pgraph: semaphore: Add lock around semaphore map

I forgot about the `concurrent map write` race, but now it's fixed. I
suppose we could probably pre-create all semaphores in the graph at once
before Start, and remove this lock, but that's an optimization for a
later day.

pgraph/pgraph.go

@@ -61,6 +61,7 @@ type Graph struct {
 	mutex     *sync.Mutex // used when modifying graph State variable
 	wg        *sync.WaitGroup
 	semas     map[string]*semaphore.Semaphore
+	slock     *sync.Mutex // semaphore mutex
 
 	prometheus *prometheus.Prometheus // the prometheus instance
 }
@@ -89,6 +90,7 @@ func NewGraph(name string) *Graph {
 		mutex: &sync.Mutex{},
 		wg:    &sync.WaitGroup{},
 		semas: make(map[string]*semaphore.Semaphore),
+		slock: &sync.Mutex{},
 	}
 }
 
@@ -126,6 +128,7 @@ func (g *Graph) Copy() *Graph {
 		mutex:     g.mutex,
 		wg:        g.wg,
 		semas:     g.semas,
+		slock:     g.slock,
 
 		prometheus: g.prometheus,
 	}

pgraph/semaphore.go

@@ -46,11 +46,13 @@ func (g *Graph) SemaLock(semas []string) error {
 			}
 		}
 
+		g.slock.Lock()          // semaphore creation lock
 		sema, ok := g.semas[id] // lookup
 		if !ok {
 			g.semas[id] = semaphore.NewSemaphore(size)
 			sema = g.semas[id]
 		}
+		g.slock.Unlock()
 
 		if err := sema.P(1); err != nil { // lock!
 			reterr = multierr.Append(reterr, err) // list of errors