Mateusz Mandera 7ff1227984 presence: Improve INSERT conflict handling in do_update_user_presence. ...

This was discussed in the review of #29999:
https://github.com/zulip/zulip/pull/29999#discussion_r1620818568

The previous way of handling wasn't entirely correct, as unnecessary
events were omitted, with a bad guarantee of even being in the correct
order.

This is an improvement as now the function detects that it ended up
doing nothing and can skip sending an event.

The race condition is hard to make up in an automated test, but we can
hackily simulate it by injecting a side_effect which will create a
conflicting UserPresence row when the function requests a cursor. Aside
of that, the actual race was simulated in manual testing to verify the
expected behavior.

2024-06-16 11:57:47 -07:00

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