engine: graph: Lock around frequent read races

These are all "safe" in terms of not ever conflicting, but the golang
memory model weirdness requires an actual lock to avoid race detector
errors.

engine/graph/engine.go

@@ -75,6 +75,7 @@ type Engine struct {
 	graph     *pgraph.Graph
 	nextGraph *pgraph.Graph
 	state     map[pgraph.Vertex]*State
+	tlock     *sync.RWMutex                     // lock around state map
 	waits     map[pgraph.Vertex]*sync.WaitGroup // wg for the Worker func
 	wlock     *sync.Mutex                       // lock around waits map
 
@@ -116,6 +117,7 @@ func (obj *Engine) Init() error {
 	}
 
 	obj.state = make(map[pgraph.Vertex]*State)
+	obj.tlock = &sync.RWMutex{}
 	obj.waits = make(map[pgraph.Vertex]*sync.WaitGroup)
 	obj.wlock = &sync.Mutex{}
 
@@ -345,7 +347,9 @@ func (obj *Engine) Commit() error {
 
 		// delete to free up memory from old graphs
 		fn := func() error {
+			obj.tlock.Lock()
 			delete(obj.state, vertex)
+			obj.tlock.Unlock()
 			delete(obj.waits, vertex)
 			return nil
 		}