`_abc._abc_subclasscheck` has very poor performance and (I think) a memory leak

首先執行提供的 Python 重現程式，並比較 ABCMeta 與 type，接著檢視連結的 PR gh-131914、gh-141171、gh-144941 和 gh-150540。完成的標準是識別並解決已回報的 issubclass 效能與記憶體增長行為，並提供來自重現程式的證據。

Bug report

I'm been hunting a memory leak in pydantic and I keep seeing _abc_subclasscheck popping up in the memray flamegraph.

Locally I've seen memray report that _abc_subclasscheck is using 2.8GB of memory in some cases!

I can't get anything that bad in a minimal example, but I have the following:

from abc import ABCMeta
from datetime import datetime


class MyMetaclass(ABCMeta):
    pass


class MyClass(metaclass=MyMetaclass):
    pass


def main():
    class Foobar(MyClass):
        pass

    assert issubclass(Foobar, MyClass)
    assert not issubclass(int, MyClass)
    assert not issubclass(str, MyClass)
    assert not issubclass(datetime, MyClass)
    t = type('A', (), {})
    assert not issubclass(t, MyClass)


if __name__ == '__main__':
    import os, psutil
    process = psutil.Process(os.getpid())
    mb = 1024 * 1024
    last = 0
    for i in range(5_000):
        main()
        # mem = process.memory_info().rss
        # print(f'{i + 1:>4d} {mem / mb:8.2f}MB {(mem - last) / mb:+8.2f}MB | {"━" * int(mem / 8_000_000)}')
        # last = mem

A few things to note:

• the commented out last few lines print current memory and change in memory over time
• I'm not sure exactly which issubclass calls are necessary, certainly I don't see a rise in memory with just the last one
• some memory drops (presumably related to clearing the abc caches?) is happening, but overall memory is increasing
• The performance of issubclass on an abc is very poor indeed - this script takes 14seconds, if i switch ABCMeta to type it takes 121ms!

Your environment

• Python 3.10.0 installed with venv
• Ubuntu 21.10

Linked PRs

• gh-131914
• gh-141171
• gh-144941
• gh-150540