bufio: use underlying ReadFrom even when data is buffered

When (*bufio.Writer).ReadFrom is called with a partially filled buffer,
fill out and flush the buffer and then call the underlying writer's
ReadFrom method if present.

Fixes #44815.

Change-Id: I15b3ef0746d0d60fd62041189a9b9df11254dd29
Reviewed-on: https://go-review.googlesource.com/c/go/+/340530
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 10 insertions, 9 deletions

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index a58df25494..063a7785f3 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -745,19 +745,14 @@ func (b *Writer) WriteString(s string) (int, error) {
 }
 
 // ReadFrom implements io.ReaderFrom. If the underlying writer
-// supports the ReadFrom method, and b has no buffered data yet,
-// this calls the underlying ReadFrom without buffering.
+// supports the ReadFrom method, this calls the underlying ReadFrom.
+// If there is buffered data and an underlying ReadFrom, this fills
+// the buffer and writes it before calling ReadFrom.
 func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
 	if b.err != nil {
 		return 0, b.err
 	}
-	if b.Buffered() == 0 {
-		if w, ok := b.wr.(io.ReaderFrom); ok {
-			n, err = w.ReadFrom(r)
-			b.err = err
-			return n, err
-		}
-	}
+	readerFrom, readerFromOK := b.wr.(io.ReaderFrom)
 	var m int
 	for {
 		if b.Available() == 0 {
@@ -765,6 +760,12 @@ func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
 				return n, err1
 			}
 		}
+		if readerFromOK && b.Buffered() == 0 {
+			nn, err := readerFrom.ReadFrom(r)
+			b.err = err
+			n += nn
+			return n, err
+		}
 		nr := 0
 		for nr < maxConsecutiveEmptyReads {
 			m, err = r.Read(b.buf[b.n:])