Allow disabling WAL sync

This way Litestream can be used to only snapshot at regular intervals.

Initial snapshot on new generation is also disabled if snapshot interval
is set to zero.

replica.go

@@ -181,9 +181,15 @@ func (r *Replica) Sync(ctx context.Context) (err error) {
 
 	r.Logger().Log(ctx, LogLevelTrace, "replica sync", "position", dpos.String())
 
-	// Create a new snapshot and update the current replica position if
-	// the generation on the database has changed.
-	if r.Pos().Generation != generation {
+	// Prevent initial snapshot if both WAL sync and automatic snapshots are disabled.
+	if r.SyncInterval == 0 && r.SnapshotInterval == 0 {
+		r.mu.Lock()
+		r.pos = dpos
+		r.mu.Unlock()
+	} else if r.Pos().Generation != generation {
+		// Create a new snapshot and update the current replica position if
+		// the generation on the database has changed.
+
 		// Create snapshot if no snapshots exist for generation.
 		snapshotN, err := r.snapshotN(ctx, generation)
 		if err != nil {
@@ -228,6 +234,18 @@ func (r *Replica) syncWAL(ctx context.Context) (err error) {
 	}
 	defer rd.Close()
 
+	// If WAL sync is disabled, just advance our position.
+	if r.SyncInterval == 0 {
+		if _, err = io.Copy(io.Discard, rd); err != nil {
+			return err
+		}
+
+		r.mu.Lock()
+		r.pos = rd.Pos()
+		r.mu.Unlock()
+		return nil
+	}
+
 	// Copy shadow WAL to client write via io.Pipe().
 	pr, pw := io.Pipe()
 	defer func() { _ = pw.CloseWithError(err) }()
@@ -680,7 +698,12 @@ func (r *Replica) deleteWALSegmentsBeforeIndex(ctx context.Context, generation s
 
 // monitor runs in a separate goroutine and continuously replicates the DB.
 func (r *Replica) monitor(ctx context.Context) {
-	ticker := time.NewTicker(r.SyncInterval)
+	monitorInterval := r.SyncInterval
+	if monitorInterval == 0 {
+		monitorInterval = 10 * time.Second
+	}
+
+	ticker := time.NewTicker(monitorInterval)
 	defer ticker.Stop()
 
 	// Continuously check for new data to replicate.