Skip locked may not be working correctly

[saward]

For a separate project, using a similar query, it appeared that a second process trying to pull tasks off would sometimes get none back, instead of the next unlocked one. The following query, similar to queue's query, did not work well (the returning

WITH u AS (
 SELECT
 repository_id AS target_repository_id,
 format(full_path, path) AS final_path
 FROM site.repository
 JOIN site.provider p ON p.provider_id = provider_id_provider
 WHERE last_checked IS NULL or last_checked < Now() - INTERVAL '1 hour'
 ORDER BY last_checked asc
 FOR UPDATE SKIP LOCKED
 LIMIT 1
)
UPDATE site.repository
SET last_checked = Now()
FROM u
WHERE repository_id = target_repository_id
RETURNING repository_id, final_path

Whereas this one worked as expected:

with selected AS (
 UPDATE site.repository
 SET last_checked = Now()
 WHERE repository_id = (
 SELECT
 repository_id
 FROM site.repository
 WHERE last_checked IS NULL or last_checked < Now() - INTERVAL '1 hour'
 ORDER BY last_checked, repository_id asc
 FOR UPDATE SKIP LOCKED
 LIMIT 1
 )
 RETURNING repository_id
)
SELECT
 repository_id AS target_repository_id,
 format(full_path, path) AS final_path
FROM site.repository
JOIN site.provider p ON p.provider_id = provider_id_provider
WHERE repository_id = (select repository_id from selected)