In a threaded environment, Python calls must be guarded by
PyGILState_Ensure() and PyGILState_Release() that
ultimately lock/unlock a mutex. Unfortunately there is no
PyGILState_TryEnsure() and therefore we may create deadlocks when
Prolog locks are involved. This may either apply to explicit Prolog
locks from with_mutex/2
and friends or implicit locks on e.g. I/O streams. The classical
scenario is thread A holding the Python GIL and wanting to
call Prolog code that locks a mutex M, while thread
B holds M and wishes to make a Python call and
this tries to lock the GIL. The predicate py_gil_owner/1
can be used to help diagnosing such issues.
