Ensure PyEval_GetGlobals() doesn't set an exception when returning NULL

[ncoghlan]

BPO	47204
Nosy	@ncoghlan

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ncoghlan'
closed_at = None
created_at = <Date 2022-04-03.06:41:46.197>
labels = ['type-feature']
title = "Ensure PyEval_GetGlobals() doesn't set an exception when returning NULL"
updated_at = <Date 2022-04-03.06:48:44.524>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2022-04-03.06:48:44.524>
actor = 'ncoghlan'
assignee = 'ncoghlan'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2022-04-03.06:41:46.197>
creator = 'ncoghlan'
dependencies = []
files = []
hgrepos = []
issue_num = 47204
keywords = []
message_count = 2.0
messages = ['416611', '416612']
nosy_count = 1.0
nosy_names = ['ncoghlan']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue47204'
versions = []

[ncoghlan]

While working on the first PR for bpo-44800 I provoked an obscure failure in PyImport_ImportModule by making PyEval_GetGlobals set an exception when returning NULL (as PyEval_GetLocals has done since bpo-18408 was implemented for Python 3.4).

This ticket covers adding an embedding test case that:

• ensures PyEval_GetGlobals() returns NULL without an exception when no Python frame is active
• ensures PyEval_GetLocals() returns NULL and sets an exception when no Python frame is active
• ensures PyImport_ImportModule still works when no Python frame is active

There's an option to slightly change the behaviour of PyEval_GetLocals() to NOT set an exception in the "no Python frame" case, and instead only set the error when there is a Python frame, but something goes wrong when attempting to access the fast locals array (such as a memory allocation failure).

[ncoghlan]

Core dev forum thread for the C API question: https://discuss.python.org/t/subtle-c-api-discrepancy-pyeval-getlocals-vs-pyeval-getglobals-with-no-python-frame/14797