ensure uid for calendar objects is unique

[georgehrke]

At the moment it's possible to have the same UID for multiple calendar objects in a calendar. This doesn't comply with the calendar standard.

The UID property value of the calendar components contained in a
calendar object resource MUST be unique in the scope of the calendar
collection in which they are stored.

Calendar components in a calendar collection that have different UID
property values MUST be stored in separate calendar object resources.

Calendar components with the same UID property value, in a given
calendar collection, MUST be contained in the same calendar object
resource.

https://tools.ietf.org/html/rfc4791#section-4.1

--

Yes, have have to add a unique key for calendarid and uid in the database, BUT:
Before we can do that, we have to fix existing calendar data of a user, otherwise the database migration will fail.
We can only do pre-db-change migrations with Nextcloud 13+, right now we need something we can backport as well.

--

To test this:

• create an event in the calendar app
• export the event
• import it in the same calendar

It will work in master (wrong behavior) and will return a 400 on this branch (correct behavior).

[nickvergessen]

$result->closeCursor();

[georgehrke]

done

[codecov]

Codecov Report

Merging #7042 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #7042      +/-   ##
============================================
+ Coverage     50.61%   50.62%   +<.01%     
- Complexity    24297    24298       +1     
============================================
  Files          1577     1577              
  Lines         92922    92932      +10     
  Branches       1359     1359              
============================================
+ Hits          47037    47046       +9     
- Misses        45885    45886       +1

Impacted Files	Coverage Δ	Complexity Δ
apps/dav/lib/CalDAV/CalDavBackend.php	85.71% <100%> (+0.15%)	219 <0> (+1)	⬆️
lib/private/Files/Cache/Propagator.php	94.93% <0%> (-1.27%)	16% <0%> (ø)

[MorrisJobke]

Tested and works 👍

[LukasReschke]

@georgehrke Please open backports if you consider this backport worthy. Thanks! :)

[LukasReschke]

That said, how big is the real-life risk here? 😄 – Does this existing behavior lead to data loss or so?

[georgehrke]

@LukasReschke Already done ;)
#7096

[georgehrke]

Does this existing behavior lead to data loss or so?

No data loss, just not standard compliant and might provoke errors if clients request events by UID.
Also allows you to have duplicate events in your calendar, if u import them twice. => unexpected behavior