add comments around resendMutex logic

Author: hlibman-connamara

in_session.go

@@ -230,6 +230,8 @@ func (state inSession) resendMessages(session *session, beginSeqNo, endSeqNo int
 		return state.generateSequenceReset(session, beginSeqNo, endSeqNo+1, inReplyTo)
 	}
 
+	// resendMutex must always be locked before sendMutex to prevent a potential deadlock
+	// sendMutex is locked below in session.EnqueueBytesAndSend()
 	session.resendMutex.Lock()
 	defer session.resendMutex.Unlock()
 

session.go

@@ -40,7 +40,9 @@ type session struct {
 	toSend [][]byte
 
 	// Mutex for access to toSend.
-	sendMutex   sync.Mutex
+	sendMutex sync.Mutex
+	// Mutex to prevent messages being sent when resendRequest is active
+	// Must be locked before sendMutex to prevent a potential deadlock
 	resendMutex sync.RWMutex
 
 	sessionEvent chan internal.Event
@@ -303,6 +305,7 @@ func (s *session) sendInReplyTo(msg *Message, inReplyTo *Message) error {
 		return s.queueForSend(msg)
 	}
 
+	// resendMutex must always be locked before sendMutex to prevent a potential deadlock
 	s.resendMutex.RLock()
 	defer s.resendMutex.RUnlock()