rules: replace error strings with sentinel errors for duplicate labelsets (#17620)

Signed-off-by: Nikos Angelopoulos <nikolaos.angelopoulos@grafana.com>

rules/alerting.go

@@ -46,6 +46,10 @@ const (
 	alertStateLabel = "alertstate"
 )
 
+// ErrDuplicateAlertLabelSet is returned when an alerting rule evaluation produces
+// metrics with identical labelsets after applying alert labels.
+var ErrDuplicateAlertLabelSet = errors.New("vector contains metrics with the same labelset after applying alert labels")
+
 // AlertState denotes the state of an active alert.
 type AlertState int
 
@@ -441,7 +445,7 @@ func (r *AlertingRule) Eval(ctx context.Context, queryOffset time.Duration, ts t
 		resultFPs[h] = struct{}{}
 
 		if _, ok := alerts[h]; ok {
-			return nil, errors.New("vector contains metrics with the same labelset after applying alert labels")
+			return nil, ErrDuplicateAlertLabelSet
 		}
 
 		alerts[h] = &Alert{

rules/alerting_test.go

@@ -612,7 +612,7 @@ func TestAlertingRuleDuplicate(t *testing.T) {
 	)
 	_, err := rule.Eval(ctx, 0, now, EngineQueryFunc(engine, storage), nil, 0)
 	require.Error(t, err)
-	require.EqualError(t, err, "vector contains metrics with the same labelset after applying alert labels")
+	require.ErrorIs(t, err, ErrDuplicateAlertLabelSet)
 }
 
 func TestAlertingRuleLimit(t *testing.T) {

rules/recording.go

@@ -30,6 +30,10 @@ import (
 	"github.com/prometheus/prometheus/promql/parser"
 )
 
+// ErrDuplicateRecordingLabelSet is returned when a recording rule evaluation produces
+// metrics with identical labelsets after applying rule labels.
+var ErrDuplicateRecordingLabelSet = errors.New("vector contains metrics with the same labelset after applying rule labels")
+
 // A RecordingRule records its vector expression into new timeseries.
 type RecordingRule struct {
 	name   string
@@ -104,7 +108,7 @@ func (rule *RecordingRule) Eval(ctx context.Context, queryOffset time.Duration,
 	// Check that the rule does not produce identical metrics after applying
 	// labels.
 	if vector.ContainsSameLabelset() {
-		return nil, errors.New("vector contains metrics with the same labelset after applying rule labels")
+		return nil, ErrDuplicateRecordingLabelSet
 	}
 
 	numSeries := len(vector)

rules/recording_test.go

@@ -176,7 +176,7 @@ func TestRuleEvalDuplicate(t *testing.T) {
 	rule := NewRecordingRule("foo", expr, labels.FromStrings("test", "test"))
 	_, err := rule.Eval(ctx, 0, now, EngineQueryFunc(engine, storage), nil, 0)
 	require.Error(t, err)
-	require.EqualError(t, err, "vector contains metrics with the same labelset after applying rule labels")
+	require.ErrorIs(t, err, ErrDuplicateRecordingLabelSet)
 }
 
 func TestRecordingRuleLimit(t *testing.T) {