[New-bugs-announce] [issue36533] logging regression with threading + fork are mixed in 3.7.1rc2 (deadlock potential)

[Gregory P. Smith]

New submission from Gregory P. Smith <greg at krypto.org>:

I'm spawning a dicussion buried in the way too long thread of https://bugs.python.org/issue6721 over here into its own specific issue to treat as a 3.7 release blocker for a rollback or repair decision before 3.7.4.

https://github.com/python/cpython/commit/3b699932e5ac3e76031bbb6d700fbea07492641d

I believe that was released in 3.7.1 is leading to a behavior regression for an application (the Fedora installer's libreswan kvmrunner?).  Full details can be found in the messages of the other issue starting with:
  https://bugs.python.org/issue6721#msg329474

TL;DR - logging.Handler instances each have their own threading.Rlock. 
libreswan implements at least one logging.Handler subclass.  That subclass's custom emit() implementation directly calls potentially many other sub-handlers emit() methods.  Some of those emit() methods (such as logging.StreamHandler) call flush() which acquires the handler's lock.

So they've got a dependency between these two locks, the first's must be acquired before the second.

But the logging module APIs have no concept of sub-handlers and lock ordering.

I see many flaws with the libreswan code's design (I'm already ignoring the futility of threading + fork) but this still caused a behavior regression in the stable 3.7 release.

(more comments coming as followups to avoid a wall of text with too many topics)

----------
assignee: gregory.p.smith
components: Library (Lib)
keywords: 3.7regression
messages: 339472
nosy: cagney, gregory.p.smith, ned.deily, vstinner
priority: release blocker
severity: normal
status: open
title: logging regression with threading + fork are mixed in 3.7.1rc2 (deadlock potential)
type: behavior
versions: Python 3.7

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36533>
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