I have encountered a minor bug with the new `threading.get_native_id()` featureset in Python 3.8. The bug occurs when creating a new multiprocessing.Process object on Unix (or on any platform where the multiprocessing start_method is 'fork' or 'forkserver').

When creating a new process via fork, the Native ID in the new MainThread is incorrect. The new forked process' threading.MainThread object inherits the Native ID from the parent process' MainThread instead of capturing/updating its own (new) Native ID.

See the following snippet:

>>> import threading, multiprocessing
>>> multiprocessing.set_start_method('fork') # or 'forkserver'
>>> def proc(): print(threading.get_native_id(), threading.main_thread().native_id) # get_native_id(), mainthread.native_id
>>> proc()
22605 22605 # get_native_id(), mainthread.native_id
>>> p = multiprocessing.Process(target=proc)
>>> p.start()
22648 22605 # get_native_id(), mainthread.native_id
>>>
>>> def update(): threading.main_thread()._set_native_id()
>>> def print_and_update(): proc(); update(); proc()
>>> print_and_update()
22605 22605 # get_native_id(), mainthread.native_id
22605 22605 
>>> p2=multiprocessing.Process(target=print_and_update); p2.start()
22724 22605 # get_native_id(), mainthread.native_id
22724 22724
>>> print_and_update()
22605 22605 # get_native_id(), mainthread.native_id
22605 22605

As you can see, the new Process object's MainThread.native_id attribute matches that of the MainThread of its parent process. 

Unfortunately, I'm not too familiar with the underlying mechanisms that Multiprocessing uses to create forked processes. 
I believe this behavior occurs because (AFAIK) a forked multiprocessing.Process copies the MainThread object from its parent process, rather than reinitializing a new one. Looking further into the multiprocessing code, it appears the right spot to fix this would be in the multiprocessing.Process.bootstrap() function. 

I've created a branch containing a working fix - I'm also open to suggestions of how a fix might otherwise be implemented. 
If it looks correct I'll create a PR against the CPython 3.8 branch. 

See the branch here: https://github.com/jaketesler/cpython/tree/fix-mp-native-id

Thanks all!
-Jake

> See the branch here: https://github.com/jaketesler/cpython/tree/fix-mp-native-id

Can you please create a PR?

@vstinner PR created :)
https://github.com/python/cpython/pull/17088

PR was updated with tests and is ready for core developer review and then the merge to cpython:master. After that (if I understand correctly) a backport will automatically get picked into the 3.8 branch if there aren't any conflicts.

New changeset c6b20be85c0de6f2355c67ae6e7e578941275cc0 by Miss Islington (bot) (Jake Tesler) in branch 'master':
bpo-38707: Fix for multiprocessing.Process MainThread.native_id (GH-17088)
https://github.com/python/cpython/commit/c6b20be85c0de6f2355c67ae6e7e578941275cc0

New changeset 829593a9262e67c72167c6cb20d383203b2ea410 by Miss Islington (bot) in branch '3.8':
bpo-38707: Fix for multiprocessing.Process MainThread.native_id (GH-17088)
https://github.com/python/cpython/commit/829593a9262e67c72167c6cb20d383203b2ea410

Thank you Jake for the report and PR!

Thanks for the fix. That was an interesting bug ;-) I like the simplicity of the fix.