only allow a single concurrent dav write to a file

[icewind1991]

While concurrent writes wont lead to file corruption it's a waste of bandwith and multiple chunked upload assemblies can lead to the server running out of space

[codecov]

Codecov Report

Merging #9355 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@ Coverage Diff @@## master #9355 +/- ##
============================================
+ Coverage 51.93% 51.93% +<.01% 
Complexity 25394 25394 ============================================
Files 1608 1608 Lines 95444 95448 +4 Branches 1393 1393 ============================================
+ Hits 49565 49569 +4 
Misses 45879 45879

Impacted Files	Coverage Δ	Complexity Δ
apps/dav/lib/Connector/Sabre/Directory.php	66.45% <100%> (+0.85%)	65 <0> (ø)	⬇️
apps/files_trashbin/lib/Trashbin.php	72.46% <0%> (-0.25%)	136% <0%> (ø)
apps/files_trashbin/lib/Expiration.php	91.93% <0%> (+1.61%)	29% <0%> (ø)	⬇️

[MorrisJobke]

@icewind1991 Could you explain a bit more the use case and scenario here?

[icewind1991]

Something was causing a sync client to have send multiple requests to do chunk assembly for a 100gb file at the same time.
The different part files were filling up the disk, causing the assemblies to fail and the whole thing to start over.

With this we can be sure that only one assembly can run at the same time

[rullzer]

So you just lock some specific file (which doesn't exist at that point) right? Just to prevent multiple trying to do it at the same time right?

[icewind1991]

Yes, locking a dummy file allows blocking concurrent access while still allowing read access to the file while writing the part file

[MorrisJobke]

@icewind1991@rullzer Should we backport this to stable13?

[rullzer]

Probably!