drf_decorator.py

def exclusive_lock(method):
    @wraps(method)
    def wrapped_method(self, request, *args, document_pk=None, **kwargs):
        document = self.assert_user_has_write_permission(document_pk)
        document_state = self.get_document_state(document=document)
        # currently there is no transaction,
        # the context managers will create/release as needed
        with document_state.get_exclusive_lock():
            # we have exclusive access
            try:
                # call the method we're wrapping
                return method(
                    self, request, *args,
                    document_pk=document_pk,
                    document=document,
                    document_state=document_state,
                    **kwargs)

            except NodeRepoException:
                # there was an error, reload the document and try again
                document_reloader = DocumentReloader(document_state)
                # start the cleaner
                task = document_reloader.run_task()
        # the document's  reload should be taking a little longer for the next lock
        with document_state.get_exclusive_lock(
                timeout=settings.PG_LOCKING_TIMEOUT_MS * 5):
            # task ran in another thread
            # by fetching the result, 
            # we will raise any exception it had here
            task.result()
            # we need to get the state again, it might have change?
            document_state = DocumentState.objects.get(document=document)
            # if there is an error, we raise it back to the client
            return method(
                self, request, *args,
                document=document,
                document_state=document_state,
                document_pk=document_pk, **kwargs)

    return wrapped_method