Expire pending silence and move to expired state

Instead of setting endsAt to startsAt we can set both to now. Thereby
the Silence will get the expired state by default.

silence/silence.go

@@ -455,7 +455,9 @@ func (s *Silences) expire(id string) error {
 	case StateActive:
 		sil.EndsAt = now
 	case StatePending:
-		sil.EndsAt = sil.StartsAt
+		// Set both to now to make Silence move to "expired" state
+		sil.StartsAt = now
+		sil.EndsAt = now
 	}
 
 	return s.setSilence(sil)

silence/silence_test.go

@@ -23,6 +23,7 @@ import (
 	"time"
 
 	pb "github.com/prometheus/alertmanager/silence/silencepb"
+	"github.com/prometheus/alertmanager/types"
 	"github.com/prometheus/common/model"
 	"github.com/stretchr/testify/require"
 	"github.com/weaveworks/mesh"
@@ -752,10 +753,14 @@ func TestSilenceExpire(t *testing.T) {
 	require.Equal(t, &pb.Silence{
 		Id:        "pending",
 		Matchers:  []*pb.Matcher{m},
-		StartsAt:  now.Add(time.Minute),
-		EndsAt:    now.Add(time.Minute),
+		StartsAt:  now,
+		EndsAt:    now,
 		UpdatedAt: now,
 	}, sil)
+	// Expiring a pending Silence should make the API return the
+	// SilenceStateExpired Silence state.
+	silenceState := types.CalcSilenceState(sil.StartsAt, sil.EndsAt)
+	require.Equal(t, silenceState, types.SilenceStateExpired)
 
 	sil, err = s.QueryOne(QIDs("active"))
 	require.NoError(t, err)