Fix UniqueConstraintViolationException while insert into oc_file_locks

[MorrisJobke]

• fixes Race condition with locks in database - unique constraint violation on oc_file_locks #9305 by not being prone to the race condition in insertIfNotExists
• fixes Serialization failure: 1213 Deadlock found when trying to get lock; try restarting transaction #6899 by not using a query that can result in a deadlock
• replaces the insertIfNotExists call with an insert which is wrapped into a try-catch block
• followup to Catch UniqueConstraintViolationException inside insertIfNotExist #12371

I would back port this one to stable13 and stable14 as well because it is a smaller and not that invasive version of #12371 and it also fixes the deadlocks.

[blizzz]

make sense

[rcdailey]

Since this will be backported, is there a PR or issue I can follow to track when this makes it into v14?

[kesselb]

#12433 will be part of https://github.com/nextcloud/server/milestone/57

[wazoox]

HI, just upgraded to Nextcloud 15.0.4, the problem persists. The postgresql logs are polluted by these 5 times a second:

2019-02-27 18:15:01 UTC [29519-8] nextcloud@nextcloud DÉTAIL:  La clé « (key)=(files/a2847fe27d8c3fc0b2a6a889de5f4a6c) » existe déjà.
2019-02-27 18:15:01 UTC [29519-9] nextcloud@nextcloud INSTRUCTION :  INSERT INTO "oc_file_locks" ("key", "lock", "ttl") VALUES($1, $2, $3)

Everything seems to work fine, but this is slowing down the system a lot which crawls under postgresql load.

[MorrisJobke]

HI, just upgraded to Nextcloud 15.0.4, the problem persists. The postgresql logs are polluted by these 5 times a second:

Fix for this is in #13721

[AndyXheli]

Issue is backup again in #6899