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

• Submitted: 26 Jan 19:12
• Reporter: Antonios Tsianakas
• Status: Open
• Category: MySQL Package Repos
• Severity: S3 (Non-critical)
• Version: 8.4.8
• OS: Ubuntu
• CPU Architecture: x86

[26 Jan 19:12] Antonios Tsianakas

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