pgraph: Build the sync group into the graph structure

This hides the sync/wait logic inside the graph itself.

pgraph/actions.go

@@ -450,7 +450,7 @@ func (g *Graph) Worker(v *Vertex) error {
 
 // Start is a main kick to start the graph. It goes through in reverse topological
 // sort order so that events can't hit un-started vertices.
-func (g *Graph) Start(wg *sync.WaitGroup, first bool) { // start or continue
+func (g *Graph) Start(first bool) { // start or continue
 	log.Printf("State: %v -> %v", g.setState(graphStateStarting), g.getState())
 	defer log.Printf("State: %v -> %v", g.setState(graphStateStarted), g.getState())
 	t, _ := g.TopologicalSort()
@@ -459,11 +459,11 @@ func (g *Graph) Start(wg *sync.WaitGroup, first bool) { // start or continue
 	for _, v := range Reverse(t) {
 
 		if !v.Res.IsWatching() { // if Watch() is not running...
-			wg.Add(1)
+			g.wg.Add(1)
 			// must pass in value to avoid races...
 			// see: https://ttboj.wordpress.com/2015/07/27/golang-parallelism-issues-causing-too-many-open-files-error/
 			go func(vv *Vertex) {
-				defer wg.Done()
+				defer g.wg.Done()
 				// TODO: if a sufficient number of workers error,
 				// should something be done? Will these restart
 				// after perma-failure if we have a graph change?
@@ -502,6 +502,11 @@ func (g *Graph) Start(wg *sync.WaitGroup, first bool) { // start or continue
 	}
 }
 
+// Wait waits for all the graph vertex workers to exit.
+func (g *Graph) Wait() {
+	g.wg.Wait()
+}
+
 // Pause sends pause events to the graph in a topological sort order.
 func (g *Graph) Pause() {
 	log.Printf("State: %v -> %v", g.setState(graphStatePausing), g.getState())

pgraph/pgraph.go

@@ -55,7 +55,8 @@ type Graph struct {
 	Adjacency map[*Vertex]map[*Vertex]*Edge // *Vertex -> *Vertex (edge)
 	Flags     Flags
 	state     graphState
-	mutex     sync.Mutex // used when modifying graph State variable
+	mutex     *sync.Mutex // used when modifying graph State variable
+	wg        *sync.WaitGroup
 }
 
 // Vertex is the primary vertex struct in this library.
@@ -78,6 +79,8 @@ func NewGraph(name string) *Graph {
 		Name:      name,
 		Adjacency: make(map[*Vertex]map[*Vertex]*Edge),
 		state:     graphStateNil,
+		// ptr b/c: "A WaitGroup must not be copied after first use."
+		wg: &sync.WaitGroup{},
 	}
 }
 
@@ -112,6 +115,8 @@ func (g *Graph) Copy() *Graph {
 		Adjacency: make(map[*Vertex]map[*Vertex]*Edge, len(g.Adjacency)),
 		Flags:     g.Flags,
 		state:     g.state,
+		mutex:     g.mutex,
+		wg:        g.wg,
 	}
 	for k, v := range g.Adjacency {
 		newGraph.Adjacency[k] = v // copy