black behaves differently in duplicated directories

[srittau]

This came up in python/typeshed#5250 and I am a bit stumped. Using Python 3.9.4 on Debian (also tried with Ubuntu), black 21.4b0 I get two different results using two identical directories:

srittau@blitz:~/Projekte$ diff -u --recursive typeshed typeshed2
srittau@blitz:~/Projekte$ 

(.venv-3.9) srittau@blitz:~/Projekte/typeshed$ black stubs/ stdlib/ --check
All done! ✨ 🍰 ✨
1334 files would be left unchanged.
(.venv-3.9) srittau@blitz:~/Projekte/typeshed$ 

(.venv-3.9) srittau@blitz:~/Projekte/typeshed2$ black stubs/ stdlib/ --check
would reformat stdlib/logging/config.pyi
would reformat stdlib/importlib/util.pyi
would reformat stdlib/distutils/filelist.pyi
would reformat stdlib/smtpd.pyi
would reformat stdlib/_typeshed/__init__.pyi
would reformat stubs/click/click/formatting.pyi
would reformat stubs/click-spinner/click_spinner/__init__.pyi
would reformat stubs/protobuf/google/protobuf/json_format.pyi
would reformat stdlib/ftplib.pyi
would reformat stubs/redis/redis/lock.pyi
would reformat stubs/waitress/waitress/compat.pyi
would reformat stubs/protobuf/google/protobuf/descriptor.pyi
would reformat stdlib/@python2/logging/__init__.pyi
would reformat stubs/JACK-Client/jack/__init__.pyi
would reformat stdlib/asyncio/base_events.pyi
would reformat stdlib/typing.pyi
would reformat stubs/redis/redis/client.pyi
Oh no! 💥 💔 💥
17 files would be reformatted, 1317 files would be left unchanged.
(.venv-3.9) srittau@blitz:~/Projekte/typeshed2$ 

Each using the venv in the respective directory, same result when re-initializing the venv or when using a common venv and deleting the local venvs.

In the typeshed case, i.e. where running black --check passes, it also shows a diff when running black --check --diff, meaning there is an inconsistency between --diff and --diff --check. I am stumped about what is going on here, but it's quite strange. Any pointers at what could be going here or how to debug this are appreciated.

[ambv]

This is in all likelihood this -> #2130

We will be releasing a hotfix soon for this.

[srittau]

I suspected something like this, but wasn't able to find the cache. But now I did. I can confirm this problem goes away with a cache wipe.