[Bug 2087935] [NEW] DeprecationWarning for multi-threaded fork() under Python 3.12

[Miles Prower]

Public bug reported:

We run unattended-upgrades on a lot of servers, but on Ubuntu noble (Ubuntu 24.04.1 LTS) we're getting a lot of cron spam that doesn't appear elsewhere:
> /usr/bin/unattended-upgrade:567: DeprecationWarning: This process (pid=475626) is multi-threaded, use of fork() may lead to deadlocks in the child.
> pid = os.fork()

The above message appears several times during an upgrade; the count is
different each week, presumably relating to what operations are needed.

lsb_release -rd:
No LSB modules are available.
Description:	Ubuntu 24.04.1 LTS
Release:	24.04

Relevant packages:
unattended-upgrades  2.9.1+nmu4ubuntu1
python3.12           3.12.3-1ubuntu0.2

The warning appears to be introduced in Python 3.12 - https://docs.python.org/3/whatsnew/3.12.html
> On POSIX platforms, os.fork() can now raise a DeprecationWarning when it can detect being called from a multithreaded process

** Affects: unattended-upgrades (Ubuntu)
     Importance: Undecided
         Status: Confirmed

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https://bugs.launchpad.net/bugs/2087935

Title:
  DeprecationWarning for multi-threaded fork() under Python 3.12

Status in unattended-upgrades package in Ubuntu:
  Confirmed

Bug description:
  We run unattended-upgrades on a lot of servers, but on Ubuntu noble (Ubuntu 24.04.1 LTS) we're getting a lot of cron spam that doesn't appear elsewhere:
  > /usr/bin/unattended-upgrade:567: DeprecationWarning: This process (pid=475626) is multi-threaded, use of fork() may lead to deadlocks in the child.
  > pid = os.fork()

  The above message appears several times during an upgrade; the count
  is different each week, presumably relating to what operations are
  needed.

  lsb_release -rd:
  No LSB modules are available.
  Description:	Ubuntu 24.04.1 LTS
  Release:	24.04

  Relevant packages:
  unattended-upgrades  2.9.1+nmu4ubuntu1
  python3.12           3.12.3-1ubuntu0.2

  The warning appears to be introduced in Python 3.12 - https://docs.python.org/3/whatsnew/3.12.html
  > On POSIX platforms, os.fork() can now raise a DeprecationWarning when it can detect being called from a multithreaded process

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