reset `coalesceTimer` to nil as soon as the event is consumed (#11924)

* reset `coalesceTimer` to nil as soon as the event is consumed

* add change log

* refactor to add relevant test.

* fix linter

* Apply suggestions from code review

Co-authored-by: Freddy <freddygv@users.noreply.github.com>

* remove non needed check

Co-authored-by: Freddy <freddygv@users.noreply.github.com>

.changelog/11924.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+xds: fix a deadlock when the snapshot channel already have a snapshot to be consumed.
+```

agent/proxycfg/manager.go

@@ -127,7 +127,7 @@ func (m *Manager) Run() error {
 	defer m.State.StopNotify(stateCh)
 
 	for {
-		m.syncState()
+		m.syncState(m.notifyBroadcast)
 
 		// Wait for a state change
 		_, ok := <-stateCh
@@ -140,7 +140,7 @@ func (m *Manager) Run() error {
 
 // syncState is called whenever the local state notifies a change. It holds the
 // lock while finding any new or updated proxies and removing deleted ones.
-func (m *Manager) syncState() {
+func (m *Manager) syncState(notifyBroadcast func(ch <-chan ConfigSnapshot)) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 
@@ -160,7 +160,7 @@ func (m *Manager) syncState() {
 		// know that so we'd need to set it here if not during registration of the
 		// proxy service. Sidecar Service in the interim can do that, but we should
 		// validate more generally that that is always true.
-		err := m.ensureProxyServiceLocked(svc)
+		err := m.ensureProxyServiceLocked(svc, notifyBroadcast)
 		if err != nil {
 			m.Logger.Error("failed to watch proxy service",
 				"service", sid.String(),
@@ -179,7 +179,7 @@ func (m *Manager) syncState() {
 }
 
 // ensureProxyServiceLocked adds or changes the proxy to our state.
-func (m *Manager) ensureProxyServiceLocked(ns *structs.NodeService) error {
+func (m *Manager) ensureProxyServiceLocked(ns *structs.NodeService, notifyBroadcast func(ch <-chan ConfigSnapshot)) error {
 	sid := ns.CompoundServiceID()
 
 	// Retrieve the token used to register the service, or fallback to the
@@ -227,16 +227,18 @@ func (m *Manager) ensureProxyServiceLocked(ns *structs.NodeService) error {
 	m.proxies[sid] = state
 
 	// Start a goroutine that will wait for changes and broadcast them to watchers.
-	go func(ch <-chan ConfigSnapshot) {
-		// Run until ch is closed
-		for snap := range ch {
-			m.notify(&snap)
-		}
-	}(ch)
+	go notifyBroadcast(ch)
 
 	return nil
 }
 
+func (m *Manager) notifyBroadcast(ch <-chan ConfigSnapshot) {
+	// Run until ch is closed
+	for snap := range ch {
+		m.notify(&snap)
+	}
+}
+
 // removeProxyService is called when a service deregisters and frees all
 // resources for that service.
 func (m *Manager) removeProxyServiceLocked(proxyID structs.ServiceID) {

agent/proxycfg/manager_test.go

@@ -598,7 +598,146 @@ func TestManager_SyncState_DefaultToken(t *testing.T) {
 
 	err = state.AddServiceWithChecks(srv, nil, "")
 	require.NoError(t, err)
-	m.syncState()
+	m.syncState(m.notifyBroadcast)
 
