PyErr_CheckSignals should be callable without holding the GIL

Feature or enhancement

Proposal:

Compiled-code modules that implement time-consuming operations that don’t require manipulating Python objects, are supposed to call PyErr_CheckSignals frequently throughout each such operation, so that if the user interrupts the operation with control-C, it is cancelled promptly.

In the normal case where no signals are pending, PyErr_CheckSignals is cheap; however, callers must hold the GIL, and compiled-code modules that implement time-consuming operations are also supposed to release the GIL during each such operation. The overhead of reclaiming the GIL in order to call PyErr_CheckSignals,
and then releasing it again, sufficiently often for reasonable user responsiveness, can be substantial.

If my understanding of the thread-state rules is correct, PyErr_CheckSignals only needs the GIL if it has work to do. Checking whether there is a pending signal, or a pending request to run the cycle collector, requires only a couple of atomic loads. Therefore, I propose that we should restructure PyErr_CheckSignals and its close relatives (_PyErr_CheckSignals and _PyErr_CheckSignalsTstate) so that all the “do we have anything to do” checks are done in a batch before anything that needs the GIL. If any of them are true, then the function should acquire the GIL itself, repeat the check (because another thread could have stolen the event while we were waiting for the GIL), and then actually do the work, thus enabling callers to not hold the GIL.

I have already prepared and tested an implementation of this change and will submit a PR shortly.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-133466
• gh-135358
• gh-140549

检查 gh-133466、gh-135358 和 gh-140549 中链接的工作，然后追踪 PyErr_CheckSignals 及其相关的 _PyErr_CheckSignals 和 _PyErr_CheckSignalsTstate。检查待处理工作检查和依赖 GIL 的处理是如何分离的；完成的标准是，调用方可以在不持有 GIL 的情况下进行检查，而所需工作仍会获取 GIL 并重新检查。