[manager/dispatcher] Use read-write lock for dispatcher context.

Signed-off-by: Anshul Pundir <anshul.pundir@docker.com>

manager/dispatcher/dispatcher.go

@@ -133,7 +133,8 @@ type Dispatcher struct {
 	// has finished initializing the dispatcher.
 	wg sync.WaitGroup
 	// This RWMutex synchronizes RPC handlers and the dispatcher stop().
-	// The RPC handlers use the read lock while stop() uses the write lock
+	// Used to serialize read-write access to the dispatcher context.
+	// Also, the RPC handlers use the read lock while stop() uses the write lock
 	// and acts as a barrier to shutdown.
 	rpcRW                sync.RWMutex
 	nodes                *nodeStore
@@ -265,12 +266,15 @@ func (d *Dispatcher) Run(ctx context.Context) error {
 	d.lastSeenManagers = getWeightedPeers(d.cluster)
 
 	defer cancel()
-	d.ctx, d.cancel = context.WithCancel(ctx)
-	ctx = d.ctx
 	d.wg.Add(1)
 	defer d.wg.Done()
 	d.mu.Unlock()
 
+	d.rpcRW.Lock()
+	d.ctx, d.cancel = context.WithCancel(ctx)
+	ctx = d.ctx
+	d.rpcRW.Unlock()
+
 	publishManagers := func(peers []*api.Peer) {
 		var mgrs []*api.WeightedPeer
 		for _, p := range peers {
@@ -333,14 +337,14 @@ func (d *Dispatcher) Stop() error {
 
 	log := log.G(d.ctx).WithField("method", "(*Dispatcher).Stop")
 	log.Info("dispatcher stopping")
-	d.cancel()
 	d.mu.Unlock()
 
 	// The active nodes list can be cleaned out only when all
 	// existing RPCs have finished.
 	// RPCs that start after rpcRW.Unlock() should find the context
 	// cancelled and should fail organically.
 	d.rpcRW.Lock()
+	d.cancel()
 	d.nodes.Clean()
 	d.downNodes.Clean()
 	d.rpcRW.Unlock()
@@ -364,14 +368,14 @@ func (d *Dispatcher) Stop() error {
 	return nil
 }
 
-func (d *Dispatcher) isRunningLocked() (context.Context, error) {
-	d.mu.Lock()
+// context returns the dispatcher context.
+func (d *Dispatcher) context() (context.Context, error) {
+	d.rpcRW.RLock()
+	defer d.rpcRW.RUnlock()
 	if !d.isRunning() {
-		d.mu.Unlock()
 		return nil, status.Errorf(codes.Aborted, "dispatcher is stopped")
 	}
 	ctx := d.ctx
-	d.mu.Unlock()
 	return ctx, nil
 }
 
@@ -510,7 +514,7 @@ func nodeIPFromContext(ctx context.Context) (string, error) {
 func (d *Dispatcher) register(ctx context.Context, nodeID string, description *api.NodeDescription) (string, error) {
 	logLocal := log.G(ctx).WithField("method", "(*Dispatcher).register")
 	// prevent register until we're ready to accept it
-	dctx, err := d.isRunningLocked()
+	dctx, err := d.context()
 	if err != nil {
 		return "", err
 	}
@@ -565,7 +569,7 @@ func (d *Dispatcher) UpdateTaskStatus(ctx context.Context, r *api.UpdateTaskStat
 	d.rpcRW.RLock()
 	defer d.rpcRW.RUnlock()
 
-	dctx, err := d.isRunningLocked()
+	dctx, err := d.context()
 	if err != nil {
 		return nil, err
 	}
@@ -759,7 +763,7 @@ func (d *Dispatcher) Tasks(r *api.TasksRequest, stream api.Dispatcher_TasksServe
 	d.rpcRW.RLock()
 	defer d.rpcRW.RUnlock()
 
-	dctx, err := d.isRunningLocked()
+	dctx, err := d.context()
 	if err != nil {
 		return err
 	}
@@ -885,7 +889,7 @@ func (d *Dispatcher) Assignments(r *api.AssignmentsRequest, stream api.Dispatche
 	d.rpcRW.RLock()
 	defer d.rpcRW.RUnlock()
 
-	dctx, err := d.isRunningLocked()
+	dctx, err := d.context()
 	if err != nil {
 		return err
 	}
@@ -1080,7 +1084,7 @@ func (d *Dispatcher) moveTasksToOrphaned(nodeID string) error {
 func (d *Dispatcher) markNodeNotReady(id string, state api.NodeStatus_State, message string) error {
 	logLocal := log.G(d.ctx).WithField("method", "(*Dispatcher).markNodeNotReady")
 
-	dctx, err := d.isRunningLocked()
+	dctx, err := d.context()
 	if err != nil {
 		return err
 	}
@@ -1190,7 +1194,7 @@ func (d *Dispatcher) Session(r *api.SessionRequest, stream api.Dispatcher_Sessio
 	d.rpcRW.RLock()
 	defer d.rpcRW.RUnlock()
 
-	dctx, err := d.isRunningLocked()
+	dctx, err := d.context()
 	if err != nil {
 		return err
 	}