contractcourt/chain_watcher: make breachArbiter handoff a function closure

This commit changes how the ChainWatcher notifies the breachArbiter
about a channel breach. Instead of assuming the breachArbiter is among
the clients subscibing to channel events, it will call a new method
contractBreach(), and assume the breachArbiter has reliably gotten the
breach info when this method returns with a non-nil error.

Since the breachArbiter was the only sybsystem having a sync chain
subsciption, we also remove the (now) unused syncDispatch option.

contractcourt/chain_watcher.go

@@ -51,15 +51,6 @@ type ChainEventSubscription struct {
 	// material required to bring the cheating channel peer to justice.
 	ContractBreach chan *lnwallet.BreachRetribution
 
-	// ProcessACK is a channel that will be used by the chainWatcher to
-	// synchronize dispatch and processing of the notification with the act
-	// of updating the state of the channel on disk. This ensures that the
-	// event can be reliably handed off.
-	//
-	// NOTE: This channel will only be used if the syncDispatch arg passed
-	// into the constructor is true.
-	ProcessACK chan error
-
 	// Cancel cancels the subscription to the event stream for a particular
 	// channel. This method should be called once the caller no longer needs to
 	// be notified of any on-chain events for a particular channel.
@@ -93,6 +84,12 @@ type chainWatcherConfig struct {
 	// confirmed.
 	markChanClosed func() error
 
+	// contractBreach is a method that will be called by the watcher if it
+	// detects that a contract breach transaction has been confirmed. Only
+	// when this method returns with a non-nil error it will be safe to mark
+	// the channel as pending close in the database.
+	contractBreach func(*lnwallet.BreachRetribution) error
+
 	// isOurAddr is a function that returns true if the passed address is
 	// known to us.
 	isOurAddr func(btcutil.Address) bool
@@ -219,12 +216,8 @@ func (c *chainWatcher) Stop() error {
 // SubscribeChannelEvents returns an active subscription to the set of channel
 // events for the channel watched by this chain watcher. Once clients no longer
 // require the subscription, they should call the Cancel() method to allow the
-// watcher to regain those committed resources. The syncDispatch bool indicates
-// if the caller would like a synchronous dispatch of the notification. This
-// means that the main chain watcher goroutine won't proceed with
-// post-processing after the notification until the ProcessACK channel is sent
-// upon.
-func (c *chainWatcher) SubscribeChannelEvents(syncDispatch bool) *ChainEventSubscription {
+// watcher to regain those committed resources.
+func (c *chainWatcher) SubscribeChannelEvents() *ChainEventSubscription {
 
 	c.Lock()
 	clientID := c.clientID
@@ -248,34 +241,6 @@ func (c *chainWatcher) SubscribeChannelEvents(syncDispatch bool) *ChainEventSubs
 		},
 	}
 
-	if syncDispatch {
-		sub.ProcessACK = make(chan error, 1)
-
-		// If this client is syncDispatch, we cannot safely delete it
-		// from our list of clients. This is because of a potential
-		// race at shutdown, where the client shuts down and calls
-		// Cancel(). In this case we must make sure the ChainWatcher
-		// doesn't think it has successfully handed off a contract
-		// breach to the client. We start a goroutine that will send an
-		// error on the ProcessACK channel until the ChainWatcher is
-		// shutdown.
-		sub.Cancel = func() {
-			c.wg.Add(1)
-			go func() {
-				defer c.wg.Done()
-
-				err := fmt.Errorf("cancelled")
-				for {
-					select {
-					case sub.ProcessACK <- err:
-					case <-c.quit:
-						return
-					}
-				}
-			}()
-		}
-	}
-
 	c.Lock()
 	c.clientSubscriptions[clientID] = sub
 	c.Unlock()
@@ -703,6 +668,13 @@ func (c *chainWatcher) dispatchContractBreach(spendEvent *chainntnfs.SpendDetail
 			return spew.Sdump(retribution)
 		}))
 
+	// Hand the retribution info over to the breach arbiter.
+	if err := c.cfg.contractBreach(retribution); err != nil {
+		log.Errorf("unable to hand breached contract off to "+
+			"breachArbiter: %v", err)
+		return err
+	}
+
 	// With the event processed, we'll now notify all subscribers of the
 	// event.
 	c.Lock()
@@ -713,25 +685,6 @@ func (c *chainWatcher) dispatchContractBreach(spendEvent *chainntnfs.SpendDetail
 			c.Unlock()
 			return fmt.Errorf("quitting")
 		}
-
-		// Wait for the breach arbiter to ACK the handoff before
-		// marking the channel as pending force closed in channeldb,
-		// but only if the client requested a sync dispatch.
-		if sub.ProcessACK != nil {
-			select {
-			case err := <-sub.ProcessACK:
-				// Bail if the handoff failed.
-				if err != nil {
-					c.Unlock()
-					return fmt.Errorf("unable to handoff "+
-						"retribution info: %v", err)
-				}
-
-			case <-c.quit:
-				c.Unlock()
-				return fmt.Errorf("quitting")
-			}
-		}
 	}
 	c.Unlock()