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 Notes on packaging Krita with G’MIC
 ==================================
 
 Krita 3 and later are compatible with G’MIC, an open-source digital
 image processing framework. This support is provided by G’MIC-Qt, a
 Qt-based frontend for G’MIC. Since its inception, G’MIC-Qt was shipped
 as a standalone, externally built executable that is an optional,
 runtime dependency of Krita.
 
 Krita 5 changes the way G’MIC-Qt is consumed. In order to support CentOS
 and macOS, G’MIC-Qt has been converted into a dynamically loadable
 library that is a dependent of Krita.
 
 This file reviews these changes, and how to package Krita accordingly.
 
 Rationale
 ---------
 
 We have chosen to ship G’MIC-Qt as a library because of two longstanding
 bugs.
 
 The Krita host for G’MIC-Qt relies on `QSharedMemory`, i.e. a shared
 memory segment, on wich a pipe is instantiated to pass messages to and
 from the host app. Firstly, this approach made opening two simultaneous
 G’MIC-Qt instances (each paired to its own Krita instance) impossible
 [^1]. Secondly, it also forbade using G’MIC-Qt with Krita on CentOS, as
 well as macOS, because the former doesn’t support `QSharedMemory` [^2],
 and the latter has a meager 4KB as the maximum shared segment size.
 While there’s no workaround (to our knowledge) in CentOS, the only
 workaround for macOS is to manipulate the maximum segment size via
 `sysctl` [^3], which was already difficult pre-Mojave [^4] and now, due
 to the significant security measures of recent macOS versions, is
 nothing short of a sysadmin task [^5].
 
 There were two approaches. One was to move to a `mmap`-d file, which is
 unpredictable to sync due to each canvas’s differing space requirements.
 The easiest, and the one we chose, was to move to a tighter coupled
 memory model– a dynamically loadable plugin, as shown in my proposal PR
 [^6]. This was rejected by the G’MIC developers because of the
 possibility of crashing the host app due to a G’MIC internal bug
 [^7][^8]. This decision was later enacted as part of G’MIC contributing
 policies [^9].
 
 How did you fix it?
 -------------------
 
 Due to the above, the only path forward was to fork G’MIC, which we did
 in Krita MR !581 [^10].
 
 From a source code point of view, our fork is based on top of the latest
 version’s tarball. Each tarball’s contents are committed to the `main`
 branch of the `amyspark/gmic` GitHub repository [^11]. For every covered
 release, there is a branch that in turn overlays our own plugin
 implementation, along with additional fixes that ensure that G’MIC-Qt
 doesn’t attempt to overwrite the internal state of the host application;
 namely, `QCoreApplication` settings, widget styles, and the installed
 translators.
 
 From a technical point of view, this library interfaces with Krita
 through a new, purpose specific library, `kisqmicinterface`. This
 library contains nothing more than the previous iteration of the
 communications system, but now exported through namesake APIs [^12].
 
 In short, we have reversed the dependency flow; while in Krita v4 and
 earlier G’MIC-Qt was a runtime dependency, in v5, it’s G’MIC-Qt that
 depends on Krita as a build *and* runtime dependency.
 
 Getting the source code
 -----------------------
 
 The patched version’s tarballs are GPG signed and available at the
 Releases section of the GitHub repository [^13]. Alternatively, the
 tarballs (though not the signatures) are also mirrored at our
 dependencies stash at files.kde.org [^14]. The tarballs are signed with
 the GPG key which is available at my GitHub profile. Its fingerprint is
 `4894424D2412FEE5176732A3FC00108CFD9DBF1E`.
 
 Building Krita’s G’MIC-Qt library
 ---------------------------------
 
 After building Krita with your standard process, the CMake install
 process should have put `kisqmicinterface.so` in your `lib` folder:
 
     [2022-01-09T16:21:32.589Z] -- Installing: /home/appimage/appimage-workspace/krita.appdir/usr/lib/x86_64-linux-gnu/libkritaqmicinterface.so.18.0.0
     [2022-01-09T16:21:32.589Z] -- Installing: /home/appimage/appimage-workspace/krita.appdir/usr/lib/x86_64-linux-gnu/libkritaqmicinterface.so.18
     [2022-01-09T16:21:32.589Z] -- Set runtime path of "/home/appimage/appimage-workspace/krita.appdir/usr/lib/x86_64-linux-gnu/libkritaqmicinterface.so.18.0.0" to "/home/appimage/appimage-workspace/krita.appdir/usr/lib/x86_64-linux-gnu:/home/appimage/appimage-workspace/deps/usr/lib:/home/appimage/appimage-workspace/deps/usr/lib/x86_64-linux-gnu"
     [2022-01-09T16:21:32.589Z] -- Installing: /home/appimage/appimage-workspace/krita.appdir/usr/lib/x86_64-linux-gnu/libkritaqmicinterface.so
 
