Prevent checkpoints during snapshots (#477)

db.go

@@ -50,6 +50,7 @@ type DB struct {
 	rtx      *sql.Tx       // long running read transaction
 	pageSize int           // page size, in bytes
 	notify   chan struct{} // closes on WAL change
+	chkMu    sync.Mutex    // checkpoint lock
 
 	fileInfo os.FileInfo // db info cached during init
 	dirInfo  os.FileInfo // parent dir info cached during init
@@ -1247,6 +1248,12 @@ func (db *DB) Checkpoint(ctx context.Context, mode string) (err error) {
 // checkpointAndInit performs a checkpoint on the WAL file and initializes a
 // new shadow WAL file.
 func (db *DB) checkpoint(ctx context.Context, generation, mode string) error {
+	// Try getting a checkpoint lock, will fail during snapshots.
+	if !db.chkMu.TryLock() {
+		return nil
+	}
+	defer db.chkMu.Unlock()
+
 	shadowWALPath, err := db.CurrentShadowWALPath(generation)
 	if err != nil {
 		return err
@@ -1482,6 +1489,19 @@ func (db *DB) CRC64(ctx context.Context) (uint64, Pos, error) {
 	return h.Sum64(), pos, nil
 }
 
+// BeginSnapshot takes an internal snapshot lock preventing checkpoints.
+//
+// When calling this the caller must also call EndSnapshot() once the snapshot
+// is finished.
+func (db *DB) BeginSnapshot() {
+	db.chkMu.Lock()
+}
+
+// EndSnapshot releases the internal snapshot lock that prevents checkpoints.
+func (db *DB) EndSnapshot() {
+	db.chkMu.Unlock()
+}
+
 // DefaultRestoreParallelism is the default parallelism when downloading WAL files.
 const DefaultRestoreParallelism = 8
 

replica.go

@@ -463,6 +463,10 @@ func (r *Replica) Snapshot(ctx context.Context) (info SnapshotInfo, err error) {
 	r.muf.Lock()
 	defer r.muf.Unlock()
 
+	// Prevent checkpoints during snapshot.
+	r.db.BeginSnapshot()
+	defer r.db.EndSnapshot()
+
 	// Issue a passive checkpoint to flush any pages to disk before snapshotting.
 	if _, err := r.db.db.ExecContext(ctx, `PRAGMA wal_checkpoint(PASSIVE);`); err != nil {
 		return info, fmt.Errorf("pre-snapshot checkpoint: %w", err)