Biased reference counting maintains two refcount fields in each object:
`ob_ref_local` and `ob_ref_shared`. The true refcount is the sum of these two
fields. In some cases, when refcounting operations are split across threads,
the ob_ref_shared field can be negative (although the total refcount must be
at least zero). In this case, the thread that decremented the refcount
requests that the owning thread give up ownership and merge the refcount
fields.
Joining a thread now ensures the underlying OS thread has exited. This is required for safer fork() in multi-threaded processes.
---------
Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
concurrent.futures: The *executor manager thread* now catches
exceptions when adding an item to the *call queue*. During Python
finalization, creating a new thread can now raise RuntimeError. Catch
the exception and call terminate_broken() in this case.
Add test_python_finalization_error() to test_concurrent_futures.
concurrent.futures._ExecutorManagerThread changes:
* terminate_broken() no longer calls shutdown_workers() since the
call queue is no longer working anymore (read and write ends of
the queue pipe are closed).
* terminate_broken() now terminates child processes, not only
wait until they complete.
* _ExecutorManagerThread.terminate_broken() now holds shutdown_lock
to prevent race conditons with ProcessPoolExecutor.submit().
multiprocessing.Queue changes:
* Add _terminate_broken() method.
* _start_thread() sets _thread to None on exception to prevent
leaking "dangling threads" even if the thread was not started
yet.