 	require.Equal(t, "default-token", m.proxies[srv.CompoundServiceID()].serviceInstance.token)
 }
+
+func TestManager_SyncState_No_Notify(t *testing.T) {
+	types := NewTestCacheTypes(t)
+	c := TestCacheWithTypes(t, types)
+	logger := testutil.Logger(t)
+	tokens := new(token.Store)
+	tokens.UpdateUserToken("default-token", token.TokenSourceConfig)
+
+	state := local.NewState(local.Config{}, logger, tokens)
+	state.TriggerSyncChanges = func() {}
+
+	m, err := NewManager(ManagerConfig{
+		Cache:  c,
+		Health: &health.Client{Cache: c, CacheName: cachetype.HealthServicesName},
+		State:  state,
+		Tokens: tokens,
+		Source: &structs.QuerySource{Datacenter: "dc1"},
+		Logger: logger,
+	})
+	require.NoError(t, err)
+	defer m.Close()
+
+	srv := &structs.NodeService{
+		Kind:    structs.ServiceKindConnectProxy,
+		ID:      "web-sidecar-proxy",
+		Service: "web-sidecar-proxy",
+		Port:    9999,
+		Meta:    map[string]string{},
+		Proxy: structs.ConnectProxyConfig{
+			DestinationServiceID:   "web",
+			DestinationServiceName: "web",
+			LocalServiceAddress:    "127.0.0.1",
+			LocalServicePort:       8080,
+			Config: map[string]interface{}{
+				"foo": "bar",
+			},
+		},
+	}
+
+	err = state.AddServiceWithChecks(srv, nil, "")
+	require.NoError(t, err)
+
+	readEvent := make(chan bool, 1)
+	snapSent := make(chan bool, 1)
+
+	m.syncState(func(ch <-chan ConfigSnapshot) {
+		for {
+			<-readEvent
+			snap := <-ch
+			m.notify(&snap)
+			snapSent <- true
+		}
+	})
+
+	// Get the relevant notification Channel, should only have 1
+	notifyCH := m.proxies[srv.CompoundServiceID()].ch
+
+	// update the leaf certs
+	roots, issuedCert := TestCerts(t)
+	notifyCH <- cache.UpdateEvent{
+		CorrelationID: leafWatchID,
+		Result:        issuedCert,
+		Err:           nil,
+	}
+	// at this point the snapshot should not be valid and not be sent
+	after := time.After(200 * time.Millisecond)
+	select {
+	case <-snapSent:
+		t.Fatal("snap should not be valid")
+	case <-after:
+
+	}
+
+	// update the root certs
+	notifyCH <- cache.UpdateEvent{
+		CorrelationID: rootsWatchID,
+		Result:        roots,
+		Err:           nil,
+	}
+
+	// at this point the snapshot should not be valid and not be sent
+	after = time.After(200 * time.Millisecond)
+	select {
+	case <-snapSent:
+		t.Fatal("snap should not be valid")
+	case <-after:
+
+	}
+
+	// prepare to read a snapshot update as the next update should make the snapshot valid
+	readEvent <- true
+
+	// update the intentions
+	notifyCH <- cache.UpdateEvent{
+		CorrelationID: intentionsWatchID,
+		Result:        &structs.IndexedIntentionMatches{},
+		Err:           nil,
+	}
+
+	// at this point we have a valid snapshot
+	after = time.After(500 * time.Millisecond)
+	select {
+	case <-snapSent:
+	case <-after:
+		t.Fatal("snap should be valid")
+
+	}
+
+	// send two snapshots back to back without reading them to overflow the snapshot channel and get to the default use case
+	for i := 0; i < 2; i++ {
+		time.Sleep(250 * time.Millisecond)
+		notifyCH <- cache.UpdateEvent{
+			CorrelationID: leafWatchID,
+			Result:        issuedCert,
+			Err:           nil,
+		}
+	}
+
+	// make sure that we are not receiving any snapshot and wait for the snapshots to be processed
+	after = time.After(500 * time.Millisecond)
+	select {
+	case <-snapSent:
+		t.Fatal("snap should not be sent")
+	case <-after:
+	}
+
+	// now make sure that both snapshots got propagated
+	for i := 0; i < 2; i++ {
+
+		readEvent <- true
+		after = time.After(500 * time.Millisecond)
+		select {
+		case <-snapSent:
+		case <-after:
+			t.Fatal("snap should be valid")
+
+		}
+	}
+}

agent/proxycfg/state.go

@@ -294,6 +294,8 @@ func (s *state) run(ctx context.Context, snap *ConfigSnapshot) {
 			}
 
 		case <-sendCh:
+			// Allow the next change to trigger a send
+			coalesceTimer = nil
 			// Make a deep copy of snap so we don't mutate any of the embedded structs
 			// etc on future updates.
 			snapCopy, err := snap.Clone()
@@ -307,9 +309,6 @@ func (s *state) run(ctx context.Context, snap *ConfigSnapshot) {
 			case s.snapCh <- *snapCopy:
 				s.logger.Trace("Delivered new snapshot to proxy config watchers")
 
-				// Allow the next change to trigger a send
-				coalesceTimer = nil
-
 				// Skip rest of loop - there is nothing to send since nothing changed on
 				// this iteration
 				continue
@@ -320,11 +319,9 @@ func (s *state) run(ctx context.Context, snap *ConfigSnapshot) {
 				s.logger.Trace("Failed to deliver new snapshot to proxy config watchers")
 
 				// Reset the timer to retry later. This is to ensure we attempt to redeliver the updated snapshot shortly.
-				if coalesceTimer == nil {
-					coalesceTimer = time.AfterFunc(coalesceTimeout, func() {
-						sendCh <- struct{}{}
-					})
-				}
+				coalesceTimer = time.AfterFunc(coalesceTimeout, func() {
+					sendCh <- struct{}{}
+				})
 
 				// Do not reset coalesceTimer since we just queued a timer-based refresh
 				continue