Refactor backoff again (#1431)

* Tweak backoffs

* Refactor backoff some more, remove BackoffIfRequired as it adds unnecessary complexity

* Ignore 404s

federationsender/statistics/statistics.go

@@ -44,6 +44,7 @@ func (s *Statistics) ForServer(serverName gomatrixserverlib.ServerName) *ServerS
 		server = &ServerStatistics{
 			statistics: s,
 			serverName: serverName,
+			interrupt:  make(chan struct{}),
 		}
 		s.servers[serverName] = server
 		s.mutex.Unlock()
@@ -68,6 +69,7 @@ type ServerStatistics struct {
 	backoffStarted atomic.Bool                  // is the backoff started
 	backoffUntil   atomic.Value                 // time.Time until this backoff interval ends
 	backoffCount   atomic.Uint32                // number of times BackoffDuration has been called
+	interrupt      chan struct{}                // interrupts the backoff goroutine
 	successCounter atomic.Uint32                // how many times have we succeeded?
 }
 
@@ -76,15 +78,24 @@ func (s *ServerStatistics) duration(count uint32) time.Duration {
 	return time.Second * time.Duration(math.Exp2(float64(count)))
 }
 
+// cancel will interrupt the currently active backoff.
+func (s *ServerStatistics) cancel() {
+	s.blacklisted.Store(false)
+	s.backoffUntil.Store(time.Time{})
+	select {
+	case s.interrupt <- struct{}{}:
+	default:
+	}
+}
+
 // Success updates the server statistics with a new successful
 // attempt, which increases the sent counter and resets the idle and
 // failure counters. If a host was blacklisted at this point then
 // we will unblacklist it.
 func (s *ServerStatistics) Success() {
-	s.successCounter.Add(1)
-	s.backoffStarted.Store(false)
+	s.cancel()
+	s.successCounter.Inc()
 	s.backoffCount.Store(0)
-	s.blacklisted.Store(false)
 	if s.statistics.DB != nil {
 		if err := s.statistics.DB.RemoveServerFromBlacklist(s.serverName); err != nil {
 			logrus.WithError(err).Errorf("Failed to remove %q from blacklist", s.serverName)
@@ -99,10 +110,30 @@ func (s *ServerStatistics) Success() {
 // whether we have blacklisted and therefore to give up.
 func (s *ServerStatistics) Failure() (time.Time, bool) {
 	// If we aren't already backing off, this call will start
-	// a new backoff period. Reset the counter to 0 so that
-	// we backoff only for short periods of time to start with.
+	// a new backoff period. Increase the failure counter and
+	// start a goroutine which will wait out the backoff and
+	// unset the backoffStarted flag when done.
 	if s.backoffStarted.CAS(false, true) {
-		s.backoffCount.Store(0)
+		if s.backoffCount.Inc() >= s.statistics.FailuresUntilBlacklist {
+			s.blacklisted.Store(true)
+			if s.statistics.DB != nil {
+				if err := s.statistics.DB.AddServerToBlacklist(s.serverName); err != nil {
+					logrus.WithError(err).Errorf("Failed to add %q to blacklist", s.serverName)
+				}
+			}
+			return time.Time{}, true
+		}
+
+		go func() {
+			until, ok := s.backoffUntil.Load().(time.Time)
+			if ok {
+				select {
+				case <-time.After(time.Until(until)):
+				case <-s.interrupt:
+				}
+			}
+			s.backoffStarted.Store(false)
+		}()
 	}
 
 	// Check if we have blacklisted this node.
@@ -136,53 +167,6 @@ func (s *ServerStatistics) BackoffInfo() (*time.Time, bool) {
 	return nil, s.blacklisted.Load()
 }
 
-// BackoffIfRequired will block for as long as the current
-// backoff requires, if needed. Otherwise it will do nothing.
-// Returns the amount of time to backoff for and whether to give up or not.
-func (s *ServerStatistics) BackoffIfRequired(backingOff atomic.Bool, interrupt <-chan bool) (time.Duration, bool) {
-	if started := s.backoffStarted.Load(); !started {
-		return 0, false
-	}
-
-	// Work out if we should be blacklisting at this point.
-	count := s.backoffCount.Inc()
-	if count >= s.statistics.FailuresUntilBlacklist {
-		// We've exceeded the maximum amount of times we're willing
-		// to back off, which is probably in the region of hours by
-		// now. Mark the host as blacklisted and tell the caller to
-		// give up.
-		s.blacklisted.Store(true)
-		if s.statistics.DB != nil {
-			if err := s.statistics.DB.AddServerToBlacklist(s.serverName); err != nil {
-				logrus.WithError(err).Errorf("Failed to add %q to blacklist", s.serverName)
-			}
-		}
-		return 0, true
-	}
-
-	// Work out when we should wait until.
-	duration := s.duration(count)
-	until := time.Now().Add(duration)
-	s.backoffUntil.Store(until)
-
-	// Notify the destination queue that we're backing off now.
-	backingOff.Store(true)
-	defer backingOff.Store(false)
-
-	// Work out how long we should be backing off for.
-	logrus.Warnf("Backing off %q for %s", s.serverName, duration)
-
-	// Wait for either an interruption or for the backoff to
-	// complete.
-	select {
-	case <-interrupt:
-		logrus.Debugf("Interrupting backoff for %q", s.serverName)
-	case <-time.After(duration):
-	}
-
-	return duration, false
-}
-
 // Blacklisted returns true if the server is blacklisted and false
 // otherwise.
 func (s *ServerStatistics) Blacklisted() bool {