Bug report

Bug description:

The PyCapsule object is not thread-safe. For example the read and write operations to the underlying pointer do not use atomics.

This can be fixed with atomic read and write operations. (same for the name field and some others)

Another potential issue is that some operations on a capsule are not atomic. For example a thread can attempt to retrieve a pointer using PyCapsule_GetPointer while a concurrent thread is updating the name and pointer using PyCapsule_SetName and PyCapsule_SetPointer. That could be addressed with locks, but I suspect that is too heavy for the capsule object.

I am leaning towards adding atomics for the read and write operations and documenting that in a PyCapsule does not guarantee its methods are atomic.

@colesbury @seberg

CPython versions tested on:

CPython main branch

Operating systems tested on:

Windows

Linked PRs

• gh-145563: Add thread-safety annotation for PyCapsule #146612
• [3.14] gh-145563: Add thread-safety annotation for PyCapsule C-API (GH-146612) #146659