add support for connection migration (#4960)

conn_id_generator.go

@@ -15,7 +15,7 @@ type connRunnerCallbacks struct {
 	ReplaceWithClosed  func([]protocol.ConnectionID, []byte)
 }
 
-type connRunners []connRunnerCallbacks
+type connRunners map[transportID]connRunnerCallbacks
 
 func (cr connRunners) AddConnectionID(id protocol.ConnectionID) {
 	for _, c := range cr {
@@ -55,6 +55,7 @@ type connIDGenerator struct {
 }
 
 func newConnIDGenerator(
+	tID transportID,
 	initialConnectionID protocol.ConnectionID,
 	initialClientDestConnID *protocol.ConnectionID, // nil for the client
 	statelessResetter *statelessResetter,
@@ -66,7 +67,7 @@ func newConnIDGenerator(
 		generator:         generator,
 		activeSrcConnIDs:  make(map[uint64]protocol.ConnectionID),
 		statelessResetter: statelessResetter,
-		connRunners:       []connRunnerCallbacks{connRunner},
+		connRunners:       map[transportID]connRunnerCallbacks{tID: connRunner},
 		queueControlFrame: queueControlFrame,
 	}
 	m.activeSrcConnIDs[0] = initialConnectionID
@@ -162,12 +163,17 @@ func (m *connIDGenerator) ReplaceWithClosed(connClose []byte) {
 	m.connRunners.ReplaceWithClosed(connIDs, connClose)
 }
 
-func (m *connIDGenerator) AddConnRunner(r connRunnerCallbacks) {
+func (m *connIDGenerator) AddConnRunner(id transportID, r connRunnerCallbacks) {
+	// The transport might have already been added earlier.
+	// This happens if the application migrates back to and old path.
+	if _, ok := m.connRunners[id]; ok {
+		return
+	}
+	m.connRunners[id] = r
 	if m.initialClientDestConnID != nil {
 		r.AddConnectionID(*m.initialClientDestConnID)
 	}
 	for _, connID := range m.activeSrcConnIDs {
 		r.AddConnectionID(connID)
 	}
-	m.connRunners = append(m.connRunners, r)
 }