This was due to an outage, we identified and fixed the root cause around 5-10 minutes ago and we should be ok now.
We are now investigating why the outage happened in the first place

As the description was overwritten: it didn't break instantly and maybe it was never strictly down, just too slow to work. For me it was extremely slow for a few minutes or so first (taking 20s to load a page) until finally I saw "upstream connect error or disconnect/reset before headers. reset reason: overflow".

Btw, Phabricator was up and performing normally, as was beta cluster. https://foundation.wikimedia.org/wiki/ was down, as was enwiki and metawiki.

Updating this task - cross posting from the original schema change task:
This query seemed to be the one that got stuck

SELECT /* MediaWiki\Extension\GlobalBlocking\GlobalBlocking::getGlobalBlockingBlock  */  gb_id,gb_address,gb_by,gb_by_wiki,gb_reason,gb_timestamp,gb_anon_only,gb_expiry,gb_range_start,gb_range_end  FROM `globalblocks`WHERE (gb_range_start  LIKE '5B85%' ESCAPE '`' ) AND (gb_range_start <= '5B85B2D2') AND (gb_range_end >= '5B85B2D2') AND (gb_expiry > '20220505012805');

However the explain output (and the optimizer trace) shows no difference on query plans between the original schema change and the new one (T307501)

@AlexisJazz Full doc will come later, but for clarification, the impact was the following:

• Cached requests (anonymous users' reads) were mostly unaffected- only 15 % to 20% of total requests got affected- mostly authenticated and power users, edits and certain kinds of special requests
• Uncached requests started to get slower at 5:36, reaching highest level of slowness (full outage) at 5:39.
• Issue started to get better at 5:51, reducing the average latency
• Issue fully solved at 5:55

A most recent test on one of the most affected hosts during the outage, does show a different query plan:

https://phabricator.wikimedia.org/T307501#7906462

Just for greater visibility and awareness, there is T301505 for the upstream connect error or disconnect/reset before headers. reset reason: overflow". error. As pointed out in that task that error message is a symptom and not the cause.