blockchain/v2: fix excessive CPU usage due to spinning on closed channels (#4761)

The event loop uses a `select` on multiple channels. However, reading from a closed channel in Go always yields the channel's zero value. The processor and scheduler close their channels when done, and since these channels are always ready to receive, the event loop keeps spinning on them.

This changes `routine.terminate()` to not close the channel, and also removes `stopDemux` and instead uses `events` channel closure to signal event loop termination.

Fixes #4687.

blockchain/v2/routine.go

@@ -69,9 +69,11 @@ func (rt *Routine) start() {
 
 	for {
 		events, err := rt.queue.Get(1)
-		if err != nil {
-			rt.logger.Info(fmt.Sprintf("%s: stopping\n", rt.name))
-			rt.terminate(fmt.Errorf("stopped"))
+		if err == queue.ErrDisposed {
+			rt.terminate(nil)
+			return
+		} else if err != nil {
+			rt.terminate(err)
 			return
 		}
 		oEvent, err := rt.handle(events[0].(Event))
@@ -131,6 +133,7 @@ func (rt *Routine) final() chan error {
 
 // XXX: Maybe get rid of this
 func (rt *Routine) terminate(reason error) {
-	close(rt.out)
+	// We don't close the rt.out channel here, to avoid spinning on the closed channel
+	// in the event loop.
 	rt.fin <- reason
 }