_PyEval_SetTrace() fails with RuntimeError: Cannot install a trace function while another trace function is being installed

[vstinner]

The test_concurrency() of test_import fails randomly on sys.settrace(). Example:

$ ./python -m test test_import -m test_concurrency -v -j30 -F --fail-env-changed 
(...)
0:00:13 load avg: 12.46 [ 97/1] test_import failed (env changed)
test_concurrency (test.test_import.ImportTests) ... ok

----------------------------------------------------------------------
Ran 1 test in 0.487s

OK
Warning -- Uncaught thread exception: RuntimeError
Exception in thread Thread-13 (run):
Traceback (most recent call last):
  File "/home/vstinner/python/3.10/Lib/threading.py", line 1016, in _bootstrap_inner
    self.run()
  File "/home/vstinner/python/3.10/Lib/threading.py", line 953, in run
    self._target(*self._args, **self._kwargs)
  File "/home/vstinner/python/3.10/Lib/test/test_import/__init__.py", line 457, in run
    sys.settrace(None)
RuntimeError: Cannot install a trace function while another trace function is being installed
(...)

The problem is that the _PyEval_SetTrace() function fails with RuntimeError("Cannot install a trace function while another trace function is being installed") on Py_XDECREF(traceobj); indirectly calls sys.settrace().

IMO Py_XDECREF(traceobj); should be called after reentrant = 0;, at the end of the function.

[vstinner]

Fixed by a8fe4bb