delete signal listeners should use on_commit

The delete signal handlers like https://github.com/wagtail/wagtail/blob/master/wagtail/wagtailimages/signal_handlers.py#L9 should use transaction.on_commit() so that they wait to delete until the sql delete is successful

Something like:

from django.db import transaction
def post_delete_file_cleanup(instance, **kwargs):
    transaction.on_commit(lambda: instance.file.delete(False))

[thenewguy]

@BertrandBordage Sure. I will try to get to it tomorrow. I’ve already written tests that I should be able to copy and paste

[thenewguy]

This test is failing because the test uses a file name that clashes with other tests that do not clean up their test documents.

The other failing tests occur on databases other than sqllite are slightly more difficult to fix because the vagrant-wagtail-development doesn’t seem to offer testing on the different databases and travis tests are slow. Seems to be related to serialized_rollback = True which wasn’t required when I originally wrote the tests against postgres. But copying them over to the vagrant-wagtail-develop box seems to force sqlite