Any lensfun/darktable expert here?

I’m on macOS, the database is directly included in the app bundle, and I simply edit the mil-canon.xml. No lens-fun-update-data.
Of course a custom lensfun.xml is better, as the standard files might be overwritten with a lensfun-update-data, should this run somewhere in the background.

Anyway, I simply appended the ” + EXTENDER RF1.4x” to the model tag of the “Canon RF 100-500mm F4.5-7.1 L IS USM”, saved the file and done.
There’s no “en” version of the tag in mil-canon.xml, so no need to bother with it (but as I said, the code in lensfun would use the “en” entry if present and not the generic one, so you should delete it).
BTW, I modified the version_1 file, not the version_2 – not sure if this plays a role.

With this, the lens is already automatically recognized, but shown in a bit weird way in the lens correction module as “Canon, Canon RF 100 …”.
So I deleted the maker tag for that lens in the database, and now it shows up as expected “Canon RF 100…” automatically.