hasher: fix error from trying to stop an already-stopped db

Before this change, Hasher would sometimes try to stop a bolt db that was
already stopped, resulting in an error. This change fixes the issue by checking
first whether the db is already stopped.

https://forum.rclone.org/t/hasher-with-gdrive-backend-does-not-return-sha1-sha256-for-old-files/44680/11?u=nielash

backend/hasher/hasher.go

@@ -439,7 +439,7 @@ func (f *Fs) DirMove(ctx context.Context, src fs.Fs, srcRemote, dstRemote string
 
 // Shutdown the backend, closing any background tasks and any cached connections.
 func (f *Fs) Shutdown(ctx context.Context) (err error) {
-	if f.db != nil {
+	if f.db != nil && !f.db.IsStopped() {
 		err = f.db.Stop(false)
 	}
 	if do := f.Fs.Features().Shutdown; do != nil {

lib/kv/bolt.go

@@ -293,6 +293,11 @@ func (db *DB) Stop(remove bool) error {
 	return db.Do(false, &opStop{remove: remove})
 }
 
+// IsStopped returns true if db is already stopped
+func (db *DB) IsStopped() bool {
+	return len(dbMap) == 0
+}
+
 // opStop: close database and stop operation loop
 type opStop struct {
 	remove bool

lib/kv/unsupported.go

@@ -36,5 +36,10 @@ func (*DB) Stop(remove bool) error {
 	return ErrUnsupported
 }
 
+// IsStopped returns true if db is already stopped
+func (db *DB) IsStopped() bool {
+	return true
+}
+
 // Exit stops all databases
 func Exit() {}