Added wikidata-bugs as a CC as adding Wikidata as a project isn't possible for some reason (@Aklapper)

[offtopic] @JohnLewis: ➔ T76732

Also see discussion and comments on https://phabricator.wikimedia.org/T89962

Investigating this took me deep into the wonderful land of ParserOptions, ParserOutput, ParserCache and OutputPage.

So the issue is that the parser cache should be split on whether "editsection" links are present in the output. But MediaWiki doesn't do that. So here is what I found.

• MediaWiki core actually has provisions for it: ParserOptions::optionsHash considers a flag called "editsection".
• But that flag is only considered if it was registered via ParserOutput::registerOption. This is where it gets arcane...
• Parser::clearState registers ParserOutput::registerOption as a "watcher" with ParserOptions
• ParserOptions calls back to the watcher whenever a "relevant" option is read, e.g. getUserLang() is called.

So, Parser::clearState enables the automatic splitting of the cache by registering the ParserOutput was a watcher to the ParserOptions. But Wikibase doesn't use the parser, and EntityParserOutputGerator doesn't register the watcher. So no automated splitting.

Now the splitting by user language is automatic (we do that somewhere explicitly - that's no longer needed then, I suppose). But it still doesn't work for editsection.

As it turns out, ParserOptions::getEditSection does not call $this->optionUsed( 'editsection' ), as I expected, following the example of getUserLang(), getThumbSize(), etc. The reason it doesn't is apparently another magic trick:

• Parser emits special <mw:editsection> tags and registers a replacement token in the ParserOutput
• These get replaced by the actual HTML for the editsection by ParserOutput::getText, using thetokens it got via ParserOutput::setEditSectionTokens.
• ParserOutput::getText delegates the generation of the actual section link to Skin::doEditSectionLink

We could try to use that mechanism by putting an <mw:editsection> tag into the wikibase-toolbar-bracketed template, and then customizing the output via the DoEditSectionLink hook called by Skin::doEditSectionLink.

But the simples solution would be to just call $parserOutput->recordOption( 'editsection' ) in EntityParserOutputGenerator. We don't even need to set up the "split cache automatically via watcher" magic for that to work, though I think we should do that anyway.

Change 192976 had a related patch set uploaded (by Tobias Gritschacher):
Split parser cache by editsection flag

https://gerrit.wikimedia.org/r/192976

Change 192976 merged by jenkins-bot:
Split parser cache by editsection flag

https://gerrit.wikimedia.org/r/192976