Fix FindMinSnapshotByGeneration() loop ref bug

Previously, a bug was introduced that added a `LIST` operation
on every replica sync which significantly increased the cost of
running Litestream against S3. This changes the behavior to only
issue the `LIST` operation when the generation has changed.

litestream.go

@@ -207,11 +207,13 @@ func FilterSnapshotsAfter(a []SnapshotInfo, t time.Time) []SnapshotInfo {
 // FindMinSnapshotByGeneration finds the snapshot with the lowest index in a generation.
 func FindMinSnapshotByGeneration(a []SnapshotInfo, generation string) *SnapshotInfo {
 	var min *SnapshotInfo
-	for _, snapshot := range a {
+	for i := range a {
+		snapshot := &a[i]
+
 		if snapshot.Generation != generation {
 			continue
 		} else if min == nil || snapshot.Index < min.Index {
-			min = &snapshot
+			min = snapshot
 		}
 	}
 	return min

litestream_test.go

@@ -128,6 +128,16 @@ func TestWALSegmentPath(t *testing.T) {
 	})
 }
 
+func TestFindMinSnapshotByGeneration(t *testing.T) {
+	infos := []litestream.SnapshotInfo{
+		{Generation: "29cf4bced74e92ab", Index: 0},
+		{Generation: "5dfeb4aa03232553", Index: 24},
+	}
+	if got, want := litestream.FindMinSnapshotByGeneration(infos, "29cf4bced74e92ab"), &infos[0]; got != want {
+		t.Fatalf("info=%#v, want %#v", got, want)
+	}
+}
+
 func MustDecodeHexString(s string) []byte {
 	b, err := hex.DecodeString(s)
 	if err != nil {

replica.go

@@ -159,22 +159,21 @@ func (r *Replica) Sync(ctx context.Context) (err error) {
 
 	Tracef("%s(%s): replica sync: db.pos=%s", r.db.Path(), r.Name(), dpos)
 
-	// Create snapshot if no snapshots exist for generation.
-	snapshotN, err := r.snapshotN(generation)
-	if err != nil {
-		return err
-	} else if snapshotN == 0 {
-		if info, err := r.Snapshot(ctx); err != nil {
-			return err
-		} else if info.Generation != generation {
-			return fmt.Errorf("generation changed during snapshot, exiting sync")
-		}
-		snapshotN = 1
-	}
-	replicaSnapshotTotalGaugeVec.WithLabelValues(r.db.Path(), r.Name()).Set(float64(snapshotN))
-
-	// Determine position, if necessary.
+	// Create a new snapshot and update the current replica position if
+	// the generation on the database has changed.
 	if r.Pos().Generation != generation {
+		// Create snapshot if no snapshots exist for generation.
+		snapshotN, err := r.snapshotN(generation)
+		if err != nil {
+			return err
+		} else if snapshotN == 0 {
+			if info, err := r.Snapshot(ctx); err != nil {
+				return err
+			} else if info.Generation != generation {
+				return fmt.Errorf("generation changed during snapshot, exiting sync")
+			}
+		}
+
 		pos, err := r.calcPos(ctx, generation)
 		if err != nil {
 			return fmt.Errorf("cannot determine replica position: %s", err)
@@ -1307,13 +1306,6 @@ func (r *Replica) downloadWAL(ctx context.Context, generation string, index int,
 
 // Replica metrics.
 var (
-	replicaSnapshotTotalGaugeVec = promauto.NewGaugeVec(prometheus.GaugeOpts{
-		Namespace: "litestream",
-		Subsystem: "replica",
-		Name:      "snapshot_total",
-		Help:      "The current number of snapshots",
-	}, []string{"db", "name"})
-
 	replicaWALBytesCounterVec = promauto.NewCounterVec(prometheus.CounterOpts{
 		Namespace: "litestream",
 		Subsystem: "replica",