wtclient: make sessionQueueSet thread safe

In preparation for an upcoming commit where multiple threads will have
access to the TowerClient sessionQueueSet, we turn it into a thread safe
struct.

watchtower/wtclient/client.go

@@ -306,7 +306,7 @@ type TowerClient struct {
 	negotiator        SessionNegotiator
 	candidateTowers   TowerCandidateIterator
 	candidateSessions map[wtdb.SessionID]*ClientSession
-	activeSessions    sessionQueueSet
+	activeSessions    *sessionQueueSet
 
 	sessionQueue *sessionQueue
 	prevTask     *wtdb.BackupID
@@ -378,7 +378,7 @@ func New(config *Config) (*TowerClient, error) {
 		log:                  plog,
 		pipeline:             queue,
 		chanCommitHeights:    make(map[lnwire.ChannelID]uint64),
-		activeSessions:       make(sessionQueueSet),
+		activeSessions:       newSessionQueueSet(),
 		summaries:            chanSummaries,
 		closableSessionQueue: newSessionCloseMinHeap(),
 		statTicker:           time.NewTicker(DefaultStatInterval),
@@ -1609,7 +1609,7 @@ func (c *TowerClient) newSessionQueue(s *ClientSession,
 func (c *TowerClient) getOrInitActiveQueue(s *ClientSession,
 	updates []wtdb.CommittedUpdate) *sessionQueue {
 
-	if sq, ok := c.activeSessions[s.ID]; ok {
+	if sq, ok := c.activeSessions.Get(s.ID); ok {
 		return sq
 	}
 
@@ -1628,12 +1628,10 @@ func (c *TowerClient) initActiveQueue(s *ClientSession,
 	sq := c.newSessionQueue(s, updates)
 
 	// Add the session queue as an active session so that we remember to
-	// stop it on shutdown.
-	c.activeSessions.Add(sq)
-
-	// Start the queue so that it can be active in processing newly assigned
-	// tasks or to upload previously committed updates.
-	sq.Start()
+	// stop it on shutdown. This method will also start the queue so that it
+	// can be active in processing newly assigned tasks or to upload
+	// previously committed updates.
+	c.activeSessions.AddAndStart(sq)
 
 	return sq
 }

watchtower/wtclient/session_queue.go

@@ -682,19 +682,63 @@ func (q *sessionQueue) signalUntilShutdown() {
 
 // sessionQueueSet maintains a mapping of SessionIDs to their corresponding
 // sessionQueue.
-type sessionQueueSet map[wtdb.SessionID]*sessionQueue
+type sessionQueueSet struct {
+	queues map[wtdb.SessionID]*sessionQueue
+	mu     sync.Mutex
+}
 
-// Add inserts a sessionQueue into the sessionQueueSet.
-func (s *sessionQueueSet) Add(sessionQueue *sessionQueue) {
-	(*s)[*sessionQueue.ID()] = sessionQueue
+// newSessionQueueSet constructs a new sessionQueueSet.
+func newSessionQueueSet() *sessionQueueSet {
+	return &sessionQueueSet{
+		queues: make(map[wtdb.SessionID]*sessionQueue),
+	}
+}
+
+// AddAndStart inserts a sessionQueue into the sessionQueueSet and starts it.
+func (s *sessionQueueSet) AddAndStart(sessionQueue *sessionQueue) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.queues[*sessionQueue.ID()] = sessionQueue
+
+	sessionQueue.Start()
+}
+
+// StopAndRemove stops the given session queue and removes it from the
+// sessionQueueSet.
+func (s *sessionQueueSet) StopAndRemove(id wtdb.SessionID) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	queue, ok := s.queues[id]
+	if !ok {
+		return
+	}
+
+	queue.Stop()
+
+	delete(s.queues, id)
+}
+
+// Get fetches and returns the sessionQueue with the given ID.
+func (s *sessionQueueSet) Get(id wtdb.SessionID) (*sessionQueue, bool) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	q, ok := s.queues[id]
+
+	return q, ok
 }
 
 // ApplyAndWait executes the nil-adic function returned from getApply for each
 // sessionQueue in the set in parallel, then waits for all of them to finish
 // before returning to the caller.
 func (s *sessionQueueSet) ApplyAndWait(getApply func(*sessionQueue) func()) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
 	var wg sync.WaitGroup
-	for _, sessionq := range *s {
+	for _, sessionq := range s.queues {
 		wg.Add(1)
 		go func(sq *sessionQueue) {
 			defer wg.Done()