autopilot/agent: signal chanOpenFailureUpdates on own channel

We do this to avoid a huge amount of goroutines piling up when autopilot
is trying to open many channels, as they will all block trying to send
the update on the stateUpdates channel. Now we instead send them on a
buffered channel, similar to what is done with the nodeUpdates.

autopilot/agent.go

@@ -122,6 +122,12 @@ type Agent struct {
 	// at most one pending update of this type to handle at a given time.
 	nodeUpdates chan *nodeUpdates
 
+	// chanOpenFailures is a channel where updates about channel open
+	// failures will be sent. This channel will be buffered to ensure we
+	// have at most one pending update of this type to handle at a given
+	// time.
+	chanOpenFailures chan *chanOpenFailureUpdate
+
 	// totalBalance is the total number of satoshis the backing wallet is
 	// known to control at any given instance. This value will be updated
 	// when the agent receives external balance update signals.
@@ -142,6 +148,7 @@ func New(cfg Config, initialState []Channel) (*Agent, error) {
 		quit:             make(chan struct{}),
 		stateUpdates:     make(chan interface{}),
 		nodeUpdates:      make(chan *nodeUpdates, 1),
+		chanOpenFailures: make(chan *chanOpenFailureUpdate, 1),
 	}
 
 	for _, c := range initialState {
@@ -265,15 +272,10 @@ func (a *Agent) OnChannelPendingOpen() {
 // autopilot has attempted to open a channel, but failed. In this case we can
 // retry channel creation with a different node.
 func (a *Agent) OnChannelOpenFailure() {
-	a.wg.Add(1)
-	go func() {
-		defer a.wg.Done()
-
 	select {
-		case a.stateUpdates <- &chanOpenFailureUpdate{}:
-		case <-a.quit:
+	case a.chanOpenFailures <- &chanOpenFailureUpdate{}:
+	default:
 	}
-	}()
 }
 
 // OnChannelClose is a callback that should be executed each time a prior
@@ -388,14 +390,6 @@ func (a *Agent) controller() {
 
 				updateBalance()
 
-			// The channel we tried to open previously failed for
-			// whatever reason.
-			case *chanOpenFailureUpdate:
-				log.Debug("Retrying after previous channel " +
-					"open failure.")
-
-				updateBalance()
-
 			// A new channel has been opened successfully. This was
 			// either opened by the Agent, or an external system
 			// that is able to drive the Lightning Node.
@@ -433,6 +427,14 @@ func (a *Agent) controller() {
 				updateBalance()
 			}
 
+		// The channel we tried to open previously failed for whatever
+		// reason.
+		case <-a.chanOpenFailures:
+			log.Debug("Retrying after previous channel open " +
+				"failure.")
+
+			updateBalance()
+
 		// New nodes have been added to the graph or their node
 		// announcements have been updated. We will consider opening
 		// channels to these nodes if we haven't stabilized.