Fix race condition crash on concurrent accesses to the same file

https://github.com/dutchcoders/transfer.sh/issues/380

server/handlers.go

@@ -625,23 +625,18 @@ func (metadata Metadata) remainingLimitHeaderValues() (remainingDownloads, remai
 	return remainingDownloads, remainingDays
 }
 
-func (s *Server) Lock(token, filename string) error {
+func (s *Server) Lock(token, filename string) {
 	key := path.Join(token, filename)
 
-	if _, ok := s.locks[key]; !ok {
-		s.locks[key] = &sync.Mutex{}
-	}
-
-	s.locks[key].Lock()
-
-	return nil
+	lock, _ := s.locks.LoadOrStore(key, &sync.Mutex{})
+	lock.(*sync.Mutex).Lock()
 }
 
-func (s *Server) Unlock(token, filename string) error {
+func (s *Server) Unlock(token, filename string) {
 	key := path.Join(token, filename)
-	s.locks[key].Unlock()
 
-	return nil
+	lock, _ := s.locks.LoadOrStore(key, &sync.Mutex{})
+	lock.(*sync.Mutex).Unlock()
 }
 
 func (s *Server) CheckMetadata(token, filename string, increaseDownload bool) (Metadata, error) {

server/server.go

@@ -282,7 +282,7 @@ type Server struct {
 
 	profilerEnabled bool
 
-	locks map[string]*sync.Mutex
+	locks sync.Map
 
 	maxUploadSize     int64
 	rateLimitRequests int
@@ -321,7 +321,7 @@ type Server struct {
 
 func New(options ...OptionFn) (*Server, error) {
 	s := &Server{
-		locks: map[string]*sync.Mutex{},
+		locks: sync.Map{},
 	}
 
 	for _, optionFn := range options {