Revert "io: allocate copy buffers from a pool"

This reverts CL 456555.

Reason for revert: This seems too likely to exercise race conditions
in code where a Write call continues to access its buffer after
returning. The HTTP/2 ResponseWriter is one such example.

Reverting this change while we think about this some more.

For #57202

Change-Id: Ic86823f81d7da410ea6b3f17fb5b3f9a979e3340
Reviewed-on: https://go-review.googlesource.com/c/go/+/467095
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/io/io.go

@@ -400,13 +400,6 @@ func CopyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 	return copyBuffer(dst, src, buf)
 }
 
-var bufPool = sync.Pool{
-	New: func() any {
-		b := make([]byte, 32*1024)
-		return &b
-	},
-}
-
 // copyBuffer is the actual implementation of Copy and CopyBuffer.
 // if buf is nil, one is allocated.
 func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
@@ -420,9 +413,15 @@ func copyBuffer(dst Writer, src Reader, buf []byte) (written int64, err error) {
 		return rt.ReadFrom(src)
 	}
 	if buf == nil {
-		bufp := bufPool.Get().(*[]byte)
-		defer bufPool.Put(bufp)
-		buf = *bufp
+		size := 32 * 1024
+		if l, ok := src.(*LimitedReader); ok && int64(size) > l.N {
+			if l.N < 1 {
+				size = 1
+			} else {
+				size = int(l.N)
+			}
+		}
+		buf = make([]byte, size)
 	}
 	for {
 		nr, er := src.Read(buf)
@@ -638,14 +637,21 @@ func (discard) WriteString(s string) (int, error) {
 	return len(s), nil
 }
 
+var blackHolePool = sync.Pool{
+	New: func() any {
+		b := make([]byte, 8192)
+		return &b
+	},
+}
+
 func (discard) ReadFrom(r Reader) (n int64, err error) {
-	bufp := bufPool.Get().(*[]byte)
+	bufp := blackHolePool.Get().(*[]byte)
 	readSize := 0
 	for {
 		readSize, err = r.Read(*bufp)
 		n += int64(readSize)
 		if err != nil {
-			bufPool.Put(bufp)
+			blackHolePool.Put(bufp)
 			if err == EOF {
 				return n, nil
 			}

src/net/http/server.go

@@ -567,12 +567,16 @@ type writerOnly struct {
 // to a *net.TCPConn with sendfile, or from a supported src type such
 // as a *net.TCPConn on Linux with splice.
 func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
+	bufp := copyBufPool.Get().(*[]byte)
+	buf := *bufp
+	defer copyBufPool.Put(bufp)
+
 	// Our underlying w.conn.rwc is usually a *TCPConn (with its
 	// own ReadFrom method). If not, just fall back to the normal
 	// copy method.
 	rf, ok := w.conn.rwc.(io.ReaderFrom)
 	if !ok {
-		return io.Copy(writerOnly{w}, src)
+		return io.CopyBuffer(writerOnly{w}, src, buf)
 	}
 
 	// Copy the first sniffLen bytes before switching to ReadFrom.
@@ -580,7 +584,7 @@ func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
 	// source is available (see golang.org/issue/5660) and provides
 	// enough bytes to perform Content-Type sniffing when required.
 	if !w.cw.wroteHeader {
-		n0, err := io.Copy(writerOnly{w}, io.LimitReader(src, sniffLen))
+		n0, err := io.CopyBuffer(writerOnly{w}, io.LimitReader(src, sniffLen), buf)
 		n += n0
 		if err != nil || n0 < sniffLen {
 			return n, err
@@ -598,7 +602,7 @@ func (w *response) ReadFrom(src io.Reader) (n int64, err error) {
 		return n, err
 	}
 
-	n0, err := io.Copy(writerOnly{w}, src)
+	n0, err := io.CopyBuffer(writerOnly{w}, src, buf)
 	n += n0
 	return n, err
 }
@@ -795,6 +799,13 @@ var (
 	bufioWriter4kPool sync.Pool
 )
 
+var copyBufPool = sync.Pool{
+	New: func() any {
+		b := make([]byte, 32*1024)
+		return &b
+	},
+}
+
 func bufioWriterPool(size int) *sync.Pool {
 	switch size {
 	case 2 << 10: