Enforce stricter validation on restart.

Previously, the sync would validate the last page written to ensure
that replication picked up from the last position. However, a large
WAL file followed by a series of shorter checkpointed WAL files means
that the last page could be the same even if multiple checkpoints
have occurred.

To fix this, the WAL header must match the shadow WAL header when
starting litestream since there are no guarantees about checkpoints.

db.go

@@ -419,6 +419,16 @@ func (db *DB) init() (err error) {
 		return err
 	}
 
+	// If we have an existing shadow WAL, ensure the headers match.
+	if ok, err := db.checkHeadersMatch(); err != nil {
+		return err
+	} else if !ok {
+		log.Printf("%s: cannot determine last wal position, clearing generation", db.path)
+		if err := os.Remove(db.GenerationNamePath()); err != nil && !os.IsNotExist(err) {
+			return fmt.Errorf("remove generation name: %w", err)
+		}
+	}
+
 	// Clean up previous generations.
 	if err := db.clean(); err != nil {
 		return fmt.Errorf("clean: %w", err)
@@ -432,6 +442,39 @@ func (db *DB) init() (err error) {
 	return nil
 }
 
+// checkHeadersMatch returns true if the primary WAL and last shadow WAL header match.
+func (db *DB) checkHeadersMatch() (bool, error) {
+	// Determine current generation.
+	generation, err := db.CurrentGeneration()
+	if err != nil {
+		return false, err
+	} else if generation == "" {
+		return false, nil // no generation
+	}
+
+	// Find current generation & latest shadow WAL.
+	shadowWALPath, err := db.CurrentShadowWALPath(generation)
+	if err != nil {
+		return false, fmt.Errorf("cannot determine current shadow wal path: %w", err)
+	}
+
+	hdr0, err := readWALHeader(db.WALPath())
+	if os.IsNotExist(err) {
+		return false, nil // no primary wal
+	} else if err != nil {
+		return false, fmt.Errorf("cannot read wal header: %w", err)
+	}
+
+	hdr1, err := readWALHeader(shadowWALPath)
+	if os.IsNotExist(err) {
+		return false, nil // no shadow wal
+	} else if err != nil {
+		return false, fmt.Errorf("cannot read shadow wal header: %w", err)
+	}
+
+	return bytes.Equal(hdr0, hdr1), nil
+}
+
 // clean removes old generations & WAL files.
 func (db *DB) clean() error {
 	if err := db.cleanGenerations(); err != nil {