Adding StructuredDiscussions just because I see Flow in that stacktrace a lot. Feel free to retag.

T194731: Show diffs for all slots [MCR] might be related. You can no longer view a diff on a talk page, e.g. at https://www.mediawiki.org/w/index.php?title=Talk:MediaWiki_1.31&action=history, try clicking on "prev" on a revision (e.g. https://www.mediawiki.org/w/index.php?title=Topic:Uhlvrgan1cuqzpdt&action=compare-post-revisions&topic_newRevision=uhlvvrmnceuctzk1)

It looks to me like rMWd31580eeb0e9: [MCR] Render multi-slot diffs made a breaking change to DifferenceEngine. It has a lot of constructor parameters, but they're all marked optional. Flow does something like:

$differenceEngine = new \DifferenceEngine;
$differenceEngine->showDiffStyle();

(actual code here)

This worked in wmf.16 but fails in wmf.18:

catrope@mwmaint1001:~$ mwscript eval.php testwiki
> $de = new DifferenceEngine; $de->showDiffStyle();
[Tue Aug 21 21:49:57 2018] [hphp] [8336:7f5b6de923c0:0:000002] [] 
Catchable fatal error: Argument 1 passed to Revision::newFromTitle() must implement interface MediaWiki\Linker\LinkTarget, null given in /srv/mediawiki/php-1.32.0-wmf.18/includes/diff/DifferenceEngine.php on line 1662

catrope@mwmaint1001:~$ mwscript eval.php enwiki
> $de = new DifferenceEngine; $de->showDiffStyle();

>

It looks like [[https://github.com/wikimedia/mediawiki/commit/d31580eeb0e9a88f46439301e27db273b7c63a7e#diff-6ce4bad77eaf1d8bd47947c6b8ccc3e4R888|the change to showDiffStyle()]] made it depend on the state of the object where it previously didn't. Flow tries to (ab)use DifferenceEngine as a stateless diff renderer, and that doesn't work anymore.

Change 454425 had a related patch set uploaded (by Catrope; owner: Catrope):
[mediawiki/extensions/Flow@master] Work around exception in DifferenceEngine::showDiffStyle()

https://gerrit.wikimedia.org/r/454425

new DifferenceEngine() is not an "empty" diff object, it's a diff between the two last revisions of the current page. That did not change; what changed is that some methods including showDiffStyle did not care about what revisions are being diffed, and now they do since they have to delegate to the diff renderer of each slot, so they need to know what slots are there. And when it tries to load the state, somehow RequestContext::getTitle() returns null and things blow up. Not sure how that can happen; from the stack trace it seems MediaWiki is fully set up by this point so $wgTitle should always be a title, Special:BadTitle if nothing else.

I suppose there are two things to fix here:

• DifferenceEngine should handle the context title being null
• There should be a way to create an "empty" DifferenceEngine. Technically there is (something like $t = Title::newFromText( 'Special:BadTitle' ); $de = new DifferenceEngine(); $de->setRevisions( new MutableRevisionRecord( $t ), new MutableRevisionRecord( $t ) ); would probably work) but it's a bit obscure. So either have static factory method for that, or just assume that calling the constructor with no arguments means asking for an empty DifferenceEngine (it seems reasonable to assume that no one uses it for getting a diff of the last edit).

Change 454484 had a related patch set uploaded (by Gergő Tisza; owner: Gergő Tisza):
[mediawiki/core@master] DifferenceEngine: use a fake title when there's no real title

https://gerrit.wikimedia.org/r/454484

@Tgr just noting that the patch in 454484 doesn't fix the exception for Flow topic diff. To test, enable Flow on your user page and add a topic, then edit it, and attempt to view the revision. When I'm looking at this locally I'm not seeing the issue as being with RequestContext::getTitle(), but rather that getPrevious() in includes/Revision.php fails to load a previous revision for a Flow topic.

So it seems like reverting the code that caused this would be too onerous. There is some doubt about the RequestContext::getTitle being the root cause. I'd be fine with a workaround in Flow to unblock train over the short-term @Catrope if you could get someone to review your change to Master branch of Flow, I can backport in the train window.

@thcipriani I'll +2 the Flow patch as soon as the tests finish running

Change 454626 had a related patch set uploaded (by Thcipriani; owner: Catrope):
[mediawiki/extensions/Flow@wmf/1.32.0-wmf.18] Work around exception in DifferenceEngine::showDiffStyle()

https://gerrit.wikimedia.org/r/454626

Change 454425 merged by jenkins-bot:
[mediawiki/extensions/Flow@master] Work around exception in DifferenceEngine::showDiffStyle()

https://gerrit.wikimedia.org/r/454425

Change 454626 merged by jenkins-bot:
[mediawiki/extensions/Flow@wmf/1.32.0-wmf.18] Work around exception in DifferenceEngine::showDiffStyle()

https://gerrit.wikimedia.org/r/454626

Mentioned in SAL (#wikimedia-operations) [2018-08-22T19:38:52Z] <thcipriani@deploy1001> Synchronized php-1.32.0-wmf.18/extensions/Flow/includes/TemplateHelper.php: [[gerrit:454626|Work around exception in DifferenceEngine::showDiffStyle()]] T202454 (duration: 00m 57s)

The same issue might affect other extensions that use DifferenceEngine in a similar way (Translate, at least).

Change 454638 had a related patch set uploaded (by Gergő Tisza; owner: Gergő Tisza):
[mediawiki/core@master] DifferenceEngine: unbreak getSlotContents when mOldRev is missing

https://gerrit.wikimedia.org/r/454638

Change 454638 merged by jenkins-bot:
[mediawiki/core@master] Fix DifferenceEngine revision loading logic

https://gerrit.wikimedia.org/r/454638

Change 455254 had a related patch set uploaded (by Jforrester; owner: Gergő Tisza):
[mediawiki/core@wmf/1.32.0-wmf.18] Fix DifferenceEngine revision loading logic

https://gerrit.wikimedia.org/r/455254

In T202454#4524463, @Tgr wrote:

The same issue might affect other extensions that use DifferenceEngine in a similar way (Translate, at least).

That was reported as T202809 and marked as duplicate of this and this bug reopened. It is unclear to me whether the core changes are enough, or whether Translate needs a separate patch similar to Flow's https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Flow/+/454425.

Change 455254 merged by jenkins-bot:
[mediawiki/core@wmf/1.32.0-wmf.18] Fix DifferenceEngine revision loading logic

https://gerrit.wikimedia.org/r/455254

Mentioned in SAL (#wikimedia-operations) [2018-08-27T11:40:08Z] <tgr@deploy1001> Synchronized php-1.32.0-wmf.18/includes/diff/DifferenceEngine.php: SWAT: [[gerrit:455254|Fix DifferenceEngine revision loading logic (T201218, T202454)]] (duration: 00m 49s)

Please check on delete "[W4T9wApAIC0AAIMtO9sAAABO] 2018-08-28 07:46:09"

In T202454#4537342, @Wargo wrote:

Please check on delete "[W4T9wApAIC0AAIMtO9sAAABO] 2018-08-28 07:46:09"

That's T200417: TranslateRenderJob.php: Cannot render translation page.

Change 454484 merged by jenkins-bot:
[mediawiki/core@master] DifferenceEngine: use a fake title when there's no real title

https://gerrit.wikimedia.org/r/454484