Issue 29590: Incorrect stack traces when re-entering a generator/coroutine stack via .throw()

The attached script sets up a stack of generators calling each other via 'yield from': f yields from g yield from h. Then the generator at the bottom of the stack yields.

Before they yield, sys._getframe shows the expected stack (or if you put in traceback.print_stack() you get the same thing).

After they yield, it depends: if the generator is resumed via 'gen.send(None)', then the stack looks sensible. But if the generator is resumed via 'gen.throw(...)', then the stack is weird: Objects/genobject.c:_gen_throw implements 'yield from' in an weird manual way, where it first directly resumes the innermost generator frame, and then propagates any result to the next generator frame, etc. So the output I get from the sample script is:

~$ python3.6 weird-throw-stack.py
-- f before yielding --
f
<module>
-- g before yielding --
g
f
<module>
-- h before yielding --
h
g
f
<module>
-- h after yielding --
h
<module>
-- g after yielding --
g
<module>
-- f after yielding --
f
<module>

This causes problems for stack-based profiling (https://github.com/vmprof/vmprof-python/issues/117), debuggers, and other tools that need to introspect the stack.

I proposed a PR to fix this: https://github.com/python/cpython/pull/19896

I just filed a related issue to this that's also similar to issue 29587:
https://bugs.python.org/issue40694

New changeset 8b33961e4bc4020d8b2d5b949ad9d5c669300e89 by Chris Jerdonek in branch 'master':
bpo-29590: fix stack trace for gen.throw() with yield from (#19896)
https://github.com/python/cpython/commit/8b33961e4bc4020d8b2d5b949ad9d5c669300e89

New changeset e92219d8f864a1a8eb381d98d5df4f1aa767dacb by Miss Islington (bot) in branch '3.9':
bpo-29590: fix stack trace for gen.throw() with yield from (GH-19896)
https://github.com/python/cpython/commit/e92219d8f864a1a8eb381d98d5df4f1aa767dacb