autopilot: use updateBalance rather than tracking balance explicitly

In this commit, we modify the balanceUpdate autopilot signal to update
the balance according to what's returned to the WalletBalance callback
rather than explicitly tracking the balance. This gives the agent a
better sense of what the wallet's balance actually is.

autopilot/agent.go

@@ -154,13 +154,8 @@ func (a *Agent) Start() error {
 
 	log.Infof("Autopilot Agent starting")
 
-	startingBalance, err := a.cfg.WalletBalance()
-	if err != nil {
-		return err
-	}
-
 	a.wg.Add(1)
-	go a.controller(startingBalance)
+	go a.controller()
 
 	return nil
 }
@@ -183,7 +178,6 @@ func (a *Agent) Stop() error {
 // balanceUpdate is a type of external state update that reflects an
 // increase/decrease in the funds currently available to the wallet.
 type balanceUpdate struct {
-	balanceDelta btcutil.Amount
 }
 
 // nodeUpdates is a type of external state update that reflects an addition or
@@ -214,13 +208,13 @@ type chanCloseUpdate struct {
 
 // OnBalanceChange is a callback that should be executed each time the balance of
 // the backing wallet changes.
-func (a *Agent) OnBalanceChange(delta btcutil.Amount) {
+func (a *Agent) OnBalanceChange() {
 	a.wg.Add(1)
 	go func() {
 		defer a.wg.Done()
 
 		select {
-		case a.stateUpdates <- &balanceUpdate{balanceDelta: delta}:
+		case a.stateUpdates <- &balanceUpdate{}:
 		case <-a.quit:
 		}
 	}()
@@ -342,12 +336,12 @@ func mergeChanState(pendingChans map[NodeID]Channel,
 // and external state changes as a result of decisions it  makes w.r.t channel
 // allocation, or attributes affecting its control loop being updated by the
 // backing Lightning Node.
-func (a *Agent) controller(startingBalance btcutil.Amount) {
+func (a *Agent) controller() {
 	defer a.wg.Done()
 
 	// We'll start off by assigning our starting balance, and injecting
 	// that amount as an initial wake up to the main controller goroutine.
-	a.OnBalanceChange(startingBalance)
+	a.OnBalanceChange()
 
 	// TODO(roasbeef): do we in fact need to maintain order?
 	//  * use sync.Cond if so
@@ -388,15 +382,16 @@ func (a *Agent) controller(startingBalance btcutil.Amount) {
 			// up an additional channel, or splice in funds to an
 			// existing one.
 			case *balanceUpdate:
-				log.Debugf("Applying external balance state "+
-					"update of: %v", update.balanceDelta)
+				log.Debug("Applying external balance state " +
+					"update")
 
-				a.totalBalance += update.balanceDelta
+				updateBalance()
 
 			// The channel we tried to open previously failed for
 			// whatever reason.
 			case *chanOpenFailureUpdate:
-				log.Debug("Retrying after previous channel open failure.")
+				log.Debug("Retrying after previous channel " +
+					"open failure.")
 
 				updateBalance()