lib, gapi: Next method of GAPI should generate first event

This puts the generation of the initial event into the Next method of
the GAPI. If it does not happen, then we will never get a graph. This is
important because this notifies the GAPI when we're actually ready to
try and generate a graph, rather than blocking on the Graph method if we
have a long compile for example.

This is also required for the etcd watch cleanup.

examples/lib/libmgmt3.go

@@ -129,9 +129,6 @@ func (obj *MyGAPI) Graph() (*pgraph.Graph, error) {
 
 // Next returns nil errors every time there could be a new graph.
 func (obj *MyGAPI) Next() chan error {
-	if obj.data.NoWatch || obj.Interval <= 0 {
-		return nil
-	}
 	ch := make(chan error)
 	obj.wg.Add(1)
 	go func() {
@@ -141,19 +138,32 @@ func (obj *MyGAPI) Next() chan error {
 			ch <- fmt.Errorf("libmgmt: MyGAPI is not initialized")
 			return
 		}
+		startChan := make(chan struct{}) // start signal
+		close(startChan)                 // kick it off!
 
-		// arbitrarily change graph every interval seconds
-		ticker := time.NewTicker(time.Duration(obj.Interval) * time.Second)
-		defer ticker.Stop()
+		ticker := make(<-chan time.Time)
+		if obj.data.NoStreamWatch || obj.Interval <= 0 {
+			ticker = nil
+		} else {
+			// arbitrarily change graph every interval seconds
+			t := time.NewTicker(time.Duration(obj.Interval) * time.Second)
+			defer t.Stop()
+			ticker = t.C
+		}
 		for {
 			select {
-			case <-ticker.C:
-				log.Printf("libmgmt: Generating new graph...")
-				select {
-				case ch <- nil: // trigger a run
-				case <-obj.closeChan:
-					return
-				}
+			case <-startChan: // kick the loop once at start
+				startChan = nil // disable
+				// pass
+			case <-ticker:
+				// pass
+			case <-obj.closeChan:
+				return
+			}
+
+			log.Printf("libmgmt: Generating new graph...")
+			select {
+			case ch <- nil: // trigger a run
 			case <-obj.closeChan:
 				return
 			}