gh-87135: Hang non-main threads that attempt to acquire the GIL during finalization #105805
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… during finalization
2 tasks
My problem with that and reason why we need this PR is that the answer to "who is going to use that ability" is unclear. End users of Python applications don't have a way to debug random thread crashes and know that they need it. Extension module authors do not know that they need it. A few big application owners will simply blindly turn it on for everything because they happen to have people aware of the consequences of not going it. But there is no good point to turn such a conditional of "hang threads rather than allow them to randomly crash or refuse to run C++ finalizers despite releasing the associated memory meaning other threads can crash because multiple other things are using the previously allocated and then freed memory". Those concurrency scribbling messes are far worse things to attempt to debug than a simple hung thread, hung within a clear C symbol saying "hi there, i'm going to hang this thread". So I'm all for undoing our mistaken thread exiting API and use of it in favor of this least bad option of hanging threads that would otherwise have failed to cleanup and left ambiguious process state behind as we have had in the past. I do still think we want APIs to allow error handling upon re-entering an interpreter or acquiring the GIL and actually failing instead of this... but we don't have those. and most of the world's Python C API using code won't be using such new APIs for a looong time while the random crashing problem would persist. So this remains the least bad option that makes a viable improvement. |
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@colesbury and/or @mpage, and @ericsnowcurrently - can you take a look at this PR? (FWIW, Eric's already gone over it with my on screen at the core dev sprint here) |
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Blocking daemon threads after the VM has been finalized seems less error prone than forcing the threads to exit. I believe this is also how the JVM handles the same situation, so there's some precedence for this approach. While we're here, I think this might be a good time to also fix the issues with threads referencing their |
vstinner
reviewed
Sep 27, 2024
if any more, i'll just skip the test on wasi.
Linking to the filed pre-existing known issue.
That goes beyond the scope of what I want to accomplish with this PR - which I believe might mostly be a candidate for backporting as a bug fix (I'll see what the RMs think). I think it is a good followup issue though - #124878 for starters but that might turn into a couple of different things to chase down as well. |
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@Yhg1s what's your opinion on the viability of backporting this as a bugfix to 3.13 and 3.12 patch releases? People who actually encounter this in the form of strange crashes from threads during shutdown have been wanting it for years. I wouldn't call it common, just a thorn in some classes of applications side. (In terms of impact I'd elide marking the deprecated C API as deprecated in the backports - otherwise I expect most of this to apply relatively easily given that it was originally authored on a |
gpshead
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needs backport to 3.12
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Sorry, @jbms and @gpshead, I could not cleanly backport this to |
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Sorry, @jbms and @gpshead, I could not cleanly backport this to |
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I expected the backport automation to fail and need edits regardless even if it hadn't. I'll be taking care of them. :) |
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Triage: @gpshead reminder about the backports :) |
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Please don't forget about backport. @gpshead |
gpshead
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Aug 15, 2025
…the GIL during finalization (pythonGH-105805) Instead of surprise crashes and memory corruption, we now hang threads that attempt to re-enter the Python interpreter after Python runtime finalization has started. These are typically daemon threads (our long standing mis-feature) but could also be threads spawned by extension modules that then try to call into Python. This marks the `PyThread_exit_thread` public C API as deprecated as there is no plausible safe way to accomplish that on any supported platform in the face of things like C++ code with finalizers anywhere on a thread's stack. Doing this was the least bad option. (cherry picked from commit 8cc5aa4) Co-authored-by: Jeremy Maitin-Shepard <[email protected]> Co-authored-by: Gregory P. Smith <[email protected]>
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GH-137827 is a backport of this pull request to the 3.13 branch. |
gpshead
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Aug 17, 2025
…L during finalization (GH-105805) (GH-137827) * [3.13] gh-87135: Hang non-main threads that attempt to acquire the GIL during finalization (GH-105805) Instead of surprise crashes and memory corruption, we now hang threads that attempt to re-enter the Python interpreter after Python runtime finalization has started. These are typically daemon threads (our long standing mis-feature) but could also be threads spawned by extension modules that then try to call into Python. This marks the `PyThread_exit_thread` public C API as deprecated as there is no plausible safe way to accomplish that on any supported platform in the face of things like C++ code with finalizers anywhere on a thread's stack. Doing this was the least bad option. (cherry picked from commit 8cc5aa4) Co-authored-by: Jeremy Maitin-Shepard <[email protected]> Co-authored-by: Gregory P. Smith <[email protected]> * state "3.13.7 and earlier" * backport: do not add the deprecated marker * fix Py_IsFinalizing doc ref --------- Co-authored-by: Jeremy Maitin-Shepard <[email protected]>
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This splits off the change from #28525 to hang threads that attempt to acquire the GIL during interpreter shutdown, but does not introduce any new public APIs.
📚 Documentation preview 📚: https://cpython-previews--105805.org.readthedocs.build/