Dynamic linking of Quaternion with libqmatrixclient

[KitsuneRal]

We're gradually coming to the state when libqmatrixclient has not only API but also ABI guarantees. Once this is done, it should be possible to build Quaternion separately from libqmatrixclient and link the two at runtime. This will please Linux distributions (they are rather unhappy when a library cannot be packaged separately); and who knows, maybe we'll start getting other clients and bindings in other languages thanks to that.

[awilfox]

The library is still built statically into Quaternion; if you'd like to take part in uncoupling the two, you're welcome in #239.

How can I help? I'm the project lead of one of those aforementioned Linux distributions that would love to have the library decoupled; I'm a moderately skilled Qt programmer; and I am also a regular user of Quaternion.

[awilfox]

Okay, so packaging Quaternion for Adélie instead of just running it from the build directory:

• It defaults to building a .so instead of an .a, at least here (GCC 6.4.0, binutils 2.29, ppc64 and x86_64)
• The .so file is not installed to /usr/lib, so the software fails to run due to a missing library:

awilcox on reynard [pts/6 Sat 30 16:25] ~: quaternion
Error loading shared library libqmatrixclient.so.0: No such file or directory (needed by /usr/bin/quaternion)
Error relocating /usr/bin/quaternion: _ZN13QMatrixClient4RoomD2Ev: symbol not found

After applying a small patch to install the library, it seems to work fine. If this patch would be helpful I would be happy to put it in a pull request.

[KitsuneRal]

Thanks a lot! As far as I understand it, there are two parts here:

1. Make sure the library gets installed to the right location so that it can be packaged. Your patch does that - please submit it to the QMatrixClient/libqmatrixclient repo.
2. Make sure that versioning of the library is correct (not only .so but also .so.* files are generated). As far as I see above, that seems to work as well.

[KitsuneRal]

Oh, sorry, there's one more thing - but that's really about compiling against the dynamically linked library rather than making the library itself. The headers should be installed to the standard place (/usr/include or whatever the distribution uses). So while you're at it, I'd appreciate if you add one more line to your patch that takes all .h files from lib/ and installs them.

[KitsuneRal]

As @alexandergraul found it the harder way, .h files should be taken from lib/ and its subdirectories.

[KitsuneRal]

The "install" part has been done; this now depends on quotient-im/libQuotient#195. @awilfox, you're welcome to help there.

[KitsuneRal]

As of libQMatrixClient 0.2, the library can be separately packaged for dynamic linkage. Next step is having a Quaternion package linked with an externally installed library rather than the in-tree source code.

[KitsuneRal]

@alexandergraul, @awilfox, is there anything I should additionally do or does Quaternion already build (and install and run) with a separately installed libQMatrixClient?

[arunisaac]

I am packaging quaternion for Guix. I was able to dynamically link quaternion to libqmatrixclient by removing the add_subdirectory(lib) line from CMakeLists.txt and fixing all #include header lines to not refer to lib.

[KitsuneRal]

Thanks for the effort, @arunisaac!
Whichever #include lines refer to lib should be patched - a PR for that is very welcome.
As for add_subdirectory(), a CMake option should be introduced that would switch between that and find_package(libqmatrixclient) (generally, one can't rely on libqmatrixclient being installed to standard paths). For that, a PR is also (of course) welcome :)

[arunisaac]

Can we just abandon static linking and move to dynamic linking? That way, we wouldn't have to introduce a new CMake option. If I understand correctly, CMake is capable of finding libqmatrixclient wherever it is installed. I don't know much about CMake. And, in case we do decide to support both static and dynamic linking, I don't feel confident enough to send a PR. If we are supporting only dynamic linking, I'd be happy to send a PR.

[arunisaac]

Meanwhile, I have packaged libqmatrixclient and quaternion for Guix. https://git.savannah.gnu.org/cgit/guix.git/commit/?id=9a3da9859be2c7b89c344c7aea5b682093932530 https://git.savannah.gnu.org/cgit/guix.git/commit/?id=e81c952a4da21c7772f8104f4fcc44af3f403daf