Fix PP callback bug

[DPeterK]

Fixes a bug (see #2052) where the IgnoreCubeException was not being respected in loading the loading of PP files with a user callback.

No tests yet, primarily because there are no tests for this behaviour at all (which is probably why this slipped through in the first place).

Closes #2052

[DPeterK]

I've now added a test! Its an integration test, because I couldn't figure out how to break the functionality of fileformats/rules.load_cubes down into unit-test-sized chunks. Also the behaviour caught here is integrated into the whole PP load framework, so I figure it fits into the integration tests.

[DPeterK]

Hopefully this will get the tests passing...

[DPeterK]

ping @bjlittle - passing tests!

[bjlittle]

@dkillick On it now ...

[bjlittle]

@dkillick This private functions doesn't appear to have changed, apart from its position in the file.

Sorry for being a pain, but could you paste it back, as it introduces unnecessary noise, thanks.

[DPeterK]

Oops! Swap did occur 😒 Will shift it back!

[bjlittle]

@dkillick For me, this is the nub of the problem, and it's not your doing.

I believe lines +1034-1035 should now simply be the following:

result = cube
if user_callback is not None:
    result = user_callback(cube, field, filename)
return result

I believe the intent of the loadcubes_user_callback_wrapper was simply to

1. raise the deprecation warning
2. fire any registered custom user rules, then
3. execute the user_callback.

The user_callback should not be executed here by iris.io.run_callback. That's the problem.

Rather, it is the loadcubes_user_callback_wrapper that should be executed by iris.io.run_callback downstream within _load_pairs_from_fields_and_filenames, as was prior to your changes.

So make the above change, then back-out your patch to _load_pairs_from_fields_and_filenames and replace it with

cube = iris.io.run_callback(user_callback, cube, field, filename)
if cube is None:
    continue

[bjlittle]

@dkillick Great, thanks!