add test which is broken with gevent

[davidhewitt]

Minimal reproduction of the case described in #3668

Here is a pretty typical output from the test:

start 0 0
gevent sleep 0 0
start 1 0
gevent sleep 1 0
after gevent sleep 0 0
dropping: ['key', ArbitraryClass(0, 0), 'ArbitraryClass(0, 0)', 'key', ArbitraryClass(1, 0), 'ArbitraryClass(1, 0)', None, 'ArbitraryClass(0, 0)']
del 0 0
del 1 0
end 0 0
start 0 1
gevent sleep 0 1
after gevent sleep 1 0
Fatal Python error: Segmentation fault

... i.e. after task 0 completes, task 1's pool of references is also freed. The crash when task 1 resumes is plain and simple UB; I have seen both segfaults and assertion failures.

[codspeed-hq]

CodSpeed Performance Report

Merging #3669 will not alter performance

_{Comparing davidhewitt:gevent (49d7718) with main (e99058a)}

Summary

✅ 78 untouched benchmarks