 It should also install these headers, as illustrated below:
 
 -   `kis_qmic_plugin_interface.h` exports a G’MIC-alike `launch` entry
     point that the plugin will implement
 -   `kis_qmic_interface.h` implements the G’MIC request-response APIs
 -   `kritaqmicinterface_export.h` is the CMake auto-generated export
     decoration header
 
 <!-- -->
 
     [2022-01-09T16:21:32.589Z] -- Installing: /home/appimage/appimage-workspace/krita.appdir/usr/include/kis_qmic_interface.h
     [2022-01-09T16:21:32.589Z] -- Installing: /home/appimage/appimage-workspace/krita.appdir/usr/include/kis_qmic_plugin_interface.h
     [2022-01-09T16:21:32.589Z] -- Installing: /home/appimage/appimage-workspace/krita.appdir/usr/include/kritaqmicinterface_export.h
 
 The three headers, along with the `libkritaqmicinterface.a` archive
 library (if building for Windows under MinGW), comprise a `krita-gmic-dev`
 package that’ll be a build dependency of the new G’MIC-Qt plugin.
 Please note that `libkritaqmicinterface.so` is consumed
 by Krita and MUST NOT be placed inside this dev package.
 
 Now, download the G’MIC-Qt tarball from one of the sources listed
 previously, and unpack it to an isolated directory. Then, you can build
 it with these lines (adjust them as described):
 
     mkdir build
     cmake -S ./gmic-$<the tarball's G'MIC version>-patched/gmic-qt \
           -B ./build \
           -DCMAKE_PREFIX_PATH=$<installation prefix of krita-gmic-dev> \
           -DCMAKE_INSTALL_PREFIX=$<installation prefix of krita itself> \ 
           -DENABLE_SYSTEM_GMIC=$<false if you don't want to use your system's G'MIC> \
           -DGMIC_QT_HOST=krita-plugin
     cmake --build . --config $<your desired build type> --target install
 
 The changes from a standard G’MIC build are:
 
 -   the new `GMIC_QT_HOST` value, `krita-plugin`
 -   the requirement for the `krita-gmic-dev` package to be available in
     `CMAKE_PREFIX_PATH`
 
 This process is illustrated in any of our official build scripts for
 Windows [^15] and for macOS/Linux [^16]. You can also check the
 `3rdparty_plugins` section of our source tree [^17] to see what other
 hardening we apply to the build.
 
 ------------------------------------------------------------------------
 
 [^1]: Bug \#44 on c-koi/gmic-qt: “CentOS7: Krita 4.0.4 + gmic\_krita\_qt
     2.3.0/2.2.3 - QSharedMemory::attach”.
     <https://github.com/c-koi/gmic-qt/issues/44>
 
 [^2]: Bug 424514 on krita: “Guaranteed crash when opening 2 G’MIC-qt”.
     <https://bugs.kde.org/show_bug.cgi?id=424514>
 
 [^3]: <https://www.ssec.wisc.edu/mcidas/doc/users_guide/2017.1/SharedMemory.html>
 
 [^4]: e.g. <https://stackoverflow.com/questions/2017004/setting-shmmax-etc-values-on-mac-os-x-10-6-for-postgresql>
 
 [^5]: See <https://developer.apple.com/forums/thread/669625> for an
     approach applicable after `com.apple.rootless` was fully enforced.
 
 [^6]: <https://github.com/c-koi/gmic-qt/pull/102>
 
 [^7]: <https://github.com/c-koi/gmic-qt/pull/102>
 
 [^8]: Note that this was already possible, simply by crashing G’MIC the
     host app would deadlock, waiting forever for a response.
 
 [^9]: <https://github.com/c-koi/gmic-qt/blob/master/NEW_HOST_HOWTO.md>
 
 [^10]: <https://invent.kde.org/graphics/krita/-/merge_requests/581>
 
 [^11]: <https://github.com/amyspark/gmic>
 
 [^12]: An older version of what’s
     <https://github.com/c-koi/gmic-qt/blob/master/src/Host/GmicQtHost.h>
     nowadays.
 
 [^13]: <https://github.com/amyspark/gmic/releases>
 
 [^14]: <https://files.kde.org/krita/build/dependencies/>
 
 [^15]: <https://invent.kde.org/graphics/krita/-/tree/master/build-tools/windows>
 
 [^16]: <https://invent.kde.org/graphics/krita/-/blob/master/packaging>
 
 [^17]: <https://invent.kde.org/graphics/krita/-/tree/master/3rdparty_plugins>