chore(db): use shorter read transactions and periodic checkpoint for smaller WAL (#10027)

Also make sure our journal size limit is in effect when checkpointing.

lib/model/indexhandler.go

@@ -295,7 +295,6 @@ func (s *indexHandler) sendIndexTo(ctx context.Context) error {
 	var f protocol.FileInfo
 	previousWasDelete := false
 
-	t0 := time.Now()
 	for fi, err := range itererr.Zip(s.sdb.AllLocalFilesBySequence(s.folder, protocol.LocalDeviceID, s.localPrevSequence+1, 5000)) {
 		if err != nil {
 			return err
@@ -304,15 +303,7 @@ func (s *indexHandler) sendIndexTo(ctx context.Context) error {
 		// Even if the batch is full, we allow a last delete to slip in, we do this by making sure that
 		// the batch ends with a non-delete, or that the last item in the batch is already a delete
 		if batch.Full() && (!fi.IsDeleted() || previousWasDelete) {
-			if err := batch.Flush(); err != nil {
-				return err
-			}
-			if time.Since(t0) > 5*time.Second {
-				// minor hack -- avoid very long running read transactions
-				// during index transmission, to help prevent excessive
-				// growth of database WAL file
-				break
-			}
+			break
 		}
 
 		if fi.SequenceNo() < s.localPrevSequence+1 {
@@ -348,11 +339,7 @@ func (s *indexHandler) sendIndexTo(ctx context.Context) error {
 
 		batch.Append(f)
 	}
-	if err := batch.Flush(); err != nil {
-		return err
-	}
-
-	return nil
+	return batch.Flush()
 }
 
 func (s *indexHandler) receive(fs []protocol.FileInfo, update bool, op string, prevSequence, lastSequence int64) error {