fix(gqlParser): Handle strings with only whitespace in parseID (#6615)

ParseID function would panic if the input consists of only whitespace characters. This PR fixes it.
Fixes GRAPHQL-720

(cherry picked from commit d994cb3)

Conflicts:
	gql/parser_test.go

gql/parser.go

@@ -371,9 +371,6 @@ func substituteVariables(gq *GraphQuery, vmap varMap) error {
 
 	idVal, ok := gq.Args["id"]
 	if ok && len(gq.UID) == 0 {
-		if idVal == "" {
-			return errors.Errorf("Id can't be empty")
-		}
 		uids, err := parseID(idVal)
 		if err != nil {
 			return err
@@ -486,9 +483,6 @@ func substituteVariablesFilter(f *FilterTree, vmap varMap) error {
 				if !ok {
 					return errors.Errorf("Couldn't find value for GraphQL variable: [%s]", v.Value)
 				}
-				if idVal.Value == "" {
-					return errors.Errorf("Id can't be empty")
-				}
 				uids, err := parseID(idVal.Value)
 				if err != nil {
 					return err
@@ -2267,8 +2261,11 @@ loop:
 // Parses ID list. Only used for GraphQL variables.
 // TODO - Maybe get rid of this by lexing individual IDs.
 func parseID(val string) ([]uint64, error) {
-	var uids []uint64
 	val = x.WhiteSpace.Replace(val)
+	if val == "" {
+		return nil, errors.Errorf("ID can't be empty")
+	}
+	var uids []uint64
 	if val[0] != '[' {
 		uid, err := strconv.ParseUint(val, 0, 64)
 		if err != nil {

gql/parser_test.go

@@ -5275,3 +5275,13 @@ func TestFilterWithEmpty(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, gq.Query[0].Filter.Func.Args[0].Value, "")
 }
+
+func TestEmptyId(t *testing.T) {
+	q := "query me($a: string) { q(func: uid($a)) { name }}"
+	r := Request{
+		Str:       q,
+		Variables: map[string]string{"$a": "   "},
+	}
+	_, err := Parse(r)
+	require.Error(t, err, "ID cannot be empty")
+}