bpo-1635741: Port _thread to multiphase init

[vstinner]

Port the _thread extension module to the multiphase initialization
API (PEP 489) and convert its static types to heap types.

https://bugs.python.org/issue1635741

[vstinner]

Sadly, the following test leaks:

 def test_leak(self): code = textwrap.dedent(r""" import os import _thread lock = _thread.allocate_lock() def _after_fork(): pass os.register_at_fork(after_in_child=_after_fork) """) test.support.run_in_subinterp(code) 

Output:

$ ./python -m test test_threading -R 3:3 -m test_leak 0:00:00 load avg: 0.77 Run tests sequentially 0:00:00 load avg: 0.77 [1/1] test_threading beginning 6 repetitions 123456 ...... test_threading leaked [56, 56, 56] references, sum=168 test_threading leaked [21, 21, 21] memory blocks, sum=63 test_threading failed == Tests result: FAILURE == 1 test failed: test_threading Total duration: 724 ms Tests result: FAILURE 

I will investigate later why it leaks.

[vstinner]

Workaround for the leak:

diff --git a/Python/pystate.c b/Python/pystate.c index ead25b08d7..66ad5d6514 100644 --- a/Python/pystate.c +++ b/Python/pystate.c @@ -311,6 +311,7 @@ interpreter_clear(PyInterpreterState *interp, PyThreadState *tstate) /* Last garbage collection on this interpreter */ _PyGC_CollectNoFail(tstate); + _PyGC_CollectNoFail(tstate); _PyGC_Fini(tstate); /* We don't clear sysdict and builtins until the end of this function. 

[vstinner]

Example of leaking tests:

./python -m test test_threading -R 3:3 -m test_threads_join -m test_threads_join_2 

These tests run import threading in a subinterpreter, and Lib/threading.py calls os.register_at_fork() which keeps the threading module dictionary alive longer than expected: I created https://bugs.python.org/issue42671 to try to fix this issue.

[vstinner]

I fixed the leak by adding a traverse function to the lock type (and convert it to a GC type).