pytest: do not store .pytest_cache in $out

[FRidh]

Issue description

In #40259 a collision of .pytest_cache files was reported. These files should not exist in the first place.

They only seem to be introduced when running pytest with $out as target, so

pytest $out

[FRidh]

Occurrences:

$ git grep 'py.test $out'
pkgs/development/python-modules/autobahn/default.nix:    py.test $out
pkgs/development/python-modules/cairocffi/default.nix:    py.test $out/${python.sitePackages}
pkgs/development/python-modules/hyperlink/default.nix:    py.test $out
pkgs/development/python-modules/pandas/default.nix:    py.test $out/${python.sitePackages}/pandas --skip-slow --skip-network \
pkgs/development/python-modules/pyzmq/default.nix:    py.test $out/${python.sitePackages}/zmq/ -k "not test_socket \
pkgs/development/python-modules/spake2/default.nix:    py.test $out
pkgs/development/python-modules/xarray/default.nix:    py.test $out/${python.sitePackages}
pkgs/top-level/python-packages.nix:      py.test $out/${python.sitePackages}
pkgs/top-level/python-packages.nix:      py.test $out
pkgs/top-level/python-packages.nix:      py.test $out
pkgs/top-level/python-packages.nix:      py.test $out/${python.sitePackages}

[FRidh]

Quoting from #40259 (comment):

Just adding for searchability/reference: et_xmlfile still includes this, and pytest could alternatively be called like py.test -p no:cacheprovider $out if we don't want this cache dir created at all.

[FRidh]

Fix in 610485f

[FRidh]

Seems like the issue is still present (#44479):

nix-shell -p 'python3.withPackages(ps: with ps; [ cairocffi tinycss ])' -I nixpkgs=.

[FRidh]

Looks like a typo; it should be .pytest_cache and not .pytest-cache.

[lopsided98]

This doesn't seem to fix the problem, because fixupPhase is run before installCheckPhase, which is where python packages run their tests.

[FRidh]

It is indeed the incorrect phase, I forgot about the order. The solution is to use postCheckHooks instead. However, it is important that when checkPhase is overridden, the postHook is still ran. Here we could use NixOS/rfcs#32.

[FRidh]

New solution adding a new phase: #45898

[FRidh]

Latest fix landed in master. Now, it still needs to be backported.

[FRidh]

I've pushed the fixes to staging-18.09. Now we need to wait for that to land in release-18.09.

[matthewbauer]

Looks to now have landed.

[alexeymuranov]

@matthewbauer , are you sure? I do not see staging-18.09 merged into release-18.09 since a while...

[alexeymuranov]

The related #47285 is still an issue on 18.09.