Update

algo/uidlist.go

@@ -46,7 +46,12 @@ func (u *UIDList) Get(i int) uint64 {
 
 // Size returns size of UIDList.
 func (u *UIDList) Size() int {
-	x.Assert(u != nil)
+	if u == nil {
+		// In a subgraph node, in processGraph, sometimes we might fan out to zero
+		// nodes, i.e., sg.destUIDs is empty. In this case, child subgraph might not
+		// have its srcUIDs initialized.
+		return 0
+	}
 	if u.list != nil {
 		return u.list.UidsLength()
 	}

query/query.go

@@ -164,6 +164,7 @@ func mergeInterfaces(i1 interface{}, i2 interface{}) interface{} {
 
 // postTraverse traverses the subgraph recursively and returns final result for the query.
 func postTraverse(sg *SubGraph) (map[uint64]interface{}, error) {
+	// No need to check for nil as Size() will return 0 in that case.
 	if sg.srcUIDs.Size() == 0 {
 		return nil, nil
 	}
@@ -819,7 +820,7 @@ func ProcessGraph(ctx context.Context, sg *SubGraph, taskQuery []byte, rch chan
 	}
 
 	if sg.destUIDs.Size() == 0 {
-		// Looks like we're done here.
+		// Looks like we're done here. Be careful with nil srcUIDs!
 		x.Trace(ctx, "Zero uids. Num attr children: %v", len(sg.Children))
 		rch <- nil
 		return