffer to be provided to the writer"
+    );
+}
+
+#[cfg(unix)]
+mod io_benches {
+    use crate::fs::File;
+    use crate::fs::OpenOptions;
+    use crate::io::prelude::*;
+    use crate::io::BufReader;
+
+    use test::Bencher;
+
+    #[bench]
+    fn bench_copy_buf_reader(b: &mut Bencher) {
+        let mut file_in = File::open("/dev/zero").expect("opening /dev/zero failed");
+        // use dyn to avoid specializations unrelated to readbuf
+        let dyn_in = &mut file_in as &mut dyn Read;
+        let mut reader = BufReader::with_capacity(256 * 1024, dyn_in.take(0));
+        let mut writer =
+            OpenOptions::new().write(true).open("/dev/null").expect("opening /dev/null failed");
+
+        const BYTES: u64 = 1024 * 1024;
+
+        b.bytes = BYTES;
+
+        b.iter(|| {
+            reader.get_mut().set_limit(BYTES);
+            crate::io::copy(&mut reader, &mut writer).unwrap()
+        });
+    }
+}

library/std/src/io/util/tests.rs

@@ -1,67 +1,8 @@
-use crate::cmp::{max, min};
 use crate::io::prelude::*;
-use crate::io::{
-    copy, empty, repeat, sink, BorrowedBuf, BufWriter, Empty, Repeat, Result, SeekFrom, Sink,
-    DEFAULT_BUF_SIZE,
-};
+use crate::io::{empty, repeat, sink, BorrowedBuf, Empty, Repeat, SeekFrom, Sink};
 
 use crate::mem::MaybeUninit;
 
-#[test]
-fn copy_copies() {
-    let mut r = repeat(0).take(4);
-    let mut w = sink();
-    assert_eq!(copy(&mut r, &mut w).unwrap(), 4);
-
-    let mut r = repeat(0).take(1 << 17);
-    assert_eq!(copy(&mut r as &mut dyn Read, &mut w as &mut dyn Write).unwrap(), 1 << 17);
-}
-
-struct ShortReader {
-    cap: usize,
-    read_size: usize,
-    observed_buffer: usize,
-}
-
-impl Read for ShortReader {
-    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
-        let bytes = min(self.cap, self.read_size);
-        self.cap -= bytes;
-        self.observed_buffer = max(self.observed_buffer, buf.len());
-        Ok(bytes)
-    }
-}
-
-struct WriteObserver {
-    observed_buffer: usize,
-}
-
-impl Write for WriteObserver {
-    fn write(&mut self, buf: &[u8]) -> Result<usize> {
-        self.observed_buffer = max(self.observed_buffer, buf.len());
-        Ok(buf.len())
-    }
-
-    fn flush(&mut self) -> Result<()> {
-        Ok(())
-    }
-}
-
-#[test]
-fn copy_specializes_bufwriter() {
-    let cap = 117 * 1024;
-    let buf_sz = 16 * 1024;
-    let mut r = ShortReader { cap, observed_buffer: 0, read_size: 1337 };
-    let mut w = BufWriter::with_capacity(buf_sz, WriteObserver { observed_buffer: 0 });
-    assert_eq!(
-        copy(&mut r, &mut w).unwrap(),
-        cap as u64,
-        "expected the whole capacity to be copied"
-    );
-    assert_eq!(r.observed_buffer, buf_sz, "expected a large buffer to be provided to the reader");
-    assert!(w.get_mut().observed_buffer > DEFAULT_BUF_SIZE, "expected coalesced writes");
-}
-
 #[test]
 fn sink_sinks() {
     let mut s = sink();