Bug #119783 mysql-common postinst fails when /etc/mysql/my.cnf.fallback is missing

Submitted:	26 Jan 19:12
Reporter:	Antonios Tsianakas	Email Updates:
Status:	Open	Impact on me:	None
Category:	MySQL Package Repos	Severity:	S3 (Non-critical)
Version:	8.4.8	OS:	Ubuntu
Assigned to:		CPU Architecture:	x86

Description:
The mysql-common package postinst script unconditionally attempts to register 
/etc/mysql/my.cnf.fallback as an alternative without checking if the file exists, 
causing installation failures when the file is not present or has been removed.

AFFECTED VERSIONS:
mysql-common package from apt.mysql.com repository. Tested with Ubuntu Noble 24.04

The postinst script in mysql-common contains the following code:

if [ "$1" = "configure" ]; then
    # Low priority fallback for client use when no server is installed.
    update-alternatives --install /etc/mysql/my.cnf my.cnf /etc/mysql/my.cnf.fallback 100
fi

This code does not verify that /etc/mysql/my.cnf.fallback exists before attempting 
to register it as an alternative. If the file is missing (e.g., manually removed, 
incomplete upgrade, or system inconsistency), the installation fails with:

update-alternatives: error: alternative path /etc/mysql/my.cnf.fallback doesn't exist
dpkg: error processing package mysql-common (--configure):
 installed mysql-common package post-installation script subprocess returned error exit status 2

How to repeat:
1. Install mysql-common package
2. Remove /etc/mysql/my.cnf.fallback
3. Attempt to upgrade or reinstall eg. apt install --reinstall mysql-common

Suggested fix:
The official Ubuntu MySQL packages handle this correctly by checking for 
file existence:

if [ "$1" = "configure" ] && [ -f /etc/mysql/my.cnf.fallback ]; then
    # Low priority fallback for client use when no server is installed.
    update-alternatives --install /etc/mysql/my.cnf my.cnf /etc/mysql/my.cnf.fallback 100
fi

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