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