Merge pull request #42561 from ameyag/20.10-logbroker

[20.10] vendor: swarmkit to fix deadlock in log broker (bump_20.10)

vendor.conf

@@ -142,7 +142,7 @@ github.com/gogo/googleapis                          01e0f9cca9b92166042241267ee2
 github.com/cilium/ebpf                              1c8d4c9ef7759622653a1d319284a44652333b28
 
 # cluster
-github.com/docker/swarmkit                          17d8d4e4d8bdec33d386e6362d3537fa9493ba00
+github.com/docker/swarmkit                          c9afb5fd44bb419bae719f400f31671712bcb99e # bump_20.10
 github.com/gogo/protobuf                            5628607bb4c51c3157aacc3a50f0ab707582b805 # v1.3.1
 github.com/golang/protobuf                          84668698ea25b64748563aa20726db66a6b8d299 # v1.3.5
 github.com/cloudflare/cfssl                         5d63dbd981b5c408effbb58c442d54761ff94fbd # 1.3.2

vendor/github.com/docker/swarmkit/manager/logbroker/subscription.go

@@ -204,20 +204,31 @@ func (s *subscription) watch(ch <-chan events.Event) error {
 	}
 
 	add := func(t *api.Task) {
+		// this mutex does not have a deferred unlock, because there is work
+		// we need to do after we release it.
 		s.mu.Lock()
-		defer s.mu.Unlock()
 
 		// Un-allocated task.
 		if t.NodeID == "" {
 			s.pendingTasks[t.ID] = struct{}{}
+			s.mu.Unlock()
 			return
 		}
 
 		delete(s.pendingTasks, t.ID)
 		if _, ok := s.nodes[t.NodeID]; !ok {
 			s.nodes[t.NodeID] = struct{}{}
+
+			s.mu.Unlock()
+
+			// if we try to call Publish before we release the lock, we can end
+			// up in a situation where the receiver is trying to acquire a read
+			// lock on it. it's hard to explain.
 			s.changed.Publish(s)
+			return
 		}
+
+		s.mu.Unlock()
 	}
 
 	for {