Fix a blocking issue in buffer reader

After sniffing and buffering data, if we try to read from
the socket again, bufio.Reader may block. This breaks HTTP handlers
in go1.5.2+ if one tries on browsers or with curl. Go's HTTP client,
however, is not broken. This issue is also there with TeeReader.

Return immediately with the data in the sniffed buffer.

buffer.go

@@ -16,23 +16,30 @@ type bufferedReader struct {
 	bufferRead int
 	bufferSize int
 	sniffing   bool
+	lastErr    error
 }
 
 func (s *bufferedReader) Read(p []byte) (int, error) {
-	// Functionality of bytes.Reader.
-	bn := copy(p, s.buffer.Bytes()[s.bufferRead:s.bufferSize])
-	s.bufferRead += bn
+	if s.bufferSize > s.bufferRead {
+		// If we have already read something from the buffer before, we return the
+		// same data and the last error if any. We need to immediately return,
+		// otherwise we may block for ever, if we try to be smart and call
+		// source.Read() seeking a little bit of more data.
+		bn := copy(p, s.buffer.Bytes()[s.bufferRead:s.bufferSize])
+		s.bufferRead += bn
+		return bn, s.lastErr
+	}
 
-	p = p[bn:]
-
-	// Funtionality of io.TeeReader.
+	// If there is nothing more to return in the sniffed buffer, read from the
+	// source.
 	sn, sErr := s.source.Read(p)
 	if sn > 0 && s.sniffing {
+		s.lastErr = sErr
 		if wn, wErr := s.buffer.Write(p[:sn]); wErr != nil {
-			return bn + wn, wErr
+			return wn, wErr
 		}
 	}
-	return bn + sn, sErr
+	return sn, sErr
 }
 
 func (s *bufferedReader) reset(snif bool) {

cmux_test.go

@@ -279,7 +279,13 @@ func TestHTTP2(t *testing.T) {
 		t.Fatal(err)
 	}
 	var b [len(http2.ClientPreface)]byte
-	if _, err := muxedConn.Read(b[:]); err != io.EOF {
+	var n int
+	// We have the sniffed buffer first...
+	if n, err = muxedConn.Read(b[:]); err == io.EOF {
+		t.Fatal(err)
+	}
+	// and then we read from the source.
+	if _, err = muxedConn.Read(b[n:]); err != io.EOF {
 		t.Fatal(err)
 	}
 	if string(b[:]) != http2.ClientPreface {