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resourceloader: Replace timestamp system with version hashing

Authored by Krinkle.

Description

resourceloader: Replace timestamp system with version hashing

Modules now track their version via getVersionHash() instead of getModifiedTime().

Background

While some resources have observeable timestamps (e.g. files stored on disk),
many other resources do not. E.g. config variables, and module definitions.

For static file modules, one can e.g. revert one of more files in a module to a
previous version and not affect the max timestamp.

Wiki modules include pages only if they exist. The user module supports common.js
and skin.js. By default neither exists. If a user has both, and then the
less-recently modified one is deleted, the max-timestamp remains unchanged.

For client-side caching, batch requests use "Math.max" on the relevant timestamps.
Again, if a module changes but another module is more recent (e.g. out-of-order
deployment, or out-of-order discovery), the change would not result in a cache miss.

More scenarios can be found in the associated Phabricator tasks.

Version hash

Previously we virtually mapped these variables to a timestamp by storing the current
time alongside a hash of the value in ObjectCache. Considering the number of
possible request contexts (wikis * modules * users * skins * languages) this doesn't
work well. It results in needless cache invalidation when the first time observation
is purged due to LRU algorithms. It also has other minor bugs leading to fewer
cache hits.

All modules automatically get the benefits of version hashing with this change.
The old getDefinitionMtime() and getHashMtime() have been replaced with dummies
that return 1. These functions are often called from getModifiedTime() in subclasses.

For backward-compatibility, their respective values (definition summary and hash)
are now included in getVersionHash directly.

As examples, the following modules have been updated to use getVersionHash directly.
Other modules still work fine and can be updated later.

  • ResourceLoaderFileModule
  • ResourceLoaderEditToolbarModule
  • ResourceLoaderStartUpModule
  • ResourceLoaderWikiModule

The presence of hashes in place of timestamps increases the startup module size on
a default MediaWiki install from 4.4k to 5.8k (after gzip and minification).

ETag

Since timestamps are no longer tracked, we need a different way to implement caching
for cache proxies (e.g. Varnish) and web browsers. Previously we used the
Last-Modified header (in combination with Cache-Control and Expires).

Instead of Last-Modified (and If-Modified-Since), we use ETag (and If-None-Match).

Entity tags (new in HTTP/1.1) are much stricter than Last-Modified by default.
They instruct browsers to allow usage of partial Range requests. Since our responses
are dynamically generated, we need to use the Weak version of ETag.

While this sounds bad, it's no different than Last-Modified. As reassured by
RFC 2616 http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.3.3 the
specified behaviour behind Last-Modified follows the same "Weak" caching logic as
Entity tags. It's just that entity tags are capable of a stricter mode (whereas
Last-Modified is inherently weak).

File cache

If $wgUseFileCache is enabled, ResourceLoader uses ResourceFileCache to cache
load.php responses. While the blind TTL handling (during the allowed expiry period)
is still maxage/timestamp based, tryRespondNotModified() now requires the caller to
know the expected ETag.

For this to work, the FileCache handling had to be moved from the top of
ResoureLoader::respond() to after the expected ETag is computed.

This also allows us to remove the duplicate tryRespondNotModified() handling since
that's is already handled by ResourceLoader::respond() meanwhile.

Misc

  • Remove redundant modifiedTime cache in ResourceLoaderFileModule.
  • Change bugzilla references to Phabricator.
  • Centralised inclusion of wgCacheEpoch using getDefinitionSummary. Previously this logic was duplicated in each place the modified timestamp was used.
  • It's easy to forget calling the parent class in getDefinitionSummary(). Previously this method only tracked 'class' by default. As such, various extensions hardcoded that one value instead of calling the parent and extending the array. To better prevent this in the future, getVersionHash() now asserts that the '_cacheEpoch' property made it through.
  • tests: Don't use getDefinitionSummary() as an API. Fix ResourceLoaderWikiModuleTest to call getPages properly.
  • In tests, the default timestamp used to be 1388534400000 (which is the unix time of 20140101000000; the unit tests' CacheEpoch). The new version hash of these modules is "XyCC+PSK", which is the base64 encoded prefix of the SHA1 digest of: '{"_class":"ResourceLoaderTestModule","_cacheEpoch":"20140101000000"}'

May also resolve:

Bug: T94074
Change-Id: Ibb292d2416839327d1807a66c78fd96dac0637d0

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