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 golang#44815.

Change-Id: I15b3ef0746d0d60fd62041189a9b9df11254dd29
Reviewed-on: https://go-review.googlesource.com/c/go/+/340530
Trust: Damien Neil <[email protected]>
Run-TryBot: Damien Neil <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

src/bufio/bufio.go

@@ -745,26 +745,27 @@ 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 {
 			if err1 := b.Flush(); err1 != nil {
 				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:])

src/bufio/bufio_test.go

@@ -1351,6 +1351,54 @@ func TestWriterReadFromErrNoProgress(t *testing.T) {
 	}
 }
 
+type readFromWriter struct {
+	buf           []byte
+	writeBytes    int
+	readFromBytes int
+}
+
+func (w *readFromWriter) Write(p []byte) (int, error) {
+	w.buf = append(w.buf, p...)
+	w.writeBytes += len(p)
+	return len(p), nil
+}
+
+func (w *readFromWriter) ReadFrom(r io.Reader) (int64, error) {
+	b, err := io.ReadAll(r)
+	w.buf = append(w.buf, b...)
+	w.readFromBytes += len(b)
+	return int64(len(b)), err
+}
+
+// Test that calling (*Writer).ReadFrom with a partially-filled buffer
+// fills the buffer before switching over to ReadFrom.
+func TestWriterReadFromWithBufferedData(t *testing.T) {
+	const bufsize = 16
+
+	input := createTestInput(64)
+	rfw := &readFromWriter{}
+	w := NewWriterSize(rfw, bufsize)
+
+	const writeSize = 8
+	if n, err := w.Write(input[:writeSize]); n != writeSize || err != nil {
+		t.Errorf("w.Write(%v bytes) = %v, %v; want %v, nil", writeSize, n, err, writeSize)
+	}
+	n, err := w.ReadFrom(bytes.NewReader(input[writeSize:]))
+	if wantn := len(input[writeSize:]); int(n) != wantn || err != nil {
+		t.Errorf("io.Copy(w, %v bytes) = %v, %v; want %v, nil", wantn, n, err, wantn)
+	}
+	if err := w.Flush(); err != nil {
+		t.Errorf("w.Flush() = %v, want nil", err)
+	}
+
+	if got, want := rfw.writeBytes, bufsize; got != want {
+		t.Errorf("wrote %v bytes with Write, want %v", got, want)
+	}
+	if got, want := rfw.readFromBytes, len(input)-bufsize; got != want {
+		t.Errorf("wrote %v bytes with ReadFrom, want %v", got, want)
+	}
+}
+
 func TestReadZero(t *testing.T) {
 	for _, size := range []int{100, 2} {
 		t.Run(fmt.Sprintf("bufsize=%d", size), func(t *testing.T) {