bytes.Buffer: read of 0 bytes at EOF shouldn't be an EOF

This corner case arose doing an RPC with a empty-slice payload. Ouch.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5505073

src/pkg/bytes/buffer.go

@@ -200,13 +200,16 @@ func (b *Buffer) WriteRune(r rune) (n int, err error) {
 
 // Read reads the next len(p) bytes from the buffer or until the buffer
 // is drained.  The return value n is the number of bytes read.  If the
-// buffer has no data to return, err is io.EOF even if len(p) is zero;
+// buffer has no data to return, err is io.EOF (unless len(p) is zero);
 // otherwise it is nil.
 func (b *Buffer) Read(p []byte) (n int, err error) {
 	b.lastRead = opInvalid
 	if b.off >= len(b.buf) {
 		// Buffer is empty, reset to recover space.
 		b.Truncate(0)
+		if len(p) == 0 {
+			return
+		}
 		return 0, io.EOF
 	}
 	n = copy(p, b.buf[b.off:])

src/pkg/bytes/buffer_test.go

@@ -373,3 +373,16 @@ func TestReadBytes(t *testing.T) {
 		}
 	}
 }
+
+// Was a bug: used to give EOF reading empty slice at EOF.
+func TestReadEmptyAtEOF(t *testing.T) {
+	b := new(Buffer)
+	slice := make([]byte, 0)
+	n, err := b.Read(slice)
+	if err != nil {
+		t.Errorf("read error: %v", err)
+	}
+	if n != 0 {
+		t.Errorf("wrong count; got %d want 0", n)
+	}
+}