Configuration of existing Yocto recipes

I am currently looking for the best way to configure existing Yocto recipes for my use case. For example the MySQL server of mariadb - part of the meta-oe layer.

I “activate” PACKAGECONFIG variables using .bbappend files. However, mariadb also has a configuration file (as CONFFILES) my.cnf in the recipe. How do I sensibly change the settings there?

Can I overwrite CONFFILES? Do I overwrite the file in my recipe, or in a .bbappendfile?
Do I patch the file?
Do I put my own configuration file in /etc/my.cnf.d/ “next to it” and try to overwrite the options?

Of course I don't want to change the original sources of the recipe to minimize the effort for updates.

I have found a lot of good instructions and documentation on how to create my own layers and recipes, but nothing that describes how I can best adapt existing recipes to my needs.

Solution

my.cnf already exists in SRC_URI of the original mariadb.inc file of meta-oe.

You just need to trick BitBake to search in your custom layer before reaching the actual meta-oe's mariadb recipe files.

Your custom layer should have:

recipes-support/
└── mysql
    ├── files
    │   └── my.cnf
    └── mariadb_%.bbappend

Then, in mariadb_%.bbappend just add:

FILESEXTRAPATHS:prepend := "${THISDIR}/files:"

Now, the original mariadb recipe (NB: by original I mean the final recipe after expanding all bbappends) will look for the file://my.cnf file using FILESEXTRAPATHS, and since you have prepended your custom path, it will find it there first.

NOTE: You don't have to add my.cnf to SRC_URI again in your bbappend.