In earlier times I though about a caching mechanism where all existing unique identifiers (host and paths) would be in a single cache file, with associated existence configuration (variables and config to recompile the LocalSettings); but I prefered create separated files instead of such a big file to avoid too much writes in a single file (a cache with too much writes is no more a real cache). This could be a solution, with the variation such a big file could be only a dictionary 'host' => [ paths ].

I see another solution where the existing cached files (by host only) have some indirection towards cached files with host and path. I think it would be complicated to implement.

My prefered solution is to create an 'existence' dictionary 'host' => [ list of path prefixes ], and then keep the current mechanism for the other per-wiki configuration (variables and list of config files for LocalSettings). Additionnally I’m not really comfortable with the current mechanism where the wiki name is directly searched against the file name; it could create security issues, and the proposed solution is much more robust: if the host doesn’t exist => 404, if the path doesn’t exist => 404, and this check is contained in a proper PHP array instead of the file system.

In the original description I was mistaken about the cached files in /cache/composer and /cache/LocalSettings, it was correctly implemented - edited the description accordingly.

Change 376690 had a related patch set uploaded (by Seb35; owner: Seb35):
[mediawiki/extensions/MediaWikiFarm@master] Cache files in /cache/wikis were never used

https://gerrit.wikimedia.org/r/376690

Change 376690 merged by jenkins-bot:
[mediawiki/extensions/MediaWikiFarm@master] Cache files in /cache/wikis were never used

https://gerrit.wikimedia.org/r/376690

For cross-reference I found this bug when I studied T162733.