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AMD64 Fedora Rawhide Refleaks 3.x:
https://buildbot.python.org/all/#/builders/565/builds/11
test_interpreters leaked [1424, 1422, 1424] references, sum=4270
According to git bisect, the leak was introduced by:
commit 71d1bd9569c8a497e279f2fea6fe47cd70a87ea3
Author: Mohamed Koubaa <koubaa.m@gmail.com>
Date: Thu Sep 3 03:21:06 2020 -0500
 bpo-1635741: Port _signal module to multi-phase init (PEP 489) (GH-22049)
 .../2020-09-01-17-07-20.bpo-1635741.7wSuCc.rst | 1 +
 Modules/signalmodule.c | 168 +++++++++++----------
 2 files changed, 87 insertions(+), 82 deletions(-)
Example of leak:
$ ./python -m test -R 3:3 test_interpreters -m test.test_interpreters.TestInterpreterClose.test_from_current
0:00:00 load avg: 0.72 Run tests sequentially
0:00:00 load avg: 0.72 [1/1] test_interpreters
beginning 6 repetitions
123456
......
test_interpreters leaked [237, 237, 237] references, sum=711
test_interpreters leaked [18, 18, 18] memory blocks, sum=54
test_interpreters failed
== Tests result: FAILURE ==
1 test failed:
 test_interpreters
Total duration: 1.1 sec
Tests result: FAILURE

The signal module is really special. In Python, only the main thread of the main interpreter is supposed to be allowed to run Python signal handlers (but the C signal handler can be executed by any thread).
The exec function of the _signal module runs actions each time a new instance of the _signal module is created, whereas some actions must only be done exactly once:
* modify Handlers[i].tripped
* modify Handlers[i].func
* Set SIGINT signal handler
* Initialize sigint_event event (Windows only): see bpo-41686
For example, calling signal.signal() in a subinterpreter raises ValueError("signal only works in main thread of the main interpreter").

About the leak, the following three variables are also initialized multiple times by signal_exec():
static PyObject *DefaultHandler;
static PyObject *IgnoreHandler;
static PyObject *IntHandler;

Another problem: PyOS_FiniInterrupts() is a public function of the C API which access global variables like IntHandler, DefaultHandler and IgnoreHandler.

New changeset 4b8032e5a4994a7902076efa72fca1e2c85d8b7f by Victor Stinner in branch 'master':
bpo-41713: _signal doesn't use multi-phase init (GH-22087)
https://github.com/python/cpython/commit/4b8032e5a4994a7902076efa72fca1e2c85d8b7f

Sounds like there needs to be some python-wide global state that perhaps the signal module wraps rather than owns. I could try to prototype it if you agree

See also:
* bpo-40600: "Add an option to disallow creating more than one instance of a module".
* bpo-40288: [subinterpreters] atexit module should not be loaded more than once per interpreter

New changeset 296a796951032f678d063008f588ccc6958d0df1 by Victor Stinner in branch 'master':
bpo-41713: Remove PyOS_InitInterrupts() function (GH-23342)
https://github.com/python/cpython/commit/296a796951032f678d063008f588ccc6958d0df1

New changeset 0ae323b87f1bed64a7fa70f5a41a5800aca032cc by Victor Stinner in branch 'master':
bpo-41686: Always create the SIGINT event on Windows (GH-23344)
https://github.com/python/cpython/commit/0ae323b87f1bed64a7fa70f5a41a5800aca032cc

New changeset 05a5d697f4f097f37c5c1e2ed0e2338a33c3fb6a by Victor Stinner in branch '3.9':
bpo-41686: Always create the SIGINT event on Windows (GH-23344) (GH-23347)
https://github.com/python/cpython/commit/05a5d697f4f097f37c5c1e2ed0e2338a33c3fb6a

New changeset a702bd4b921167e73f8fc987aa64ada571fdc3f8 by Victor Stinner in branch '3.8':
bpo-41686: Always create the SIGINT event on Windows (GH-23344) (GH-23347) (GH-23349)
https://github.com/python/cpython/commit/a702bd4b921167e73f8fc987aa64ada571fdc3f8

New changeset 29aa624047f893b3b3194f00252b2156bbbf4f9b by Victor Stinner in branch 'master':
bpo-41686: Move _Py_RestoreSignals() to signalmodule.c (GH-23353)
https://github.com/python/cpython/commit/29aa624047f893b3b3194f00252b2156bbbf4f9b 

New changeset 7f9b25a21ab95f8cf8d663396993766307be475c by Victor Stinner in branch 'master':
bpo-41713: Port _signal module to multi-phase init (GH-23355)
https://github.com/python/cpython/commit/7f9b25a21ab95f8cf8d663396993766307be475c

Done! _signal uses again the multi-phase init API.

The PyOS_InitInterrupts function is still listed in PC/python3dll.c.

Petr Viktorin:
> The PyOS_InitInterrupts function is still listed in PC/python3dll.c.
Ooops, I proposed PR 24257 to remove it.

New changeset e8e66eab941b983b6e85cd0d57cd45838880c568 by Victor Stinner in branch 'master':
bpo-41713: Remove PyOS_InitInterrupts() from python3dll.c (GH-24257)
https://github.com/python/cpython/commit/e8e66eab941b983b6e85cd0d57cd45838880c568

> bpo-41713: Port _signal module to multi-phase init (GH-23355)
This change caused a regression: bpo-43963.