Fix panic in expand(_all_) on empty database.

Author: Pawan Rawal

query/query.go

@@ -1695,14 +1695,21 @@ func (sg *SubGraph) appendDummyValues() {
 	}
 }
 
-func uniquePreds(vl []*intern.ValueList) map[string]struct{} {
-	preds := make(map[string]struct{})
+func uniquePreds(vl []*intern.ValueList) []string {
+	predMap := make(map[string]struct{})
 
 	for _, l := range vl {
 		for _, v := range l.Values {
-			preds[string(v.Val)] = struct{}{}
+			if len(v.Val) > 0 {
+				predMap[string(v.Val)] = struct{}{}
+			}
 		}
 	}
+
+	preds := make([]string, 0, len(predMap))
+	for pred := range predMap {
+		preds = append(preds, pred)
+	}
 	return preds
 }
 
@@ -1746,23 +1753,27 @@ func expandSubgraph(ctx context.Context, sg *SubGraph) ([]*SubGraph, error) {
 				x.Errorf("Cannot run expand() query when ExpandEdge(--expand_edge) is false.")
 		}
 
+		var preds []string
 		// It could be expand(_all_) or expand(val(x)).
 		if child.Params.Expand == "_all_" {
-			// Get the predicate list for expansion. Otherwise we already
-			// have the list populated from the var.
+			// Get the predicate list for expansion.
 			child.ExpandPreds, err = getNodePredicates(ctx, sg)
 			if err != nil {
 				return out, err
 			}
+			preds = uniquePreds(child.ExpandPreds)
+
 			rpreds, err := getReversePredicates(ctx)
 			if err != nil {
 				return out, err
 			}
-			child.ExpandPreds[0].Values = append(child.ExpandPreds[0].Values, rpreds...)
+			preds = append(preds, rpreds...)
+		} else {
+			// We already have the predicates populated from the var.
+			preds = uniquePreds(child.ExpandPreds)
 		}
 
-		up := uniquePreds(child.ExpandPreds)
-		for pred, _ := range up {
+		for _, pred := range preds {
 			temp := &SubGraph{
 				ReadTs:  sg.ReadTs,
 				LinRead: sg.LinRead,
@@ -1795,23 +1806,20 @@ func expandSubgraph(ctx context.Context, sg *SubGraph) ([]*SubGraph, error) {
 	return out, nil
 }
 
-func getReversePredicates(ctx context.Context) ([]*intern.TaskValue, error) {
+func getReversePredicates(ctx context.Context) ([]string, error) {
 	schs, err := worker.GetSchemaOverNetwork(ctx, &intern.SchemaRequest{})
 	if err != nil {
 		return nil, err
 	}
-	var preds []*intern.TaskValue
+	preds := make([]string, 0, len(schs))
 	for _, sch := range schs {
 		if !sch.Reverse {
 			continue
 		}
 		pred := make([]byte, 1+len(sch.Predicate))
 		pred[0] = '~'
 		copy(pred[1:], sch.Predicate)
-		preds = append(preds, &intern.TaskValue{
-			Val:     pred,
-			ValType: intern.Posting_DEFAULT,
-		})
+		preds = append(preds, string(pred))
 	}
 	return preds, nil
 }

query/query_test.go

@@ -7781,3 +7781,17 @@ func TestUidWithoutDebug2(t *testing.T) {
 	js := processToFastJsonNoErr(t, query)
 	require.JSONEq(t, `{"data":{"q":[{"uid":"0x1","friend":[{"uid":"0x17"},{"uid":"0x18"},{"uid":"0x19"},{"uid":"0x1f"},{"uid":"0x65"}]}]}}`, js)
 }
+
+func TestExpandAll_empty_panic(t *testing.T) {
+	populateGraph(t)
+
+	query := `
+		{
+			me(func: uid(0x01)) @filter(eq(name,"foobar")){
+				expand(_all_)
+			}
+		}
+	`
+	js := processToFastJsonNoErr(t, query)
+	require.JSONEq(t, `{"data":{"me":[]}}`, js)
+}