Worker: more granular locking when flushing upstream buffer

Only lock the database mutex when actual queries are performed, but not
during the entire flush loop.

internal/worker/upstream_buffer.go

@@ -227,15 +227,15 @@ func (ub *UpstreamBufferDB) QueueSize() (int, error) {
 }
 
 func (ub *UpstreamBufferDB) Flush(ctx context.Context) error {
-	ub.dbMutex.Lock()
-	defer ub.dbMutex.Unlock()
-
 	if ub.db == nil {
 		log.Panic().Msg("no database opened, unable to queue task updates")
 	}
 
 	// See if we need to flush at all.
+	ub.dbMutex.Lock()
 	queueSize, err := ub.queueSize()
+	ub.dbMutex.Unlock()
+
 	switch {
 	case err != nil:
 		return fmt.Errorf("unable to determine queue size: %w", err)
@@ -247,7 +247,9 @@ func (ub *UpstreamBufferDB) Flush(ctx context.Context) error {
 	// Keep flushing until the queue is empty or there is an error.
 	var done bool
 	for !done {
+		ub.dbMutex.Lock()
 		done, err = ub.flushFirstItem(ctx)
+		ub.dbMutex.Unlock()
 
 		if err != nil {
 			return err