Review and improve reliability of language pack upgrades

[jsnajdr]

In a ​WooCommerce GitHub issue we were investigating user reports about missing translations after upgrading the WooCommerce plugin to a new version. And we tracked it down to unreliability of language pack upgrades for the plugin.

When you upgrade WooCommerce from version 10.0.4 to 10.1.0, the new version 10.1.0 is not fully compatible with the language pack for 10.0.4. Some translations will be missing. Even if they are not new: for client-side JSON translation files, a translation can move from one JSON file to another, and then load_script_textdomain will fail to load the matching JSON file for a given JS script if the language pack is not up to date.

A plugin upgrade (at least manual one in WP Admin) should trigger also language pack upgrade (it's the LanguagePackUpgrader::async_upgrade method hooked to upgrader_process_complete) but apparently failed language pack upgrade doesn't affect the result of the plugin upgrade. It's reported as success even if languages failed to update.

We've seen cases where after a successful plugin upgrade the language pack is not updated at all (there are only old files) and the site is therefore in a broken state (wrong language in parts of frontend).

We've also seen a case where wp-content/languages/plugins/woocommerce contained files from both old and new language packs. That doesn't break the site, but it's still something that shouldn't happen: the language pack upgrader is supposed to clear the destination directory before moving the new files there. This is another sign that something is unreliable.

Looking at the code confirms that errors from LanguagePackUpgrader are generally ignored. Wherever we call the upgrader's upgrade_bulk method, we ignore the return value:

• in update-core.php handling the do-translation-upgrade form action
• in the async_upgrade method hooked to be done after a plugin upgrade
• in the REST handler for POST /wp/v2/plugins after a plugin install/upgrade

Compared to that, when calling upgrade_bulk on a Plugin_Upgrader or Theme_Upgrader the results are handled and reported.