Fix `parser.VectorSelector.String()` with empty name matcher (#14015)

The check fell into "this matcher equals vector selector's name" case when vector selector doesn't have a name and the matcher is an explicit matcher for an empty __name__ label.

To provide some context about why this is important: some downstream projects use the promql.Parse(expr.String()) to clone an expression's AST, and with this bug that matcher disappears in the cloning.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

promql/parser/printer.go

@@ -204,8 +204,8 @@ func (node *VectorSelector) String() string {
 		labelStrings = make([]string, 0, len(node.LabelMatchers)-1)
 	}
 	for _, matcher := range node.LabelMatchers {
-		// Only include the __name__ label if its equality matching and matches the name.
-		if matcher.Name == labels.MetricName && matcher.Type == labels.MatchEqual && matcher.Value == node.Name {
+		// Only include the __name__ label if its equality matching and matches the name, but don't skip if it's an explicit empty name matcher.
+		if matcher.Name == labels.MetricName && matcher.Type == labels.MatchEqual && matcher.Value == node.Name && matcher.Value != "" {
 			continue
 		}
 		labelStrings = append(labelStrings, matcher.String())

promql/parser/printer_test.go

@@ -135,6 +135,9 @@ func TestExprString(t *testing.T) {
 		{
 			in: `a[1m] @ end()`,
 		},
+		{
+			in: `{__name__="",a="x"}`,
+		},
 	}
 
 	for _, test := range inputs {
@@ -216,6 +219,16 @@ func TestVectorSelector_String(t *testing.T) {
 			},
 			expected: `{__name__="foobar"}`,
 		},
+		{
+			name: "empty name matcher",
+			vs: VectorSelector{
+				LabelMatchers: []*labels.Matcher{
+					labels.MustNewMatcher(labels.MatchEqual, labels.MetricName, ""),
+					labels.MustNewMatcher(labels.MatchEqual, "a", "x"),
+				},
+			},
+			expected: `{__name__="",a="x"}`,
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			require.Equal(t, tc.expected, tc.vs.String())