Allow querying all lang values of a predicate. (dgraph-io#2910)

This PR introduces the syntax pred@* which returns all the values of a
predicate which contain a language tag.

Author: martinmr

gql/parser.go

@@ -1561,6 +1561,10 @@ L:
 				function.Attr = val
 				attrItemsAgo = 0
 			} else if expectLang {
+				if val == "*" {
+					return nil, x.Errorf(
+						"The * symbol cannot be used as a valid language inside functions")
+				}
 				function.Lang = val
 				expectLang = false
 			} else if function.Name != uid {
@@ -2130,6 +2134,14 @@ func parseLanguageList(it *lex.ItemIterator) ([]string, error) {
 	}
 	it.Prev()
 
+	for _, lang := range langs {
+		if lang == string(star) && len(langs) > 1 {
+			return nil, x.Errorf(
+				"If * is used, no other languages are allowed in the language list. Found %v",
+				langs)
+		}
+	}
+
 	return langs, nil
 }
 

gql/parser_test.go

@@ -2567,6 +2567,22 @@ func TestLangs(t *testing.T) {
 	require.Equal(t, []string{"en", "ru", "hu"}, gq.Query[0].Children[1].Langs)
 }
 
+func TestAllLangs(t *testing.T) {
+	query := `
+	query {
+		me(func: uid(1)) {
+			name@*
+		}
+	}
+	`
+
+	gq, err := Parse(Request{Str: query})
+	require.NoError(t, err)
+	require.Equal(t, 1, len(gq.Query[0].Children))
+	require.Equal(t, "name", gq.Query[0].Children[0].Attr)
+	require.Equal(t, []string{"*"}, gq.Query[0].Children[0].Langs)
+}
+
 func TestLangsInvalid1(t *testing.T) {
 	query := `
 	query {
@@ -2665,6 +2681,36 @@ func TestLangsInvalid7(t *testing.T) {
 	require.Contains(t, err.Error(), "Expected only one dot(.) while parsing language list.")
 }
 
+func TestLangsInvalid8(t *testing.T) {
+	query := `
+	query {
+		me(func: uid(1)) {
+			name@*:en
+		}
+	}
+	`
+
+	_, err := Parse(Request{Str: query})
+	require.Error(t, err)
+	require.Contains(t, err.Error(),
+		"If * is used, no other languages are allowed in the language list")
+}
+
+func TestLangsInvalid9(t *testing.T) {
+	query := `
+	query {
+		me(func: eqs(name@*, "Amir")) {
+			name@en
+		}
+	}
+	`
+
+	_, err := Parse(Request{Str: query})
+	require.Error(t, err)
+	require.Contains(t, err.Error(),
+		"The * symbol cannot be used as a valid language inside functions")
+}
+
 func TestLangsFilter(t *testing.T) {
 	query := `
 	query {

gql/state.go

@@ -38,6 +38,7 @@ const (
 	colon       = ':'
 	lsThan      = '<'
 	grThan      = '>'
+	star        = '*'
 )
 
 // Constants representing type of different graphql lexed items.
@@ -338,7 +339,7 @@ func lexFilterFuncName(l *lex.Lexer) lex.StateFn {
 func lexDirectiveOrLangList(l *lex.Lexer) lex.StateFn {
 	r := l.Next()
 	// Check first character.
-	if !isNameBegin(r) && r != period {
+	if !isNameBegin(r) && r != period && r != star {
 		return l.Errorf("Unrecognized character in lexDirective: %#U", r)
 	}
 	l.Backup()
@@ -543,6 +544,9 @@ func isLangOrDirective(r rune) bool {
 	if r >= '0' && r <= '9' {
 		return true
 	}
+	if r == '*' {
+		return true
+	}
 	return false
 }
 

query/query.go

@@ -991,6 +991,13 @@ func createTaskQuery(sg *SubGraph) (*pb.Query, error) {
 			}
 		}
 	}
+
+	// If the lang is set to *, query all the languages.
+	if len(sg.Params.Langs) == 1 && sg.Params.Langs[0] == "*" {
+		sg.Params.expandAll = true
+		sg.Params.Langs = nil
+	}
+
 	out := &pb.Query{
 		ReadTs:       sg.ReadTs,
 		Attr:         attr,
@@ -1003,6 +1010,7 @@ func createTaskQuery(sg *SubGraph) (*pb.Query, error) {
 		FacetsFilter: sg.facetsFilter,
 		ExpandAll:    sg.Params.expandAll,
 	}
+
 	if sg.SrcUIDs != nil {
 		out.UidList = sg.SrcUIDs
 	}

query/query0_test.go

@@ -123,6 +123,18 @@ func TestQueryNamesInLanguage(t *testing.T) {
 		js)
 }
 
+func TestQueryAllLanguages(t *testing.T) {
+	query := `{
+	  people(func: eq(name@hi, "अमित")) {
+		name@*
+	  }
+	}`
+	js := processToFastJsonNoErr(t, query)
+	require.JSONEq(t,
+		`{"data":{"people": [{"name@en":"Amit", "name@hi":"अमित", "name":""}]}}`,
+		js)
+}
+
 func TestQueryNamesBeforeA(t *testing.T) {
 	query := `{
 	  people(func: lt(name, "A")) {