Lock around saving state

Prevent interleaving state syncs as it could conceivably lead to
empty state files as per #1367

client/alloc_runner.go

@@ -66,6 +66,9 @@ type AllocRunner struct {
 	destroyCh   chan struct{}
 	destroyLock sync.Mutex
 	waitCh      chan struct{}
+
+	// serialize saveAllocRunnerState calls
+	persistLock sync.Mutex
 }
 
 // allocRunnerState is used to snapshot the state of the alloc runner
@@ -179,8 +182,12 @@ func (r *AllocRunner) SaveState() error {
 }
 
 func (r *AllocRunner) saveAllocRunnerState() error {
+	r.persistLock.Lock()
+	defer r.persistLock.Unlock()
+
 	// Create the snapshot.
 	alloc := r.Alloc()
+
 	r.allocLock.Lock()
 	states := alloc.TaskStates
 	allocClientStatus := r.allocClientStatus

client/task_runner.go

@@ -69,6 +69,9 @@ type TaskRunner struct {
 	destroyLock  sync.Mutex
 	destroyEvent *structs.TaskEvent
 	waitCh       chan struct{}
+
+	// serialize SaveState calls
+	persistLock sync.Mutex
 }
 
 // taskRunnerState is used to snapshot the state of the task runner
@@ -186,6 +189,9 @@ func (r *TaskRunner) RestoreState() error {
 
 // SaveState is used to snapshot our state
 func (r *TaskRunner) SaveState() error {
+	r.persistLock.Lock()
+	defer r.persistLock.Unlock()
+
 	snap := taskRunnerState{
 		Task:               r.task,
 		Version:            r.config.Version,