BUG #15715: UPDATE using "in (subquery for update limit 1)" does not respect the limit in subquery

The following bug has been logged on the website:

Bug reference: 15715
Logged by: Lars Vonk
Email address: lars(dot)vonk(at)gmail(dot)com
PostgreSQL version: 11.2
Operating system: Ubuntu 18.04.2 LTS
Description:

Hi,

We are using a ruby library called delayed-job
(https://github.com/collectiveidea/delayed_job) that generates the following
sql

LOG: duration: 82797.554 ms execute <unnamed>: UPDATE "delayed_jobs" SET
locked_at = '2019-03-26 13:25:20.808244', locked_by = 'host:myhost pid:9958'
WHERE id IN (SELECT "delayed_jobs"."id" FROM "delayed_jobs" WHERE ((run_at
<= '2019-03-26 13:25:20.807815' AND (locked_at IS NULL OR locked_at <
'2019-03-26 09:25:20.807828') OR locked_by = 'host:myhost pid:9958') AND
failed_at IS NULL) ORDER BY priority ASC, run_at ASC LIMIT 1 FOR UPDATE)
RETURNING *

We noticed that all the "delayed_jobs" were getting locked instead of just
1. There is already a bug report (and a recent "fix") for this in
delayed_job:
https://github.com/collectiveidea/delayed_job_active_record/issues/143

We also found some related stackoverflow content which sort of hints that
this is a bug:

- https://dba.stackexchange.com/questions/69471/postgres-update-limit-1
-
https://github.com/feikesteenbergen/demos/blob/master/bugs/update_limit_bug.txt

So is this a postgres bug? Or is "limit 1" not garantueed in subqueries?

Kind regards,
Lars