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We still have a bunch of "global" C variables (static globals, static locals, maybe thread-local storage) in our code-base that break the isolation between interpreters. I added Tools/c-globals/check-c-globals.py a while back to help identify such variables, however more have crept in. I also did not take static locals or thread-locals into account.
To address the above, we should do the following:
1. update check-c-globals.py to identify static locals (and thread-locals)
2. deal with any identified globals
 * move them to _PyRuntimeState (or thread-locals to PyThreadState, etc.)
 * ignore them by adding them to Tools/c-globals/ignored-globals.txt
3. add check-c-globals.py to "make check"
4. (if "make check" isn't already there), ensure check-c-globals.py is run at some point in CI
Separately, we should move fields out of _PyRuntimeState into PyInterpreterState wherever possible. That can also be done at step 2 if it's not too much work.

Also, Tools/c-globals/ignored-globals.txt is a bit out of date (some vars have been removed, renamed, or moved to another file). That should get cleaned up. It might also make sense to update check-c-globals.py to verify that all variables in ignored-globals.txt actually exist.

New changeset ee536b2020b1f0baad1286dbd4345e13870324af by Eric Snow in branch 'master':
bpo-36876: Add a tool that identifies unsupported global C variables. (#15877)
https://github.com/python/cpython/commit/ee536b2020b1f0baad1286dbd4345e13870324af

The new test_check_c_globals.ActualChecks test is failing with an "unexpected success" on the bolen-ubuntu buildbot (under Ubuntu 18.04.3). I can reproduce the failure in a manually built tree.

@db3l, I'll take a look right away.

New changeset 64535fc6c0712caef0bc46be30e661f7ccf8280e by Eric Snow in branch 'master':
bpo-36876: Skip test_check_c_globals for now. (gh-16017)
https://github.com/python/cpython/commit/64535fc6c0712caef0bc46be30e661f7ccf8280e

New changeset 088b63ea7a8331a3e34bc93c3b873c60354b4fad by Eric Snow in branch 'master':
bpo-36876: Fix the globals checker tool. (gh-16058)
https://github.com/python/cpython/commit/088b63ea7a8331a3e34bc93c3b873c60354b4fad

New changeset e4c431ecf50def40eb93c3969c1e4eeaf7bf32f1 by Eric Snow in branch 'master':
bpo-36876: Re-organize the c-analyzer tool code. (gh-16841)
https://github.com/python/cpython/commit/e4c431ecf50def40eb93c3969c1e4eeaf7bf32f1

New changeset 9def81aa52adc3cc89554156e40742cf17312825 by Vinay Sajip in branch 'master':
bpo-36876: Moved Parser/listnode.c statics to interpreter state. (GH-16328)
https://github.com/python/cpython/commit/9def81aa52adc3cc89554156e40742cf17312825

Thanks, Vinay!

FYI, others have been tackling this in separate issues (e.g. Victor, anyone relative to PEP 384).

And I *am* still working on the c-analyzer + a test to that runs it, so we can keep from adding more globals. :)

More and more C extensions are converted to multiphase initialization API (PEP 489) and their global variables are moved into a new module state.
bpo-39465 will make _Py_IDENTIFIER compatible with subinterpreters.
See also bpo-40521 for caches like free lists and Unicode interned strings.

New changeset 345cd37abe324ad4f60f80e2c3133b8849e54e9b by Eric Snow in branch 'master':
bpo-36876: Fix the C analyzer tool. (GH-22841)
https://github.com/python/cpython/commit/345cd37abe324ad4f60f80e2c3133b8849e54e9b

New changeset 4fe72090deb7fb7bc09bfa56c92f6b3b0967d395 by Eric Snow in branch 'master':
bpo-36876: Small adjustments to the C-analyzer tool. (GH-23045)
https://github.com/python/cpython/commit/4fe72090deb7fb7bc09bfa56c92f6b3b0967d395

New changeset 9f02b479e6b6b48d0c2aad621978cff82e530b15 by Eric Snow in branch 'master':
bpo-36876: [c-analyzer tool] Tighten up the results and output. (GH-23431)
https://github.com/python/cpython/commit/9f02b479e6b6b48d0c2aad621978cff82e530b15

New changeset 7ec59d8861ef1104c3028678b2cacde4c5693e19 by Eric Snow in branch 'master':
bpo-36876: [c-analyzer tool] Add a "capi" subcommand to the c-analyzer tool. (gh-23918)
https://github.com/python/cpython/commit/7ec59d8861ef1104c3028678b2cacde4c5693e19

New changeset 5ae9be68d9f1a628fdc920b647257f94afb77887 by Eric Snow in branch 'master':
bpo-36876: [c-analyzer tool] Additional CLI updates for "capi" command. (gh-23929)
https://github.com/python/cpython/commit/5ae9be68d9f1a628fdc920b647257f94afb77887

I'm getting the following FutureWarning on a certain regular expression. I think it just needs "[]" to become "\[\]".
0:05:36 load avg: 0.00 [ 53/427] test_check_c_globals
...\Tools\c-analyzer\c_common\tables.py:236: FutureWarning: Possible nested set at position 12
 _COLSPEC_RE = re.compile(textwrap.dedent(r'''

New changeset 77bab59c8a1f04922bb975cc4f11e5323d1d379d by Eric Snow in branch 'main':
bpo-36876: Update the c-analyzer whitelist. (gh-31225)
https://github.com/python/cpython/commit/77bab59c8a1f04922bb975cc4f11e5323d1d379d

New changeset cb68788dcadf43b47292bab7816a5ed9efa69730 by Eric Snow in branch 'main':
bpo-36876: Minor cleanup to c-analyzer "ignored" data.' (gh-31239)
https://github.com/python/cpython/commit/cb68788dcadf43b47292bab7816a5ed9efa69730

New changeset 80e4f262aa27a39abf3fadc19a6323fea4607a8f by Eric Snow in branch 'main':
bpo-36876: Make sure the c-analyzer is checking all the source files.' (gh-31264)
https://github.com/python/cpython/commit/80e4f262aa27a39abf3fadc19a6323fea4607a8f