qt, build, ci: add optional Qt 6 support

[CyberTailor]

Introduce USE_QT6 CMake option to build the GUI with Qt 6 instead of Qt 5. Most of porting work was done with the assistance of Clazy and somewhat carefully reading the docs.

No functional changes are required.

Rationale

Qt5 upstream (Qt Company) OSS support ended on 2020-12-08¹. Since then, bugs are only fixed if reproduced in Qt6 first, then backported. Since then, public availability of commercial 5.15 LTS releases is delayed by 1 year, including repository access for cherry-picking.

The last official LTS release will be this April/May:
https://www.qt.io/blog/qt-5.15-extended-support-for-subscription-license-holders
https://wiki.qt.io/Qt_5.15_Release#Release_Plan

Public availability of this release will be 1 year later, in April/May 2026. There will be no Qt5 releases or bugfixes going forward.

KDE's Qt5PatchCollection has no clear EOL date², but since most of KDE has moved to Qt 6, it wouldn't last for long. And this work relies on Qt company's upstream commits anyway.

Some distros already stopped accepting new packages that use Qt 5, and began actively cleaning packages with no plans for porting to Qt 6.

Footnotes

1. https://www.qt.io/blog/qt-offering-changes-2020 ↩

2. https://community.kde.org/Qt5PatchCollection#For_how_long_will_this_be_maintained? ↩

[div72]

@jamescowens What are your thoughts on porting this to automake? Upstream is moving to CMake with the soon to be released v29, we might as well move to CMake-only with the next update/mandatory too.

[CyberTailor]

Qt 6 has all necessary pkg-config files, so it can be used with autotools. In my opinion, it would be easier to switch from Qt 5 to Qt 6 unconditionally to avoid adding more complexity to the build system.

[CyberTailor]

we might as well move to CMake-only with the next update/mandatory too

There is indeed no point in maintaining two build systems, and there should be a roadmap for replacing autotools with CMake to fill the gaps. CD still uses autotools, and it would need to be also ported. For creating installers and packages I've seen CPack being used. Makefile-based "depends" can still be used for the time being, as it's out of build system scope.

[jamescowens]

I had an offline conversation with @div72 about this. I agree. I think it is time to retire both Qt5 and the autotools build system. Big change, but it is the right move. I am willing to do this for the next mandatory (Natasha) given that we have some time while the consensus-critical code is soaking in testnet.

[jamescowens]

@CyberTailor Can you rebase this PR to the head of the development branch to clear the merge conflict please?

[ZiggySchulz]

Building this branch on macOS with using Qt 6.9 and CMake version 4.0.1 I get the following error

CMake Error at src/secp256k1/CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.

Which I believe is an existing issue. But after increasing the version I get the following deprecation warnings (duplicates removed):

/Users/ziggy/Documents/Dev/Gridcoin/Gridcoin-Research/src/qt/bitcoingui.cpp:316:54: warning: 'operator+' is deprecated: Use operator| instead [-Wdeprecated-declarations]
  316 |     overviewAction->setShortcut(QKeySequence(Qt::ALT + Qt::Key_1));

/Users/ziggy/Documents/Dev/Gridcoin/Gridcoin-Research/src/qt/optionsdialog.cpp:91:85: warning: 'nativeCountryName' is deprecated: Use nativeTerritoryName() instead [-Wdeprecated-declarations]
   91 |             ui->lang->addItem(locale.nativeLanguageName() + QString(" - ") + locale.nativeCountryName() + QString(" (") + langStr + QString(")"), QVariant(langStr));

/Users/ziggy/Documents/Dev/Gridcoin/Gridcoin-Research/src/qt/sendcoinsdialog.cpp:51:56: warning: 'stateChanged' is deprecated: Use checkStateChanged() instead [-Wdeprecated-declarations]
   51 |     connect(ui->coinControlChangeCheckBox, &QCheckBox::stateChanged, this, &SendCoinsDialog::coinControlChangeChecked);

