In the previous post on Snapping KDE Applications we looked at the high-level implication and use of the KDE Frameworks 5 content snap to snapcraft snap bundles for binary distribution. Today I want to get a bit more technical and look at the actual building and inner workings of the content snap itself.
The KDE Frameworks 5 snap is a content snap. Content snaps are really just ordinary snaps that define a content interface. Namely, they expose part or all of their file tree for use by another snap but otherwise can be regular snaps and have their own applications etc.
KDE Frameworks 5’s snap is special in terms of size and scope. The whole set of KDE Frameworks 5, combined with Qt 5, combined with a large chunk of the graphic stack that is not part of the ubuntu-core snap. All in all just for the Qt5 and KF5 parts we are talking about close to 100 distinct source tarballs that need building to compose the full frameworks stack. KDE is in the fortunate position of already having builds of all these available through KDE neon. This allows us to simply repack existing work into the content snap. This is for the most part just as good as doing everything from scratch, but has the advantage of saving both maintenance effort and build resources.
I do love automation, so the content snap is built by some rather stringy proof of concept code that automatically translates the needed sources into a working snapcraft.yaml that repacks the relevant KDE neon debs into the content snap.
Looking at this snapcraft.yaml we’ll find some fancy stuff.
After the regular snap attributes the actual content-interface is defined. It’s fairly straight forward and simply exposes the entire snap tree as
kde-frameworks-5-all content. This is then used on the application snap side to find a suitable content snap so it can access the exposed content (i.e. in our case the entire file tree).
The parts of the snap itself are where the most interesting things happen. To make things easier to read and follow I’ll only show the relevant excerpts.
The content snap consists of the following parts: kf5, kf5-dev, breeze, plasma-integration.
The kf5 part is the meat of the snap. It tells snapcraft to stage the binary runtime packages of KDE Frameworks 5 and Qt 5. This effectively makes snapcraft pack the named debs along with necessary dependencies into our snap.
The kf5-dev part looks almost like the kf5 part but has entirely different functionality. Instead of staging the runtime packages it stages the buildtime packages (i.e. the -dev packages). It additionally has a tricky snap rule which excludes everything from actually ending up in the snap. This is a very cool tricky, this effectively means that the buildtime packages will be in the stage and we can build other parts against them, but we won’t have any of them end up in the final snap. After all, they would be entirely useless there.
Besides those two we also build two runtime integration parts entirely from scratch
plasma-integration. They aren’t actually needed, but ensure sane functionality in terms of icon theme selection etc. These are ordinary build parts that simply rely on the
kf5-dev parts to provide the necessary dependencies.
An important question to ask here is how one is meant to build against this now. There is this
kf5-dev part, but it does not end up in the final snap where it would be entirely useless anyway as snaps are not used at buildtime. The answer lies in one of the rigging scripts around this. In the snapcraft.yaml we configured the
kf5-dev part to stage packages but then excluded everything from being snapped. However, knowing how snapcraft actually goes about its business we can “abuse” its inner workings to make use of the part after all. Before the actual snap is created snapcraft “primes” the snap, this effectively means that all installed trees (i.e. the stages) are combined into one tree (i.e. the primed tree), the exclusion rule of the
kf5-dev part is then applied on this tree. Or in other words: the primed tree is the snap before exclusion was applied. Meaning the primed tree is everything from all parts, including the development headers and CMake configs. We pack this tree in a development tarball which we then use on the application side to stage a development environment for the KDE Frameworks 5 snap.
Specifically on the application-side we use a boilerplate part that employs the same trick of stage-everything but snap-nothing to provide the build dependencies while not having anything end up in the final snap.
Using the KDE Framworks 5 content snap KDE can create application snaps that are a fraction of the size they would be if they contained all dependencies themselves. While this does give up optimization potential by aggregating requirements in a more central fashion it quickly starts paying off given we are saving upwards of 70 MiB per snap.
Application snaps can of course still add more stuff on top or even override things if needed.
Finally, as we approach the end of the year, we begin the season of giving. What would suit the holidays better than giving to the entire world by supporting KDE with a small donation?