Flip fixed-format cache to on by default

[ilevkivskyi]

Since this is scheduled for v1.20 it is better do this sooner than later. So that people who are using mypy master can give some early feedback.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[JukkaL]

Let's do it! Maybe we should also enable sqlite cache by default (in 1.20), for even more incremental mode performance gains?

[ilevkivskyi]

@JukkaL

Maybe we should also enable sqlite cache by default (in 1.20), for even more incremental mode performance gains?

Maybe. I am thinking maybe we should also add some helper script to extract cached content from DB, or do we already have one?

[ilevkivskyi]

Btw, I just noticed that some of the scripts (like convert-cache.py, diff-cache.py, apply-cache-diff.py) will not work with fixed format. The first one is probably easy to fix, the other two are less trivial.

[JukkaL]

I am thinking maybe we should also add some helper script to extract cached content from DB, or do we already have one?

We already have it: misc/convert-cache.py can convert between file-based and sqlite caches.

The first one is probably easy to fix, the other two are less trivial.

These are all worth fixing. I think they mostly work at file level, so supporting fixed format caches shouldn't be difficult, but I haven't looked into this in detail. Maybe we can just process the binary data for each .ff file as a single entity instead of using deserialized json data. The dependency cache files have special processing, but are they even using the fixed format?

[ilevkivskyi]

These are all worth fixing.

OK, I will make a PR to fix misc/convert-cache.py soon. I guess you can try fixing the other two, since you may be actually using these?

The dependency cache files have special processing, but are they even using the fixed format?

Deps files are still JSON, and TBH I remember very little about them, so I would prefer if you take a look at the cache diff scripts.

[ilevkivskyi]

It turns out fixing convert-cache.py was as simple as #20761. Cache diff scripts still need fixing.