/Users/ziggy/Documents/Dev/Gridcoin/Gridcoin-Research/src/qt/voting/polltablemodel.cpp:268:5: warning: ignoring return value of function declared with 'nodiscard' attribute: Use QThreadPool::start(Callable&&) if you don't need the returned QFuture [-Wunused-result]
  268 |     QtConcurrent::run([this]() {

/Users/ziggy/Documents/Dev/Gridcoin/Gridcoin-Research/src/qt/bitcoin.cpp:294:36: warning: 'AA_UseHighDpiPixmaps' is deprecated: High-DPI pixmaps are always enabled. This attribute no longer has any effect. [-Wdeprecated-declarations]
  294 |     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);

/Users/ziggy/Documents/Dev/Gridcoin/Gridcoin-Research/src/qt/bitcoin.cpp:394:59: warning: 'location' is deprecated: Use path() [-Wdeprecated-declarations]
  394 |     if (qtTranslatorBase.load("qt_" + lang, QLibraryInfo::location(QLibraryInfo::TranslationsPath)))

All of which seems like easy fixes. An example of how I fixed the warnings on my branch can be seen here:
ZiggySchulz@932a26d

[CyberTailor]

Yes, fixes for these deprecation warnings should be applied when Qt 5 support is dropped.

My pull request is rather conservative as I don't know what's on the development team's roadmap and thus try to be careful.

Also I was told that your branch could be rebased on top of mine. Can I do that for you?

[ZiggySchulz]

Ah of course.

All good for the rebasing, up and rolling based on your code :)

[CyberTailor]

I've made a merge commit instead of rebasing, as it was easier. You should be able to pull from my qt6-only branch into your qt6 branch.

https://github.com/CyberTailor/Gridcoin-Research/tree/qt6-only

[ZiggySchulz]

Looks good thank you. Annoying about the _qs warning. Looks like Ubuntu 22.04 only has 6.2 in its repo so we probably can't justify bumping the version higher than that for a fair while :(

[jamescowens]

Let's get this rebased on top of development, which now includes the CI fixes and get this finalized. I want to get this merged to ensure we don't drop off the trailing edge on Qt.

[CyberTailor]

Done rebasing

[ZiggySchulz]

Currently in macOS the application gets called gridcoinreasearch in the menu bar. Adding
MACOSX_BUNDLE_BUNDLE_NAME "Gridcoin Wallet"
or similar to CMake fixes this issue.
Another macOS issue with CMake is the application logo doesn't get set. I've managed to get this going by adding

target_sources(gridcoinresearch PRIVATE
    res/icons/gridcoin.icns
)
...
    MACOSX_BUNDLE_ICON_FILE "gridcoin.icns"
...
set_source_files_properties(res/icons/gridcoin.icns PROPERTIES MACOSX_PACKAGE_LOCATION "Resources")

On another side note, which I am unsure if it is related to CMake, my builds don't respond to Command+Q or Gridcoin->Quit so I am having to force quit the app.
Can be addressed elsewhere, otherwise seems to work well :)

[CyberTailor]

Currently in macOS the application gets called gridcoinreasearch in the menu bar. Adding MACOSX_BUNDLE_BUNDLE_NAME "Gridcoin Wallet" or similar to CMake fixes this issue. Another macOS issue with CMake is the application logo doesn't get set. I've managed to get this going by adding

target_sources(gridcoinresearch PRIVATE
    res/icons/gridcoin.icns
)
...
    MACOSX_BUNDLE_ICON_FILE "gridcoin.icns"
...
set_source_files_properties(res/icons/gridcoin.icns PROPERTIES MACOSX_PACKAGE_LOCATION "Resources")

Gridcoin has a custom Info.plist template, so I changed CMake properties to use it instead of the default template. Check the recently pushed branch, does it display correctly now?

As for packaging, CMake's CPack can build DMG installers as well as other package formats. This will be implemented in follow-up PRs.

[ZiggySchulz]

Yep, looks good :)

[div72]

ACK ccf2059.