Inform Zero about tablets in posting dir

When Alpha starts, it should inform Zero about the predicates it is
serving, so that Zero can then allow it to serve those predicates. Right
now that doesn't happen, which means some other group could start
serving the predicate without having the data for it.

This PR fixes that by using the schema to determine which predicates
Alpha has in the postings dir and informs Zero about these.

worker/groups.go

@@ -142,9 +142,33 @@ func StartRaftNodes(walStore *badger.DB, bindall bool) {
 	go gr.receiveMembershipUpdates()
 	go gr.processOracleDeltaStream()
 
+	go gr.informZeroAboutTablets()
 	gr.proposeInitialSchema()
 }
 
+func (g *groupi) informZeroAboutTablets() {
+	// Before we start this Alpha, let's pick up all the predicates we have in our postings
+	// directory, and ask Zero if we are allowed to serve it. Do this irrespective of whether
+	// this node is the leader or the follower, because this early on, we might not have
+	// figured that out.
+	ticker := time.NewTicker(time.Second)
+	defer ticker.Stop()
+
+	for range ticker.C {
+		failed := false
+		preds := schema.State().Predicates()
+		for _, pred := range preds {
+			if tablet := g.Tablet(pred); tablet == nil {
+				failed = true
+			}
+		}
+		if !failed {
+			glog.V(1).Infof("Done informing Zero about the %d tablets I have", len(preds))
+			return
+		}
+	}
+}
+
 func (g *groupi) proposeInitialSchema() {
 	// propose the schema for _predicate_
 	if Config.ExpandEdge {