Use report_config_data mypy plugin hook to reset cache on changes

[sobolevn]

Now, when any of our settings will change, mypy will be clear the existing cache. This is especially useful for stateful checks: incremental, dmypy, etc.

However, I don't think that it is possible to easily test this. This is how it is tested in mypy itself:

[case testPluginConfigData]
# flags: --config-file tmp/mypy.ini
import a
import b
[file a.py]
[file b.py]
[file test.json]
{"a": false, "b": false}
[file test.json.2]
{"a": true, "b": false}

[file mypy.ini]
\[mypy]
plugins=<ROOT>/test-data/unit/plugins/config_data.py

# The config change will force a to be rechecked but not b.
[rechecked a]

The only testing I've done is manual. It seems to work.

Closes #1568

[flaeppe]

Cool stuff 👍

[flaeppe]

Would be interesting to experiment with models, etc

[sobolevn]

@flaeppe any feedback from real-life projects is welcome 😉

[flaeppe]

Sure, I'll try it out.

Additional thought: would this invalidate the whole cache if settings changes? Perhaps that is a bit drastic? Wouldn't it be enough to purge the cache for just settings file(s)?

I suppose it could be problematic to try to purge cache of any imports in a settings file though.

First thing coming to mind: a configuration declaring additional settings files to purge. Although theoretically it should be possible to automatically figure out additional settings modules to purge

[sobolevn]

Additional thought: would this invalidate the whole cache if settings changes? Perhaps that is a bit drastic? Wouldn't it be enough to purge the cache for just settings file(s)?

No, because settings can change installed apps, models, urls, etc.

[flaeppe]

Yeah, I suppose

[flaeppe]

I was looking at this like: if any contents of settings file(s) changes. But I realised now when testing it out that wasn't the case.