After upgrading from buster to bullseye, apache2 does not start any more
if libapache2-mod-jk was installed and active prior to the upgrading:

$ sudo cleanenv / /etc/init.d/apache2 start
Starting Apache httpd web server: apache2 failed!
The apache2 configtest failed. ... (warning).
Output of config test was:
AH00526: Syntax error on line 23 of /etc/apache2/mods-available/httpd-jk.conf:
JkWorkersFile only allowed once
Action 'configtest' failed.
The Apache error log may have more information.

This is caused by:

$ sudo fgrep -ri JkWorkersFile /etc/
/etc/apache2/mods-available/jk.conf:    JkWorkersFile /etc/libapache2-mod-jk/workers.properties
/etc/apache2/mods-available/httpd-jk.conf:    JkWorkersFile /etc/libapache2-mod-jk/workers.properties

Both files exist…

$ ll /etc/apache2/mods-available/jk.conf /etc/apache2/mods-available/httpd-jk.conf
-rw-r--r-- 1 root root 4802 Oct 14  2018 /etc/apache2/mods-available/httpd-jk.conf
-rw-r--r-- 1 root root 4802 Jun  4  2020 /etc/apache2/mods-available/jk.conf

… and belong to the same package(!):

$ dpkg -S /etc/apache2/mods-available/jk.conf /etc/apache2/mods-available/httpd-jk.conf
libapache2-mod-jk: /etc/apache2/mods-available/jk.conf
libapache2-mod-jk: /etc/apache2/mods-available/httpd-jk.conf


My best guess is that the conffile was renamed but the renaming
process was done improperly / violating Policy.

As both files are identical…

$ md5sum /etc/apache2/mods-available/jk.conf /etc/apache2/mods-available/httpd-jk.conf
f6ebd56d10bf0dcb17d79b2133cb9f5c  /etc/apache2/mods-available/jk.conf
f6ebd56d10bf0dcb17d79b2133cb9f5c  /etc/apache2/mods-available/httpd-jk.conf

… I guess I can manually deactivate and remove one. Looking at
https://packages.debian.org/bullseye/libapache2-mod-jk and
https://packages.debian.org/bullseye/amd64/libapache2-mod-jk/filelist
httpd-jk.conf seems to be the one that has to go (this is a hint for
other people running into this issue).


The root cause of this is that applications using mod_jk in buster
had to do a workaround to load httpd-jk.conf due to #928813 so…

# work around Debian #928813
<IfFile /etc/apache2/mods-available/httpd-jk.conf>
	# either /etc/apache2/conf-available/httpd-jk.conf with a2enconf httpd-jk
	# *or*   /etc/apache2/mods-available/jk.conf       with a2enmod jk
	# would be correct, but here we are
	Include /etc/apache2/mods-available/httpd-jk.conf
</IfFile>

… ends up loading this twice because the old file was not removed
during the upgrade.

$ sudo rm /etc/apache2/mods-available/httpd-jk.conf
$ sudo cleanenv / /etc/init.d/apache2 start
Starting Apache httpd web server: apache2 ..

This helped, indeed.

Hello,

I had only a quick look at this issue. I remember that one configuration file
was wrongly named back when Stretch was oldstable and Buster the new stable
distribution. For now I suggest to downgrade this bug to important because the
issue is not present when upgrading from Bullseye to Bookworm. I can take a
look at it later. We probably only need to use a maintscript file to remove the
obsolete configuration file on upgrade.

Regards,

Markus