Detect updated mofiles

[soulseekah]

When mofiles are updated we have to clear our caches, otherwise changed translations will not be served. New translations will, though.

First thought is parsing the update date from the matching po file, but that might not even exist. What do we do then? md5sum the mofile and match against the sum in cache? That would add some overhead, we don't want to open the files unless we really need to.

More ideas? Plugin versions? Update hooks? Not reliable.

[versusbassz]

https://wordpress.stackexchange.com/questions/270252/filter-hook-on-plugin-update
maybe this hook?

[soulseekah]

This doesn't account for manual updates (via SFTP, etc.), Loco Translate updates (which writes to mo files directly). Maybe filemtime()?

[soulseekah]

Couple of things to note with filemtime() caching (i.e. stat caching) - according to the PHP source stat caching is done in the basic globals (BG) which is destroyed on module shutdown (PHP_MSHUTDOWN_FUNCTION). This means that caches could persist for quite a while, especially in pool-based configurations where the modules are shared across subprocesses.

This needs more preliminary testing, especially with php-fpm.