resourceloader: Store relative instead of absolute paths in module_deps
Make paths stored in the module_deps table relative to $IP. This ensures that when
the MediaWiki install path changes and/or if the location of the extension or skins
directory changes, that ResourceLoader's internal model is still accurate.
Previously when these paths change, ResourceLoader would exhibit various bugs.
- Unable to detect changes in the module (if the directory no longer exists).
- Point #1 is usually preceeded by one last cache invalidation as the content hash of the file path changes (from a valid hash to an empty string).
- Unnecessary cache invalidation (if both old and new directories exist). This happens when a file is both an entry point (in the 'scripts' or 'styles' array) and also a file dependency. At first they are de-duplicated by array_unique. But after the disk path changes, the next check will result in the old path being fetched from module_deps, and the new path from the live configuration. This causes two changes that result in needless cache invalidation:
- The hash list contains one more item (T111481).
- The hash list contains both the old and new version of a file. (or even alternate versions, e.g. when a change is backported to the old wmf branch; it also affects wikis on the new branch due to the stale file path still in the database).
It seems unusual to move a MediaWiki install, and usually we recommend third
parties to run update.php if site administrators do move their wiki. However
Wikimedia's deployment system implicitly moves the MediaWiki install continously
from e.g. "/srv/mediawiki/php-1.26wmf5" to "/srv/mediawiki/php-1.26wmf6".
This caused virtually all ResourceLoader caching layers to get invalidated every
week when another wmf-branch is deployed, thus breaking these file paths, which
changes the version hash, which then invalidates the cache.