Fix tablet error handling.

query/query.go

@@ -2064,7 +2064,7 @@ func ProcessGraph(ctx context.Context, sg, parent *SubGraph, rch chan error) {
 				return
 			}
 			result, err := worker.ProcessTaskOverNetwork(ctx, taskQuery)
-			if err != nil && strings.Contains(err.Error(), worker.ErrUnservedTabletMessage) {
+			if err != nil && strings.Contains(err.Error(), worker.ErrNonExistentTabletMessage) {
 				sg.UnknownAttr = true
 			} else if err != nil {
 				rch <- err

worker/groups.go

@@ -375,6 +375,8 @@ func (g *groupi) ServesTabletReadOnly(key string) (bool, error) {
 
 // Do not modify the returned Tablet
 func (g *groupi) Tablet(key string) (*pb.Tablet, error) {
+	emptyTablet := pb.Tablet{}
+
 	// TODO: Remove all this later, create a membership state and apply it
 	g.RLock()
 	tablet, ok := g.tablets[key]
@@ -392,11 +394,16 @@ func (g *groupi) Tablet(key string) (*pb.Tablet, error) {
 	out, err := zc.ShouldServe(context.Background(), tablet)
 	if err != nil {
 		glog.Errorf("Error while ShouldServe grpc call %v", err)
-		return nil, err
+		return &emptyTablet, err
+	}
+
+	// Do not store tablets with group ID 0, as they are just dummy tablets for
+	// predicates that do no exist.
+	if out.GroupId > 0 {
+		g.Lock()
+		g.tablets[key] = out
+		g.Unlock()
 	}
-	g.Lock()
-	g.tablets[key] = out
-	g.Unlock()
 
 	if out.GroupId == groups().groupId() {
 		glog.Infof("Serving tablet for: %v\n", key)

worker/mutation.go

@@ -39,8 +39,9 @@ import (
 )
 
 var (
-	ErrUnservedTabletMessage = "Tablet isn't being served by this instance"
-	errUnservedTablet        = x.Errorf(ErrUnservedTabletMessage)
+	ErrNonExistentTabletMessage = "Requested predicate is not being served by any tablet"
+	errNonExistentTablet        = x.Errorf(ErrNonExistentTabletMessage)
+	errUnservedTablet           = x.Errorf("Tablet isn't being served by this instance")
 )
 
 func isStarAll(v []byte) bool {
@@ -533,7 +534,7 @@ func MutateOverNetwork(ctx context.Context, m *pb.Mutations) (*api.TxnContext, e
 		if gid == 0 {
 			span.Annotatef(nil, "state: %+v", groups().state)
 			span.Annotatef(nil, "Group id zero for mutation: %+v", mu)
-			return tctx, errUnservedTablet
+			return tctx, errNonExistentTablet
 		}
 		mu.StartTs = m.StartTs
 		go proposeOrSend(ctx, gid, mu, resCh)

worker/predicate_move.go

@@ -186,9 +186,11 @@ func (w *grpcWorker) MovePredicate(ctx context.Context,
 	if err := posting.Oracle().WaitForTs(ctx, in.TxnTs); err != nil {
 		return &emptyPayload, x.Errorf("While waiting for txn ts: %d. Error: %v", in.TxnTs, err)
 	}
-	if servesTablet, err := groups().ServesTablet(in.Predicate); err != nil {
+	if gid, err := groups().BelongsTo(in.Predicate); err != nil {
 		return &emptyPayload, err
-	} else if !servesTablet {
+	} else if gid == 0 {
+		return &emptyPayload, errNonExistentTablet
+	} else if gid != groups().groupId() {
 		return &emptyPayload, errUnservedTablet
 	}
 

worker/proposal.go

@@ -136,7 +136,9 @@ func (n *node) proposeAndWait(ctx context.Context, proposal *pb.Proposal) (perr
 	checkTablet := func(pred string) error {
 		if tablet, err := groups().Tablet(pred); err != nil {
 			return err
-		} else if tablet == nil || tablet.GroupId != groups().groupId() {
+		} else if tablet == nil || tablet.GroupId == 0 {
+			return errNonExistentTablet
+		} else if tablet.GroupId != groups().groupId() {
 			return errUnservedTablet
 		}
 		return nil

worker/schema.go

@@ -116,7 +116,7 @@ func populateSchema(attr string, fields []string) *api.SchemaNode {
 // empty then it adds all known groups
 func addToSchemaMap(schemaMap map[uint32]*pb.SchemaRequest, schema *pb.SchemaRequest) error {
 	for _, attr := range schema.Predicates {
-		gid, err := groups().BelongsTo(attr)
+		gid, err := groups().BelongsToReadOnly(attr)
 		if err != nil {
 			return err
 		}
@@ -195,7 +195,7 @@ func GetSchemaOverNetwork(ctx context.Context, schema *pb.SchemaRequest) ([]*api
 
 	for gid, s := range schemaMap {
 		if gid == 0 {
-			return schemaNodes, errUnservedTablet
+			return schemaNodes, errNonExistentTablet
 		}
 		go getSchemaOverNetwork(ctx, gid, s, results)
 	}

worker/task.go

@@ -141,7 +141,7 @@ func ProcessTaskOverNetwork(ctx context.Context, q *pb.Query) (*pb.Result, error
 	if err != nil {
 		return &emptyResult, err
 	} else if gid == 0 {
-		return &emptyResult, errUnservedTablet
+		return &emptyResult, errNonExistentTablet
 	}
 
 	span := otrace.FromContext(ctx)
@@ -734,13 +734,16 @@ func processTask(ctx context.Context, q *pb.Query, gid uint32) (*pb.Result, erro
 	// zero, then it wouldn't know that this group is no longer serving this
 	// predicate. There's no issue if a we are serving a particular tablet and
 	// we get partitioned away from group zero as long as it's not removed.
-	// ServesTabletReadOnly is called instead of ServesTablet to prevent this
-	// alpha from requesting to serve this tablet.
-	if servesTablet, err := groups().ServesTabletReadOnly(q.Attr); err != nil {
+	// BelongsToReadOnly is called instead of BelongsTo to prevent this alpha
+	// from requesting to serve this tablet.
+	if gid, err := groups().BelongsToReadOnly(q.Attr); err != nil {
 		return &emptyResult, err
-	} else if !servesTablet {
+	} else if gid == 0 {
+		return &emptyResult, errNonExistentTablet
+	} else if gid != groups().groupId() {
 		return &emptyResult, errUnservedTablet
 	}
+
 	var qs queryState
 	if q.Cache == UseTxnCache {
 		qs.cache = posting.Oracle().CacheAt(q.ReadTs)
@@ -1629,6 +1632,8 @@ func (w *grpcWorker) ServeTask(ctx context.Context, q *pb.Query) (*pb.Result, er
 	if err != nil {
 		return &emptyResult, err
 	} else if gid == 0 {
+		return &emptyResult, errNonExistentTablet
+	} else if gid != groups().groupId() {
 		return &emptyResult, errUnservedTablet
 	}