DifferenceEngine::getDiffBody should be returning a string or false so issue probably lies in Flow or includes/diff/DifferenceEngine.php

The error comes from

	public function generateContentDiffBody( Content $old, Content $new ) {
		Assert::parameterType( TextContent::class, $old, '$old' );
		Assert::parameterType( TextContent::class, $new, '$new' );

@Tgr added this in https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/MobileFrontend/+/452413/ as part of T201842. Can you provide some context?

Tentatively marking as blocking. It started we wmf.24 went to group0 with no instances of this error anywhere in the 30 days prior.

The code in MobileFrontend where the error first originated hasn't changed recently, although <https://gerrit.wikimedia.org/r/452413 might be related. It's possible that it is triggered by a change in core or Flow or elsewhere.

In T206090#4637209, @Jdlrobson wrote:

@Tgr added this in https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/MobileFrontend/+/452413/ as part of T201842. Can you provide some context?

As the commit summary says, the patch just moved code around. The equivalent pre-MCR code was this.

Is this a frequent error or just no one bothered to manually construct a mobile diff of a Flow edit before? There are 4 separate URLs in Logstash, could just be a developer poking around after seeing the initial error. That or MobileFrontend started to expose links somehow. Either way, doesn't seem like a train blocker to me and I doubt such URLs would have worked before. (How does Flow handle diffs at all? It has a non-text content type but it does not seem to have its own diff engine implementation.)

To be clear, the problem is not related to mobile diffs. Any extension that provides non-text content types (Content subclasses which are not TextContent subclasses) should also provide a custom DifferenceEngine or SlotDiffRenderer as the default one will throw. Flow somehow disables the normal diff URLs so something like https://www.mediawiki.org/w/index.php?diff=2382486 does not cause an error (although it doesn't do what the user would expect, either), but anything else that invokes the diff logic will throw, e.g. https://www.mediawiki.org/wiki/Special:ComparePages?rev1=2382480&rev2=2382486

Growth-Team discussed in triage this week, we're going to look at this soon and we'll start with a ~30 minute investigation.

Change 471901 had a related patch set uploaded (by Kosta Harlan; owner: Kosta Harlan):
[mediawiki/extensions/Flow@master] WIP: Return empty string for flow board diff

https://gerrit.wikimedia.org/r/471901

Change 471901 merged by jenkins-bot:
[mediawiki/extensions/Flow@master] Return false for Flow board diff

https://gerrit.wikimedia.org/r/471901