Make filter labels consistent with Prometheus (#2403)

* Make filter labels consistent with Prometheus

Filtering the alert out when the label is missing precludes a
possible match for an empty value. This change allows the
match to be evaluated.

Closes #2342

Signed-off-by: Victor Araujo <vear91@gmail.com>

* Add tests for matchFilterLabels in v2 api

Signed-off-by: Victor Araujo <vear91@gmail.com>

api/v1/api.go

@@ -662,7 +662,7 @@ func matchFilterLabels(matchers []*labels.Matcher, sms map[string]string) bool {
 			if string(m.Value) == "" && !prs {
 				continue
 			}
-			if !prs || !m.Matches(string(v)) {
+			if !m.Matches(string(v)) {
 				return false
 			}
 		}

api/v2/api.go

@@ -551,7 +551,7 @@ func matchFilterLabels(matchers []*labels.Matcher, sms map[string]string) bool {
 			if m.Value == "" && !prs {
 				continue
 			}
-			if !prs || !m.Matches(v) {
+			if !m.Matches(v) {
 				return false
 			}
 		}

api/v2/api_test.go

@@ -25,6 +25,7 @@ import (
 	open_api_models "github.com/prometheus/alertmanager/api/v2/models"
 	general_ops "github.com/prometheus/alertmanager/api/v2/restapi/operations/general"
 	"github.com/prometheus/alertmanager/config"
+	"github.com/prometheus/alertmanager/pkg/labels"
 	"github.com/prometheus/alertmanager/types"
 )
 
@@ -171,3 +172,41 @@ func TestAlertToOpenAPIAlert(t *testing.T) {
 		},
 	}, openAPIAlert)
 }
+
+func TestMatchFilterLabels(t *testing.T) {
+	sms := map[string]string{
+		"foo": "bar",
+	}
+
+	testCases := []struct {
+		matcher  labels.MatchType
+		name     string
+		val      string
+		expected bool
+	}{
+		{labels.MatchEqual, "foo", "bar", true},
+		{labels.MatchEqual, "baz", "", true},
+		{labels.MatchEqual, "baz", "qux", false},
+		{labels.MatchEqual, "baz", "qux|", false},
+		{labels.MatchRegexp, "foo", "bar", true},
+		{labels.MatchRegexp, "baz", "", true},
+		{labels.MatchRegexp, "baz", "qux", false},
+		{labels.MatchRegexp, "baz", "qux|", true},
+		{labels.MatchNotEqual, "foo", "bar", false},
+		{labels.MatchNotEqual, "baz", "", false},
+		{labels.MatchNotEqual, "baz", "qux", true},
+		{labels.MatchNotEqual, "baz", "qux|", true},
+		{labels.MatchNotRegexp, "foo", "bar", false},
+		{labels.MatchNotRegexp, "baz", "", false},
+		{labels.MatchNotRegexp, "baz", "qux", true},
+		{labels.MatchNotRegexp, "baz", "qux|", false},
+	}
+
+	for _, tc := range testCases {
+		m, err := labels.NewMatcher(tc.matcher, tc.name, tc.val)
+		require.NoError(t, err)
+
+		ms := []*labels.Matcher{m}
+		require.Equal(t, tc.expected, matchFilterLabels(ms, sms))
+	}
+}