Schema editors leak transaction state on exception

[Michal Petrucha]

When a schema editor is used as a context manager and an uncaught exception is raised within its block, BaseDatabaseSchemaEditor's __exit__ method doesn't call atomic.__exit__, which leaves the transaction opened when entering the block open.

I'm going to open a pull request shortly with a simple patch which does fix things for me, but I'm not sure about its correctness and I have no idea how to test this. An alternative approach might be to store an instance of Atomic as an attribute in BaseDatabaseSchemaEditor and call its __enter__ and __exit__ instead of creating separate instances; I have no idea whether that makes any difference or not.

[Aymeric Augustin]

I'm probably to blame for suggesting the __enter__ / __exit__ hack to Andrew, but I have to say this is very bad.

If we're using this hack, we must guarantee that __exit__ will be called with robust exception handling.

[Michal Petrucha]

The pull request can be found here: ​https://github.com/django/django/pull/1545

[tuxcanfly]

Added a simple test:

​https://github.com/koniiiik/django/pull/2

As expected, the test fails on master, but passes with the patch (by koniiiik) applied.

[Tim Graham]

Fix was committed in 630eb0564abd228da439d86ad93acb4089d795e7, but not the test.

[Andrew Godwin <andrew@…>]

In 6f7977bb63ea592faaa7b3bdf2898f8361f30260:

Fixed #21029: Test for previously-commited SchemaEditor.exit bug.