python: fix quadratic behavior in collection of items using xunit fixtures

[bluetech]

Refs #4824 (as mentioned below there is still some slowness we should look at, so not closing)

Since commit 0f918b1 pytest uses auto-generated autouse pytest fixtures for the xunit fixtures {setup,teardown}_{module,class,method,function}. All of these fixtures were given the same name.

Unfortunately, pytest fixture lookup for a name works by grabbing all of the fixtures globally declared with a name and filtering to only those which match the specific node. So each xunit-using item iterates over a list (of fixturedefs) of a size of all previous same-xunit-using items, i.e. quadratic.

Fixing this properly to use a better data structure is likely to take some effort, but we can avoid the immediate problem by just using a different name for each item's autouse fixture, so it only matches itself.

A benchmark is added to demonstrate the issue. It is still way too slow after the fix and possibly still quadratic, but for a different reason which is another matter.

Running --collect-only, before (snipped):

         202533232 function calls (201902604 primitive calls) in 86.379 seconds

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000   85.688   85.688 main.py:320(pytest_collection)
        1    0.000    0.000   85.688   85.688 main.py:567(perform_collect)
80557/556    0.021    0.000   85.050    0.153 {method 'extend' of 'list' objects}
85001/15001  0.166    0.000   85.045    0.006 main.py:785(genitems)
    10002    0.050    0.000   84.219    0.008 runner.py:455(collect_one_node)
    10002    0.049    0.000   83.763    0.008 runner.py:340(pytest_make_collect_report)
    10002    0.079    0.000   83.668    0.008 runner.py:298(from_call)
    10002    0.019    0.000   83.541    0.008 runner.py:341(<lambda>)
     5001    0.184    0.000   81.922    0.016 python.py:412(collect)
     5000    0.020    0.000   81.072    0.016 python.py:842(collect)
    30003    0.118    0.000   78.478    0.003 python.py:218(pytest_pycollect_makeitem)
    30000    0.190    0.000   77.957    0.003 python.py:450(_genfunctions)
    40001    0.081    0.000   76.664    0.002 nodes.py:183(from_parent)
    30000    0.087    0.000   76.629    0.003 python.py:1595(from_parent)
    40002    0.092    0.000   76.583    0.002 nodes.py:102(_create)
    30000    0.305    0.000   76.404    0.003 python.py:1533(__init__)
    15000    0.132    0.000   74.765    0.005 fixtures.py:1439(getfixtureinfo)
    15000    0.165    0.000   73.664    0.005 fixtures.py:1492(getfixtureclosure)
    15000    0.044    0.000   57.584    0.004 fixtures.py:1653(getfixturedefs)
    30000   18.840    0.001   57.540    0.002 fixtures.py:1668(_matchfactories)
 37507500   31.352    0.000   38.700    0.000 nodes.py:76(ischildnode)
    15000   10.464    0.001   15.806    0.001 fixtures.py:1479(_getautousenames)
112930587/112910019   7.333    0.000    7.339    0.000 {built-in method builtins.len}

After:

         51890333 function calls (51259706 primitive calls) in 27.306 seconds

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
        1    0.000    0.000   26.783   26.783 main.py:320(pytest_collection)
        1    0.000    0.000   26.783   26.783 main.py:567(perform_collect)
80557/556    0.020    0.000   26.108    0.047 {method 'extend' of 'list' objects}
85001/15001  0.151    0.000   26.103    0.002 main.py:785(genitems)
    10002    0.047    0.000   25.324    0.003 runner.py:455(collect_one_node)
    10002    0.045    0.000   24.888    0.002 runner.py:340(pytest_make_collect_report)
    10002    0.069    0.000   24.805    0.002 runner.py:298(from_call)
    10002    0.017    0.000   24.690    0.002 runner.py:341(<lambda>)
     5001    0.168    0.000   23.150    0.005 python.py:412(collect)
     5000    0.019    0.000   22.223    0.004 python.py:858(collect)
    30003    0.101    0.000   19.818    0.001 python.py:218(pytest_pycollect_makeitem)
    30000    0.161    0.000   19.368    0.001 python.py:450(_genfunctions)
    30000    0.302    0.000   18.236    0.001 python.py:1611(from_parent)
    40001    0.084    0.000   18.051    0.000 nodes.py:183(from_parent)
    40002    0.116    0.000   17.967    0.000 nodes.py:102(_create)
    30000    0.308    0.000   17.770    0.001 python.py:1549(__init__)
    15000    0.117    0.000   16.111    0.001 fixtures.py:1439(getfixtureinfo)
    15000    0.134    0.000   15.135    0.001 fixtures.py:1492(getfixtureclosure)
    15000    9.320    0.001   14.738    0.001 fixtures.py:1479(_getautousenames)

[nicoddemus]

Great finding @bluetech, thanks a lot!

It is probably good to leave a comment in the code explaining why the name of the fixture is different with a link to the issue.

[RonnyPfannschmidt]

Lovely workaround and detailing of the underlying issue

Thanks!

[bluetech]

Thanks for the reviews! I added comments "Use a unique name to speed up lookup.".

[bluetech]

I forgot to add a changelog, will add in the next PR.