gh-144766: Fix a crash in fork child process when perf support is enabled.

[yilei]

Why? After fork, _PyPerfTrampoline_Fini may leave the old code watcher registered if trampolined code objects from the parent still exist (trampoline_refcount > 0).

This change clears it unconditionally before _PyPerfTrampoline_Init registers a new one, to prevent two watchers sharing the same globals and double-decrementing trampoline_refcount.

Also added test_trampoline_works_after_fork_with_many_code_objects, and verified it fails before the fix.

• Issue: perf_trampoline crash in the fork child process #144766

[yilei]

The macOS free-threading test failures seem unrelated.

[pablogsal]

This makes sense to me! Thanks for taking the time to produce a good test for this and the excellent investigation and the fix ❤️

[miss-islington-app]

Thanks @yilei for the PR, and @pablogsal for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @yilei and @pablogsal, I could not cleanly backport this to 3.13 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 5922149a5033ec1151320864e605adf88f53f280 3.13

[bedevere-app]

GH-144816 is a backport of this pull request to the 3.14 branch.

[pablogsal]

@yilei can you try to follow the instructions to do the 3.13 backport by hand? Otherwise I can do it later today so no worries :)

[bedevere-app]

GH-144818 is a backport of this pull request to the 3.13 branch.

[yilei]

@yilei can you try to follow the instructions to do the 3.13 backport by hand? Otherwise I can do it later today so no worries :)

I can get to it ~Monday if you haven't done so then.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Arch Linux Usan Function 3.13 (no tier) has failed when building commit 0151abc.

What do you need to do:

1. Don't panic.
2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1601/builds/656) and take a look at the build logs.
4. Check if the failure is related to this commit (0151abc) or if it is a false positive.
5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1601/builds/656

Failed tests:

• test_urllib2net

Summary of the results of the build (if available):

==

Click to see traceback logs

Note: switching to '0151abcd9f580f9460269f9787af7ad883c8362b'.

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Turn off this advice by setting config variable advice.detachedHead to false

HEAD is now at 0151abcd9f5 [3.13] gh-144766: Fix a crash in fork child process when perf support is enabled. (GH-144795) (#144818)
Switched to and reset branch '3.13'

make: *** [Makefile:2252: buildbottest] Error 2