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Description of problem:
We should backport fix addressed in #20670 in order to prevent potential use-after-free bugs.


Version-Release number of selected component (if applicable):
systemd-239-45.el8

How reproducible:
unknown

Steps to Reproduce:
1. There are no clear steps to trigger this issue in systemd (as in service manager) context, but it should be possible to write synthetic reproducer in C that would use the sd-event APIs in a way that triggers the bug.

Actual results:
Use after free of sd-event object.

Expected results:
Reference count for sd-event is not dropped to 0 by user-provided callback and main sd-event loop object is not freed prematurely.

Additional info:
https://github.com/systemd/systemd/pull/20670

Created attachment 1824565 [details]
Reproducer

This short C program demonstrates the problem. Compile it and run under valgrind to see errors reported when run against unpatched version of libsystemd.

gcc -ggdb3 -O0 -o bz2005790 bz2005790.c -lsystemd