Block Sends when Resend Request is active

in_session.go

@@ -230,6 +230,11 @@ 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()
+
 	seqNum := beginSeqNo
 	nextSeqNum := seqNum
 	msg := NewMessage()

in_session_test.go

@@ -19,6 +19,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/mock"
 	"github.com/stretchr/testify/suite"
 
 	"github.com/quickfixgo/quickfix/internal"
@@ -373,6 +374,25 @@ func (s *InSessionTestSuite) TestFIXMsgInResendRequestDoNotSendApp() {
 	s.State(inSession{})
 }
 
+func (s *InSessionTestSuite) TestFIXMsgInResendRequestBlocksSend() {
+	s.MockApp.On("ToApp").Return(nil)
+	s.Require().Nil(s.session.send(s.NewOrderSingle()))
+	s.LastToAppMessageSent()
+	s.MockApp.AssertNumberOfCalls(s.T(), "ToApp", 1)
+	s.NextSenderMsgSeqNum(2)
+
+	s.MockStore.On("IterateMessages", mock.Anything, mock.Anything, mock.AnythingOfType("func([]byte) error")).
+		Run(func(_ mock.Arguments) {
+			s.Require().Nil(s.session.send(s.NewOrderSingle()))
+		}).
+		Return(nil)
+
+	s.MockApp.On("FromAdmin").Return(nil)
+	s.fixMsgIn(s.session, s.ResendRequest(1))
+
+	s.NextSenderMsgSeqNum(2)
+}
+
 func (s *InSessionTestSuite) TestFIXMsgInTargetTooLow() {
 	s.IncrNextTargetMsgSeqNum()
 

session.go

@@ -41,6 +41,9 @@ type session struct {
 
 	// Mutex for access to toSend.
 	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
 	messageEvent chan bool
@@ -302,6 +305,10 @@ 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()
+
 	s.sendMutex.Lock()
 	defer s.sendMutex.Unlock()