Filter empty matchers correctly

api/api.go

@@ -696,10 +696,16 @@ func matchFilterLabels(matchers []*labels.Matcher, sms map[string]string) bool {
 		v, prs := sms[m.Name]
 		switch m.Type {
 		case labels.MatchNotEqual, labels.MatchNotRegexp:
+			if string(v) == "" && prs {
+				return true
+			}
 			if !m.Matches(string(v)) {
 				return false
 			}
 		default:
+			if string(v) == "" && !prs {
+				return true
+			}
 			if !prs || !m.Matches(string(v)) {
 				return false
 			}

api/api_test.go

@@ -312,6 +312,38 @@ func TestReceiversMatchFilter(t *testing.T) {
 	require.False(t, receiversMatchFilter(receivers, filter))
 }
 
+func TestMatchFilterLabels(t *testing.T) {
+	testCases := []struct {
+		matcher  labels.MatchType
+		expected bool
+	}{
+		{labels.MatchEqual, true},
+		{labels.MatchRegexp, true},
+		{labels.MatchNotEqual, false},
+		{labels.MatchNotRegexp, false},
+	}
+
+	for _, tc := range testCases {
+		l, err := labels.NewMatcher(tc.matcher, "foo", "")
+		require.NoError(t, err)
+		sms := map[string]string{
+			"baz": "bar",
+		}
+		ls := []*labels.Matcher{l}
+
+		require.Equal(t, tc.expected, matchFilterLabels(ls, sms))
+
+		l, err = labels.NewMatcher(tc.matcher, "foo", "")
+		require.NoError(t, err)
+		sms = map[string]string{
+			"baz": "bar",
+			"foo": "quux",
+		}
+		ls = []*labels.Matcher{l}
+		require.NotEqual(t, tc.expected, matchFilterLabels(ls, sms))
+	}
+}
+
 func newMatcher(labelSet model.LabelSet) types.Matchers {
 	matchers := make([]*types.Matcher, 0, len(labelSet))
 	for key, val := range labelSet {

pkg/parse/parse.go

@@ -9,7 +9,7 @@ import (
 )
 
 var (
-	re      = regexp.MustCompile(`(?:\s?)(\w+)(=|=~|!=|!~)(?:\"([^"=~!]+)\"|([^"=~!]+))`)
+	re      = regexp.MustCompile(`(?:\s?)(\w+)(=|=~|!=|!~)(?:\"([^"=~!]+)\"|([^"=~!]+)|\"\")`)
 	typeMap = map[string]labels.MatchType{
 		"=":  labels.MatchEqual,
 		"!=": labels.MatchNotEqual,
@@ -66,7 +66,7 @@ func Input(s string) (name, value string, matchType labels.MatchType, err error)
 		value = ms[4]
 	}
 
-	if name == "" || value == "" || !prs {
+	if name == "" || !prs {
 		return "", "", labels.MatchEqual, fmt.Errorf("failed to parse")
 	}
 

pkg/parse/parse_test.go

@@ -101,6 +101,14 @@ func TestMatchers(t *testing.T) {
 				return append(ms, m, m2)
 			}(),
 		},
+		{
+			input: `{foo=""}`,
+			want: func() []*labels.Matcher {
+				ms := []*labels.Matcher{}
+				m, _ := labels.NewMatcher(labels.MatchEqual, "foo", "")
+				return append(ms, m)
+			}(),
+		},
 	}
 
 	for i, tc := range testCases {