[QGIS-Developer] How to handle upstream Qt fixes

[Alessandro Pasotti]

Hi Nyall,

We discussed this topic in today's PSC meeting and we have an answer about
the question if we can invest money for KDAB help in fixing Qt 5 bugs.

The answer is yes, even if the patches are against an officially
unsupported Qt 5 version, if you want to contact them please go ahead.

For windows we can apply the patches ourselves, for debian & friends there
is a good chance the KDE patches + any other patch that fixes critical bugs
will be applied anyway.

Kind regards.


On Fri, Sep 3, 2021 at 12:46 AM Nyall Dawson <nyall.dawson at gmail.com> wrote:

> Hi PSC, devs,
>
> I'd like to kick start some discussion on how best to handle the
> situation with upstream Qt and their (lack of) support for Qt 5. As a
> quick summary of the situation:
>
> - Qt Co effectively ended open source support of Qt 5 at the 5.15.2
> release, and have moved all focus to Qt 6.
> - While some preliminary work has been done, QGIS doesn't currently
> support Qt 6 based builds, and likely won't be ready for this for some
> time (even completely ignoring all the stable API questions a Qt 6
> build raises entirely!)
> - QGIS often depends on fixes and enhancements which need to be made
> upstream in Qt, and can't be resolved or worked-around in QGIS alone
> - KDE and other open source projects forked Qt 5.15 at
> https://invent.kde.org/qt/qt/qtbase/-/commits/dev/, and are actively
> backporting fixes from Qt6 to that branch. Fedora recently started
> using the KDE branch for Qt 5 library builds, so users of that
> platform once again are getting bug fixes deployed [1]. I'm unaware if
> other distributions or builds of Qt are using this currently.
> - Similarly, there's a KDE fork of Qt 3d at
> https://invent.kde.org/qt/qt/qt3d/-/commits/kde/5.15/
>
> Right now, there's a number of very frustrating issues that Qt 5.15.2
> has which impact our users. An example is #44876, which results in
> very large PDF exports from QGIS with broken hairline line rendering
> [2]. In the past QGIS has contracted KDAB as part of the QGIS bug
> fixing efforts to directly fix issues which affect QGIS users
> upstream, with good results. Unfortunately, given that we are stuck on
> Qt 5.15.2 and upstream won't release any more 5.15.x versions, we
> can't just do that same approach again to get fixes into Qt.
>
> So I'd like to raise discussions about the best way we can handle this
> situation as a downstream project.
>
> My thoughts/questions:
>
> - Are we free to change the Windows builds to use the KDE backports
> fork of 5.15 instead of the official 5.15.2 releases? (Or does that
> change lots of osgeo4w packaging things?). Similarly, are we free to
> move the MacOS builds to the KDE branch too?
> - Could we also move the Windows/MacOS builds of Qt 3d to use the KDE fork?
> - Does anyone know if Debian have plans to migrate to the KDE
> backports fork? (Last I heard, the debian Qt maintainers stepped down
> and the package is currently lacking a maintainer!)
> - If we can get the majority of our users onto builds which use the
> KDE backports branch (i.e. Windows/mac users), could we re-start the
> relationship with KDAB and contract them for bug fixes again for 3.22?
> (with the arrangement explicitly requiring them to backport fixes to
> Qt 5 via KDE's fork).
>
> Nyall
>
> [1]
> https://src.fedoraproject.org/rpms/qt5-qtbase/c/400d49b3925dc2852218289310674abd3950b4e0?branch=rawhide
> [2] https://github.com/qgis/QGIS/issues/44876
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-- 
Alessandro Pasotti
QCooperative:  www.qcooperative.net
ItOpen:   www.itopen.it
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