[QGIS-Developer] LTR 3.10: Fixing issues when content cache is used in blocking mode

[Julien Cabieces]

Hi,

Like I said in the PR, I decided to not merge this fixes (there is other
fixes related to the task manager race condition issues) in LTR because
it was dangerous modifications which have an impact on the entire
application.

Even if no one has complained about those modifications in master
and 3.12, I will vote for solution 2 because of lesser impact.

Regards,
Julien

> Hi devs,
>
> I need your point of view about fixing issues when content cache is used 
> in blocking mode.
>
> A QGIS Server user has identified an issue when content cache is used in 
> blocking mode. In some cases, the bug is timed out QGIS Server.
> This issue is due to a loop to wait for content task cache begun. In 
> some cases, the loop starts even if the task has already begun, and it 
> is never stopped.
> This issue has been fixed in master and version 3.12, by fixing the task 
> manager waitforfinished (PR 32838 [1], a 6 mounths old PR) and removing 
> the loop when content cache is used in blocking mode (part of PR 35158 [2]).
> These pull requests has not been backported to LTR, the 3.10 release.
>
> To fix, the issue in LTR, I have manually backported the 2 Pull Requests:
> * Backport Fix Task manager waitforfinished, PR 36651 [3]
> * Backport Fix issues when content cache is used in blocking mode, PR 
> 36624 [4]
> And requested Nyall Danwson reviews.
>
> After Nyall reviews, I have two options to fix the issue:
> 1. Fix Task manager waitforfinished (merge PR 36651 [3]) and removing 
> the loop (removing commit c184270 [5] from PR 36624 [4] then merge it)
> 2. Keep and fix the loop (merge PR 36624 [4] and close PR 36651 [3])
>
> So I need a collegial decision about the least bad way to fix the issue:
> 1. Merge a risky backport and remove the loop
> 2. Keep and fix the loop
>
> Thanks for your feedbacks.
>
> René-Luc
>
> [1] https://github.com/qgis/QGIS/pull/32838
> [2] https://github.com/qgis/QGIS/pull/35158
> [3] https://github.com/qgis/QGIS/pull/36651
> [4] https://github.com/qgis/QGIS/pull/36624
> [5] 
> https://github.com/qgis/QGIS/pull/36624/commits/c184270ebe2bb51164fdcd13b7ccd8ebc42e71a0
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