gh-158254: Detect keys changing during reverse dict iteration

[jab]

Reverse dict iterators only checked that the dict's size was unchanged. So after clear() and a refill to the same size that leaves live entries below the iterator's position, they kept yielding entries for the new keys. They could yield more items than the dict had when the iterator was created, and __length_hint__() then wrapped around to 2**64-1.

This adds the check the forward iterators already have (e.g. dictiter_iternextkey_lock_held): finding an entry after the expected number of items raises RuntimeError: dictionary keys changed during iteration.

The new test fails without the change ("RuntimeError not raised") and passes with it. The full test suite passes on a macOS arm64 debug build (not free-threaded).

This change was written with the help of Claude Code, an AI assistant. I have reviewed it.

🤖 Generated with Claude Code

• Issue: Reverse dict iterators don't detect keys changing during iteration #158254

[ZeroIntensity]

LGTM

[miss-islington-app]

Thanks @jab for the PR, and @ZeroIntensity for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13, 3.14, 3.15.
🐍🍒⛏🤖

[bedevere-app]

GH-158267 is a backport of this pull request to the 3.15 branch.

[bedevere-app]

GH-158268 is a backport of this pull request to the 3.14 branch.

[bedevere-app]

GH-158269 is a backport of this pull request to the 3.13 branch.

[bedevere-bot]

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Windows Server 2022 NoGIL 3.14 (tier-1) has failed when building commit f085176.

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/1717/builds/1783) and take a look at the build logs.
4. Check if the failure is related to this commit (f085176) 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/1717/builds/1783

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

==

Click to see traceback logs

Traceback (most recent call last):
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\threading.py", line 1082, in _bootstrap_inner
    self._context.run(self.run)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\threading.py", line 1024, in run
    self._target(*self._args, **self._kwargs)
    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\multiprocessing\pool.py", line 595, in _handle_results
    cache[job]._set(i, obj)
    ~~~~~~~~~~~~~~~^^^^^^^^
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\multiprocessing\pool.py", line 783, in _set
    del self._cache[self._job]
        ~~~~~~~~~~~^^^^^^^^^^^
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\multiprocessing\pool.py", line 171, in __delitem__
    self.notifier.put(None)
    ~~~~~~~~~~~~~~~~~^^^^^^
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\multiprocessing\queues.py", line 394, in put
    self._writer.send_bytes(obj)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\multiprocessing\connection.py", line 210, in send_bytes
    self._send_bytes(m[offset:offset + size])
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\bbarea\3.14.itamaro-win64-srv-22-aws.x64.nogil\build\Lib\multiprocessing\connection.py", line 297, in _send_bytes
    raise ValueError("concurrent send_bytes() calls "
                     "are not supported")
ValueError: concurrent send_bytes() calls are not supported
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