Revert "server: BroadcastMessage will no longer block until completion"

This reverts commit 6db90ef09ab974df0ff09aeaf75a3d80414d4f50.

The root cause was fixed by commit
f4e7c36c80e8e2be9edb78b3b317c69d28d6d78f. As a result, this commit is no
longer needed.

server.go

@@ -818,7 +818,9 @@ func (s *server) establishPersistentConnections() error {
 // messages to all peers other than the one specified by the `skip` parameter.
 //
 // NOTE: This function is safe for concurrent access.
-func (s *server) BroadcastMessage(skip *btcec.PublicKey, msgs ...lnwire.Message) error {
+func (s *server) BroadcastMessage(skip *btcec.PublicKey,
+	msgs ...lnwire.Message) error {
+
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
@@ -829,7 +831,9 @@ func (s *server) BroadcastMessage(skip *btcec.PublicKey, msgs ...lnwire.Message)
 // peers except the one specified by `skip`.
 //
 // NOTE: This method MUST be called while the server's mutex is locked.
-func (s *server) broadcastMessages(skip *btcec.PublicKey, msgs []lnwire.Message) error {
+func (s *server) broadcastMessages(
+	skip *btcec.PublicKey,
+	msgs []lnwire.Message) error {
 
 	srvrLog.Debugf("Broadcasting %v messages", len(msgs))
 
@@ -837,15 +841,23 @@ func (s *server) broadcastMessages(skip *btcec.PublicKey, msgs []lnwire.Message)
 	// all messages to each of peers.  We synchronize access to peersByPub
 	// throughout this process to ensure we deliver messages to exact set
 	// of peers present at the time of invocation.
+	var wg sync.WaitGroup
 	for pubStr, sPeer := range s.peersByPub {
 		if skip != nil && sPeer.addr.IdentityKey.IsEqual(skip) {
 			srvrLog.Debugf("Skipping %v in broadcast", pubStr)
 			continue
 		}
 
-		go s.sendPeerMessages(sPeer, msgs)
+		// Dispatch a go routine to enqueue all messages to this peer.
+		wg.Add(1)
+		s.wg.Add(1)
+		go s.sendPeerMessages(sPeer, msgs, &wg)
 	}
 
+	// Wait for all messages to have been dispatched before returning to
+	// caller.
+	wg.Wait()
+
 	return nil
 }
 
@@ -854,7 +866,9 @@ func (s *server) broadcastMessages(skip *btcec.PublicKey, msgs []lnwire.Message)
 // method will return a non-nil error.
 //
 // NOTE: This function is safe for concurrent access.
-func (s *server) SendToPeer(target *btcec.PublicKey, msgs ...lnwire.Message) error {
+func (s *server) SendToPeer(target *btcec.PublicKey,
+	msgs ...lnwire.Message) error {
+
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
@@ -865,7 +879,8 @@ func (s *server) SendToPeer(target *btcec.PublicKey, msgs ...lnwire.Message) err
 // particular peer comes online.
 //
 // NOTE: This function is safe for concurrent access.
-func (s *server) NotifyWhenOnline(peer *btcec.PublicKey, connectedChan chan<- struct{}) {
+func (s *server) NotifyWhenOnline(peer *btcec.PublicKey,
+	connectedChan chan<- struct{}) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
 
@@ -889,7 +904,8 @@ func (s *server) NotifyWhenOnline(peer *btcec.PublicKey, connectedChan chan<- st
 
 // sendToPeer is an internal method that delivers messages to the specified
 // `target` peer.
-func (s *server) sendToPeer(target *btcec.PublicKey, msgs []lnwire.Message) error {
+func (s *server) sendToPeer(target *btcec.PublicKey,
+	msgs []lnwire.Message) error {
 
 	// Compute the target peer's identifier.
 	targetPubBytes := target.SerializeCompressed()
@@ -908,14 +924,37 @@ func (s *server) sendToPeer(target *btcec.PublicKey, msgs []lnwire.Message) erro
 		return err
 	}
 
-	s.sendPeerMessages(targetPeer, msgs)
+	s.sendPeerMessages(targetPeer, msgs, nil)
 
 	return nil
 }
 
 // sendPeerMessages enqueues a list of messages into the outgoingQueue of the
-// `targetPeer`.
-func (s *server) sendPeerMessages(targetPeer *peer, msgs []lnwire.Message) {
+// `targetPeer`.  This method supports additional broadcast-level
+// synchronization by using the additional `wg` to coordinate a particular
+// broadcast.
+//
+// NOTE: This method must be invoked with a non-nil `wg` if it is spawned as a
+// go routine--both `wg` and the server's WaitGroup should be incremented
+// beforehand.  If this method is not spawned as a go routine, the provided
+// `wg` should be nil, and the server's WaitGroup should not be tracking this
+// invocation.
+func (s *server) sendPeerMessages(
+	targetPeer *peer,
+	msgs []lnwire.Message,
+	wg *sync.WaitGroup) {
+
+	// If a WaitGroup is provided, we assume that this method was spawned
+	// as a go routine, and that it is being tracked by both the server's
+	// WaitGroup, as well as the broadcast-level WaitGroup `wg`. In this
+	// event, we defer a call to Done on both WaitGroups to 1) ensure that
+	// server will be able to shutdown after its go routines exit, and 2)
+	// so the server can return to the caller of BroadcastMessage.
+	if wg != nil {
+		defer s.wg.Done()
+		defer wg.Done()
+	}
+
 	for _, msg := range msgs {
 		targetPeer.queueMsg(msg, nil)
 	}