From a3c11b21c7e2cae913c78c71c921b7e55d5c248d Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Wed, 19 Aug 2020 15:37:34 +0100
Subject: [PATCH] Review comments

---
 federationsender/internal/api.go          |  2 +-
 federationsender/statistics/statistics.go | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/federationsender/internal/api.go b/federationsender/internal/api.go
index dd7cb63cf..386149a33 100644
--- a/federationsender/internal/api.go
+++ b/federationsender/internal/api.go
@@ -77,7 +77,7 @@ func failBlacklistableError(err error, stats *statistics.ServerStatistics) {
 		stats.Failure()
 		return
 	}
-	if mxerr.Code == 500 {
+	if mxerr.Code >= 500 || mxerr.Code < 600 {
 		stats.Failure()
 		return
 	}
diff --git a/federationsender/statistics/statistics.go b/federationsender/statistics/statistics.go
index 0dd8da200..07b7e7b9c 100644
--- a/federationsender/statistics/statistics.go
+++ b/federationsender/statistics/statistics.go
@@ -97,11 +97,22 @@ func (s *ServerStatistics) Failure() {
 
 // 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
 	}
 
+	// Check if the interrupt channel is already closed. If it is
+	// then this call should have no side effects but still return
+	// the current duration.
+	select {
+	case <-interrupt:
+		count := s.backoffCount.Load()
+		return time.Second * time.Duration(math.Exp2(float64(count))), false
+	default:
+	}
+
 	// Work out how many times we've backed off so far.
 	count := s.backoffCount.Inc()
 	duration := time.Second * time.Duration(math.Exp2(float64(count)))