We exiting a frame (returning from a Python function) we have to release the stack allocated frame. If a heap-allocated frame object exists, we need to copy the contents of the (stack) frame into the frame object.
However, this involves allocating memory for the copy. Allocating memory can invoke GC, causing arbitrary code to be run, or the allocation can fail. Either leaves us in a precarious state, which may be unsafe.

I haven't been able to produce a crash, but I'm not sure that there isn't a potential crash lurking there either.

The fix is fairly simple. Allocate space for the copy of the frame at the end of the frame object. Then we need to copy the data, space will have already been allocated, and nothing can fail.

Since, in theory, heap-allocated frames are relatively rare, the extra memory used won't be an issue.

> Allocating memory can invoke GC

Technically, only allocating objects can trigger GC (specifically initializing them). All the malloc APIs don't currently call the GC by themselves.

Ok, so it is not as bad as I thought.

A failed allocation might leave us with an invalid frameobject, though. So it is still worth fixing.

New changeset 60929576e40038ec71d896230f69e4411c82be4b by Mark Shannon in branch 'main':
bpo-45786: Allocate space for frame in frame object. (GH-29729)
https://github.com/python/cpython/commit/60929576e40038ec71d896230f69e4411c82be4b

With https://bugs.python.org/issue45963 both frame objects and generators contain space for the frame, so no allocation occurs on exit.

New changeset 2217462bda1865a047d358306088682ee6a091ed by Victor Stinner in branch 'main':
bpo-45786: Remove _PyFrame_Fini() and _PyFrame_DebugMallocStats() (GH-31874)
https://github.com/python/cpython/commit/2217462bda1865a047d358306088682ee6a091ed