synchronize stream creation

SPDY implementations MUST synchronize stream creation (all the way
to writing the frame) as stream IDs **MUST** increase monotonically.
There was already synchronization on ID creation, but if CreateStream
was called from different goroutines, the calls could interleave
before writing the actual frames to the wire, getting out of order.
(This is _incorrect_ and should cause the other side to STREAM_RST.)

We fix this by moving the s.nextIdLock.Lock() out into the
CreateStream call itself. This way, the frame will be written
before letting the next stream grab the ID and write the frame.
There are probably better ways of doing this, but this minimizes
changes.

License: Apache 2
Signed-off-by: Juan Batiz-Benet <[email protected]>

connection.go

@@ -593,6 +593,11 @@ func (s *Connection) remoteStreamFinish(stream *Stream) {
 // the stream Wait or WaitTimeout function on the stream returned
 // by this function.
 func (s *Connection) CreateStream(headers http.Header, parent *Stream, fin bool) (*Stream, error) {
+	// MUST synchronize stream creation (all the way to writing the frame)
+	// as stream IDs **MUST** increase monotonically.
+	s.nextIdLock.Lock()
+	defer s.nextIdLock.Unlock()
+
 	streamId := s.getNextStreamId()
 	if streamId == 0 {
 		return nil, fmt.Errorf("Unable to get new stream id")
@@ -833,8 +838,6 @@ func (s *Connection) sendStream(stream *Stream, fin bool) error {
 // getNextStreamId returns the next sequential id
 // every call should produce a unique value or an error
 func (s *Connection) getNextStreamId() spdy.StreamId {
-	s.nextIdLock.Lock()
-	defer s.nextIdLock.Unlock()
 	sid := s.nextStreamId
 	if sid > 0x7fffffff {
 		return 0