Add sanity check to SaveState

Also just reuse the task states snapshot taken by `Alloc()` instead of
doing a redundant copy.

client/alloc_runner.go

@@ -180,12 +180,9 @@ func (r *AllocRunner) SaveState() error {
 
 func (r *AllocRunner) saveAllocRunnerState() error {
 	// Create the snapshot.
-	r.taskStatusLock.RLock()
-	states := copyTaskStates(r.taskStates)
-	r.taskStatusLock.RUnlock()
-
 	alloc := r.Alloc()
 	r.allocLock.Lock()
+	states := alloc.TaskStates
 	allocClientStatus := r.allocClientStatus
 	allocClientDescription := r.allocClientDescription
 	r.allocLock.Unlock()

client/util.go

@@ -92,6 +92,13 @@ func persistState(path string, data interface{}) error {
 	if err := os.Rename(tmpPath, path); err != nil {
 		return fmt.Errorf("failed to rename tmp to path: %v", err)
 	}
+
+	// Sanity check since users have reported empty state files on disk
+	if stat, err := os.Stat(path); err != nil {
+		return fmt.Errorf("unable to stat state file %s: %v", path, err)
+	} else if stat.Size() == 0 {
+		return fmt.Errorf("persisted invalid state file %s; see https://github.com/hashicorp/nomad/issues/1367")
+	}
 	return nil
 }