Fix(Dgraph): Fix how visited nodes are detected in recurse queries. (#…

…6272)

Currently a node is marked as visited if it's been visited before AND
has been visited from the same source UID. In dense graphs, the second
condition leads to exponential growth of the data and to other issues
such as responses that are too big to encode.  Removing this condition
fixes the issue.

Fixed tests and verified the new output makes sense.

Fixes DGRAPH-2337

(cherry picked from commit 3638c12)

query/query3_test.go

@@ -79,7 +79,6 @@ func TestRecurseNestedError2(t *testing.T) {
 }
 
 func TestRecurseQuery(t *testing.T) {
-
 	query := `
 		{
 			me(func: uid(0x01)) @recurse {
@@ -90,7 +89,7 @@ func TestRecurseQuery(t *testing.T) {
 		}`
 	js := processQueryNoErr(t, query)
 	require.JSONEq(t,
-		`{"data": {"me":[{"name":"Michonne", "friend":[{"name":"Rick Grimes", "friend":[{"name":"Michonne"}]},{"name":"Glenn Rhee"},{"name":"Daryl Dixon"},{"name":"Andrea", "friend":[{"name":"Glenn Rhee"}]}]}]}}`, js)
+		`{"data": {"me":[{"name":"Michonne", "friend":[{"name":"Rick Grimes", "friend":[{"name":"Michonne"}]},{"name":"Glenn Rhee"},{"name":"Daryl Dixon"},{"name":"Andrea"}]}]}}`, js)
 }
 
 func TestRecurseExpand(t *testing.T) {
@@ -132,7 +131,7 @@ func TestRecurseQueryOrder(t *testing.T) {
 		}`
 	js := processQueryNoErr(t, query)
 	require.JSONEq(t,
-		`{"data": {"me":[{"dob":"1910-01-01T00:00:00Z","friend":[{"dob":"1910-01-02T00:00:00Z","friend":[{"dob":"1910-01-01T00:00:00Z","name":"Michonne"}],"name":"Rick Grimes"},{"dob":"1909-05-05T00:00:00Z","name":"Glenn Rhee"},{"dob":"1909-01-10T00:00:00Z","name":"Daryl Dixon"},{"dob":"1901-01-15T00:00:00Z","friend":[{"dob":"1909-05-05T00:00:00Z","name":"Glenn Rhee"}],"name":"Andrea"}],"name":"Michonne"}]}}`,
+		`{"data": {"me":[{"dob":"1910-01-01T00:00:00Z","friend":[{"dob":"1910-01-02T00:00:00Z","friend":[{"dob":"1910-01-01T00:00:00Z","name":"Michonne"}],"name":"Rick Grimes"},{"dob":"1909-05-05T00:00:00Z","name":"Glenn Rhee"},{"dob":"1909-01-10T00:00:00Z","name":"Daryl Dixon"},{"dob":"1901-01-15T00:00:00Z","name":"Andrea"}],"name":"Michonne"}]}}`,
 		js)
 }
 
@@ -147,7 +146,7 @@ func TestRecurseQueryAllowLoop(t *testing.T) {
 			}
 		}`
 	js := processQueryNoErr(t, query)
-	require.JSONEq(t, `{"data":{"me":[{"friend":[{"friend":[{"dob":"1910-01-01T00:00:00Z","name":"Michonne"}],"dob":"1910-01-02T00:00:00Z","name":"Rick Grimes"},{"dob":"1909-05-05T00:00:00Z","name":"Glenn Rhee"},{"dob":"1909-01-10T00:00:00Z","name":"Daryl Dixon"},{"friend":[{"dob":"1909-05-05T00:00:00Z","name":"Glenn Rhee"}],"dob":"1901-01-15T00:00:00Z","name":"Andrea"}],"dob":"1910-01-01T00:00:00Z","name":"Michonne"}]}}`, js)
+	require.JSONEq(t, `{"data":{"me":[{"friend":[{"friend":[{"dob":"1910-01-01T00:00:00Z","name":"Michonne"}],"dob":"1910-01-02T00:00:00Z","name":"Rick Grimes"},{"dob":"1909-05-05T00:00:00Z","name":"Glenn Rhee"},{"dob":"1909-01-10T00:00:00Z","name":"Daryl Dixon"},{"dob":"1901-01-15T00:00:00Z","name":"Andrea"}],"dob":"1910-01-01T00:00:00Z","name":"Michonne"}]}}`, js)
 }
 
 func TestRecurseQueryAllowLoop2(t *testing.T) {

query/recurse.go

@@ -27,7 +27,7 @@ import (
 )
 
 func (start *SubGraph) expandRecurse(ctx context.Context, maxDepth uint64) error {
-	// Note: Key format is - "attr|fromUID|toUID"
+	// Note: Key format is - "attr|toUID"
 	reachMap := make(map[string]struct{})
 	allowLoop := start.Params.RecurseArgs.AllowLoop
 	var numEdges uint64
@@ -118,15 +118,15 @@ func (start *SubGraph) expandRecurse(ctx context.Context, maxDepth uint64) error
 				sg.updateUidMatrix()
 			}
 
-			for mIdx, fromUID := range sg.SrcUIDs.Uids {
+			for mIdx := range sg.SrcUIDs.Uids {
 				if allowLoop {
 					for _, ul := range sg.uidMatrix {
 						numEdges += uint64(len(ul.Uids))
 					}
 				} else {
 					algo.ApplyFilter(sg.uidMatrix[mIdx], func(uid uint64, i int) bool {
-						key := fmt.Sprintf("%s|%d|%d", sg.Attr, fromUID, uid)
-						_, seen := reachMap[key] // Combine fromUID here.
+						key := fmt.Sprintf("%s|%d", sg.Attr, uid)
+						_, seen := reachMap[key]
 						if seen {
 							return false
 						}