From d5924ef0b4545084d26bff4287976816adb9b944 Mon Sep 17 00:00:00 2001
From: Soheil Hassas Yeganeh <soheil.h.y@gmail.com>
Date: Sun, 24 Apr 2016 12:55:13 -0400
Subject: [PATCH] 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    | 23 +++++++++++++++--------
 cmux_test.go |  8 +++++++-
 2 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/buffer.go b/buffer.go
index 8bb66ff..a487355 100644
--- a/buffer.go
+++ b/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) {
diff --git a/cmux_test.go b/cmux_test.go
index 669b713..0039954 100644
--- a/cmux_test.go
+++ b/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 {