GSoC/2021/StatusReports/AnjaniKumar

digiKam: Port to Qt6 on Linux

digiKam is a professional photo management software. With the release of Qt6, digiKam needs to to adapt to many new improvements made possible with the new Qt6 framework such as code readability, better classes from Qt6 and new unit tests. Since digiKam's code-base is huge, a proper plan had to be agreed upon to achieve the goal in the GSoC timeline.

Mentors

• Gilles Caulier
• Maik Qualmann
• Thanh Trung Dinh

Links

• GSoC project link

• digiKam repository

• My Work branch

Goals

• To build digiKam with current Qt6 stable release.
• Prepare digiKam so that it can be built with later Qt6 releases as well.
• Write new AppImage builder scripts

Status Report

Community Bonding (May 18 - June 6)

We discussed the proposal again and made some strategies on the first task and how to test the new patches.The first task was to Port digiKam to Qt 5.15. This is a sophisticated task and involves a lot of sub-steps like:

• Identifying deprecated classes and class members/functions in Qt 5.15
• Identifying their uses in digiKam code and organizing them based on which class has most uses.
• Porting the identified code. Sometimes trivial, sometimes needs reviews from mentors.
• Writing new unit test if needed.
• Running tests and check the patched code at runtime with the digiKam stable release.

We planned a safe way to introduce patches safely in the code, otherwise regressions might be introduced. Here is a list that I made of deprecated code in digiKam and sorted it according to number of usages it had.

• QTextStream::endl
• QStringList
• qrand()
• qsrand()
• QSet
• QImage
• QWheelEvent
• QMap
• QHash
• QDateTime
• QButtonGroup
• QPixmap
• QPrinter
• QProcess
• QTabletEvent
• QTimeLine
• QMutex

After identifying and listing the code that needed to be patched, I proposed two methods to approach the problem:

• We pick a source file in digiKam and make patches to it resolving all Qt deprecation warnings.
• We pick a Qt class from our list and identify it's use in all digiKam code and patch them.

The latter was safer as dealing with a lot of Qt classes in the same file might have led to regressions which were difficult to track down.

So I started early in this time only so that I don't get stuck in the coding period. I started picking Qt classes with few usages and started porting them. These were QMap, QPrinter. I faced some difficulty but my mentor's feedback helped me a lot and learnt quickly. More about this period in the blog post.

Related blog

https://anjani.live/posts/gsoc-community-bonding/

Coding Period (June 7 - July 12)

Week 1

I patched a lot of warnings mostly from Qt classes that had fewer warnings. I also wrote a new unit test for FilterAction class. This test was related to QHash warnings. The test checked if all keys are unique in a QHash in FilterAction class. In a nutshell, there were 6693 lines of build warnings on master.With the patches on my branch these warnings reduced to 1830 lines. That’s a reduction of about 73%.

Related Patches

• Fix QMap::unite() deprecation warning in presentation plugin
• Fix QProcess deprecated warning in cameracontroller backend
• Fix QHash deprecated declarations in FilterAction class
• Fix QImage deprecated transformed() usages
• New Unit test: FilterAction Unique keys test for all filters

Related Blog

https://anjani.live/posts/gsoc21-week-1/

Week 2

Continuation of work from previous week. We also discussed about introducing STL algorithms in digiKam. Though it is huge task in its own and not related to my project so we suppressed the cppcheck warnings in report and moved on.

Related Patches

• Fix QSet issues by using only QSet constructor to make QSet objects
• Version check for QButtonGroup
• Version check for QWheelEvent
• Version check for QLabel
• Version check for QFlag
• Macro for Qt::endl
• Macro for Qt::SplitBehavior

Related Blog

https://anjani.live/posts/gsoc21-week-2/

Week 3

During this week I patched the very old random number generators and qsrand()/qrand() in digiKam. As a result we were able to drop Boost::Random dependency. I also ported Qt X11 Extras module code. A very detailed report is in the blog post.

Related patches

• Remove qsrand
• Remove qrand
• Port class RandomNumberGenerator to drop boost::random
• Remove Qt X11Extras module
• Remove deprecated QMutex usage

Related Blog

https://anjani.live/posts/gsoc21-week-3/

Week 4

In this week, digiKam was fully ported to Qt 5.15. We also started discussion on now introducing Qt6 this week. I tried building it for the first time with Qt6, it failed(no surprises here). We ran into a problem that digiKam depends on Qt WebEngine which would only be stable in Qt 6.2 in September. We discussed about possible solutions. I tried building the alpha Qt 6.2 but it doesn't build.

Related Patches

• Replace removed QDateTime constructor with new function
• Port dbjobsthread.cpp to not use deprecated const_iterator “+” operator
• Isolate rajce plugin

Week 5

I found the solution to problem from last week. Gilles also helped me to get started. This week I worked on porting the build system to Qt6. digiKam is huge and we have successfully branched the build process if no web module is found. A more detailed report is in the blog.

Related Patches

• Dummy webwidget

Related Blog

https://anjani.live/posts/gsoc21-week-5/

Week 6

This week our goal was to port the build system in such a way that it can detect a Qt installation 5/6 and adjust accordingly. Our goal was to preserve the functionality of current Qt5 pipelines and tests. We used cmake variables and patched hardcoded Qt versions in the cmake with dynamically set variables enabling cmake configuration with both Qt5 and Qt6.

Related Patches

• Port CMake files
• Link Core5Compat module
• Port cmake include dirs in bqm/metadata

Related Blog

https://anjani.live/posts/gsoc21-week-6/