classification
Title: atexit callbacks should be run at subinterpreter shutdown
Type: behavior Stage:
Components: Interpreter Core Versions: Python 3.7
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: Dormouse759, encukou, eric.snow, grahamd, ncoghlan
Priority: normal Keywords:

Created on 2017-10-30 15:44 by Dormouse759, last changed 2017-11-09 00:35 by ncoghlan.

Messages (9)
msg305233 - (view) Author: Marcel Plch (Dormouse759) * Date: 2017-10-30 15:44
`Py_FinalizeEx()` only executes callbacks from the subinterpreter from which Py_FinalizeEx is called.

What is the expected behavior here?
Should the callbacks be specific to individual subinterpreters?
msg305277 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2017-10-31 08:12
Hmm, I don't think calling Py_Finalize in any interpreter other than the main one is actually defined at all, so I'm inclined to just make it an error, rather than trying to figure out how it should work. It would then be up to the embedding application to make sure it switched back to the main interpreter before calling Py_Finalize.

(We don't have this concern with Py_Initialize, since that *creates* the main interpreter)
msg305278 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2017-10-31 08:14
For the main interpreter, Py_Finalize should be destroying all other subinterpreters as one of the first things it does, so if we're *not* currently doing that, it would make sense to fix it as part of Eric's subinterpreters PEP.
msg305279 - (view) Author: Petr Viktorin (encukou) * Date: 2017-10-31 09:28
I don't have a reason to think Py_Finalize is not *destroying* the other subinterpreters. But it's not calling their atexit callbacks.

(Is there any reason Py_Finalize couldn't switch to the main interpreter itself? Assuming it's even necessary.)
msg305280 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2017-10-31 09:46
I guess we allow an unhandled SystemExit in a child thread to propagate to (and hence terminate) the main thread, so allowing a Py_Finalize call in a subinterpreter to terminate the main interpreter would be comparable to that.

My main rationale for *requiring* that the main interpreter be active (or be made active) when shutting down is to reduce the number of scenarios we need to test (right now we only test Py_Initialize/Py_Finalize cycles with a single interpreter, and officially allowing finalization from arbitrary interpreters expands that test matrix a fair bit).
msg305298 - (view) Author: Petr Viktorin (encukou) * Date: 2017-10-31 14:06
I'm not sure where the concept of "main subinterpreter" comes in, with respect to this issue.

I thnik the issue of atexit callbacks could be solved by something like keeping info about each callback's subinterpreter, and switching subinterpreters before running each one. I can see the locking needed for that being quite hairy, but independent of which thread calls all this.
msg305859 - (view) Author: Petr Viktorin (encukou) * Date: 2017-11-08 15:49
When you destroy a subinterpreter before Py_Finalize is called, Python can't start calling its atexit callbacks – they no longer have a subinterpreter to run in.

Therefore I think callbacks for a particular subinterpreter should be called when (and only when) that subinterpreter is destroyed. Regardless of whether it's the main one or not.
msg305896 - (view) Author: Graham Dumpleton (grahamd) Date: 2017-11-08 20:07
FWIW, that atexit callbacks were not called for sub interpreters ever was a pain point for mod_wsgi.

What mod_wsgi does is override the destruction sequence so that it will first go through each sub interpreter when and shutdown threading explicitly, then call atexit handlers. When that is done, only then will it destroy the sub interpreter and the main interpreter.

I have noted this previously in discussion associated with:

https://bugs.python.org/issue6531
msg305927 - (view) Author: Nick Coghlan (ncoghlan) * (Python committer) Date: 2017-11-09 00:35
Ah, I see - yeah, that makes a lot of sense.

I've retitled the issue to make it clearer that the problem is where the responsibility for calling atexit functions lives - currently it's in Py_Finalize, but really it should be inside the interpreter object's own teardown code.
History
Date User Action Args
2017-11-09 00:35:26ncoghlansetmessages: + msg305927
components: + Interpreter Core, - Extension Modules
title: atexit callbacks only called for current subinterpreter -> atexit callbacks should be run at subinterpreter shutdown
2017-11-08 20:07:09grahamdsetmessages: + msg305896
2017-11-08 15:49:56encukousetmessages: + msg305859
2017-10-31 14:06:07encukousetmessages: + msg305298
2017-10-31 09:46:06ncoghlansetmessages: + msg305280
2017-10-31 09:28:40encukousetmessages: + msg305279
2017-10-31 08:14:02ncoghlansetmessages: + msg305278
2017-10-31 08:12:37ncoghlansetnosy: + grahamd, eric.snow
messages: + msg305277
2017-10-30 15:44:43Dormouse759create