Packaging Desktop Apps With Flatpak

As mentioned in theprevious post, the web platform is popular for creating apps that would make plenty of sense as desktop software. This is partly because it unifies many platforms, but mostly it’s because it solves the distribution problem . That is, having written a program, how do I get it into the hands of users?

Consider the process of installing a desktop app for a user with a fresh install of their operating system:

  • Download the installer for the application
  • Execute the installer
  • Run the application

On the other hand, the web platform simplifies all of that into a single step:

  • Open the application in the web browser

Package managers remove the requirement to find and execute unvetted installation code, but they have other flaws, such as requiring specific packaging for each operating system distribution (Windows, Ubuntu/Debian, Arch, Fedora, CentOS, Nix, etc). Someone has to do this work, whether or not the software developer themselves take it on.

Flatpak is one way to distribute applications in a more standardized way, and this post demonstrates how to roll up the application and all its dependencies into a Flatpak so that users can easily install it.

The Makefile

The Makefile in the last post was extremely simple. This one is a bit more general, and thus a bit more complicated. Firstly, it needs to be changed to support the Makefile conventions recommended by the GNU project, primarily by abstracting the system directories and installation programs. At the top of the new Makefile , there’s the following code:

# Install to /usr unless otherwise specified, such as `make PREFIX=/app`
PREFIX=/usr

# What to run to install various files
INSTALL=install
# Run to install the actual binary
INSTALL_PROGRAM=$(INSTALL)
# Run to install application data, with differing permissions
INSTALL_DATA=$(INSTALL) -m 644

# Directories into which to install the various files
bindir=$(DESTDIR)$(PREFIX)/bin
sharedir=$(DESTDIR)$(PREFIX)/share

Then, in the install and uninstall targets, rather than simply cp ing files into the correct places, $(INSTALL_PROGRAM) and $(INSTALL_DATA) are used, and every filepath is prefixed with $(sharedir) or $(bindir) .

This way, if someone wants to install this program onto a distribution that requires installing into a weird place like /var/<program name>/whatever , they can just set the PREFIX variable - and we can use it to do the same for the Flatpak.

How Flatpak Works

Flatpak packages use a number of Linux kernel technologies ( cgroups , namespaces, seccomp, and bind mounts) and some open standards from the Open Container Initiative to provide a universal and standardized way to distribute applications. It provides a runtime on which bundled libraries and applications can be executed (more info at the official docs ).

The critical aspects for application developers and users are security through isolation and dependability due to built-in dependency management, as well as ease of use in terms of installation.

Making a Flatpak

Making a Flatpak requres a program called flatpak-builder , in addition to the Flatpak tools themselves. In addition, we need a few extra components:

# Add flathub and the gnome-nightly repo
flatpak remote-add --user --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
flatpak remote-add --user --if-not-exists gnome-nightly https://sdk.gnome.org/gnome-nightly.flatpakrepo

# Install the gnome-nightly Sdk and Platform runtime
flatpak install --user gnome-nightly org.gnome.Sdk org.gnome.Platform

# Install the required rust-stable extension from flathub
flatpak install --user flathub org.freedesktop.Sdk.Extension.rust-stable//18.08

With these installed, we need a Flatpak metadata file. We’ll create the development version first. So, in the data directory of the repo, the codes.nora.gDiceRoller-development.json file will contain the app’s build metadata:

{
    "app-id" : "codes.nora.gDiceRoller",
    "tags" : [ "development" ],

This section establishes the name of this application ( app-id ), which must be globally unique, and any tags that should be applied to the specific version built by this Flatpak specification.

"runtime" : "org.gnome.Platform",
    "runtime-version" : "3.32",
    "sdk" : "org.gnome.Sdk",
    "sdk-extensions" : [
        "org.freedesktop.Sdk.Extension.rust-stable"
    ],

This section tells Flatpak and flatpak-builder which pieces of software and which libraries are needed to build and run the application. The runtime should pretty much always be org.gnome.Platform , while the runtime-version is dictated by your requirements. Remember that newer versions are available to fewer users, typically.

sdk , like runtime , is usually the same, but sdk-extensions depends heavily on which tools and languages you use to create the application. In this case, that’s the Rust extension we installed earlier.

"command" : "codes.nora.gDiceRoller",
    "desktop-file-name-suffix" : " ☢️",
    "finish-args" : [
        "--socket=x11",
        "--socket=wayland",
        "--device=dri"
    ],

First off, the command specifies which file should be run to actually start the application.

desktop-file-name-suffix specifies that a “radioactive” symbol should be added to the end of the application’s desktop file. This is a common practice for Flatpak devs distributing both a development and a production version of an app, so people can install both and tell them apart in their application directory.

The finish-args section specifies that the program needs access to the various desktop subsystems, so it can actually draw windows on the screen. I don’t, for instance, specify --socket=network or other external I/O, because the application doesn’t need it.

"build-options" : {
        "append-path" : "/usr/lib/sdk/rust-stable/bin",
        "build-args" : [],
        "env" : {
            "CARGO_HOME" : "/run/build/gRiceRoller/cargo",
            "RUSTFLAGS" : "--error-format=short --remap-path-prefix =../",
            "RUST_BACKTRACE" : "1"
        }
    },

build-options simply sets the appropriate location for the Rust build tools and turns on backtraces for debugging.

"modules" : [
        {
            "name" : "gDiceRoller",
            "buildsystem" : "simple",
            "build-commands": [
                "make",
                "make install PREFIX=/app" 
            ],
            "sources" : [
                {
                    "type" : "dir",
                    "path" : "../"
                }
            ]
        }
    ]
}

The modules section specifies what code actually needs to be built and how to do that. In this case, that’s a “simple” build system, which essentially means “just run the commands here”. Crucially, the PREFIX variable is set to /app , which is a Flatpak requirement.

In the previous section, we set up the Makefile so that setting the PREFIX variable will install the app in a different location. flatpak-builder will take the contents of /app and package that as the application, so that’s where it gets installed.

For the development version, we specify the local directory so that changes a developer makes are reflected immediately.

The last step for this is to add building this to the Makefile :

# Build a Flatpak package
flatpak-development: target/release/gDiceRoller
    mkdir -p flatpak-development
    flatpak-builder flatpak-development data/codes.nora.gDiceRoller-development.json

And, to clean the built package:

# Remove supplemental build files
clean :
    rm -rf flatpak/ flatpak-development/

Development vs. Production

The production version, in data/codes.nora.gDiceRoller.json , is totally identical, except that:

desktop-file-name-suffix
tags
sources

Here, sources specifies the Git repository, along with the version tag and the commit hash:

"sources" : [
  {
    "type" : "git",
    "url" : "https://gitlab.gnome.org/NoraCodes/gDiceRoller.git",
    "tag": "v1.1.3",
    "commit": "57ad7d8cc886c8d9c838e665441e347d71637b1d"
  }
]

Critically, this decouples this Flatpak specification from the code itself. This will be important when uploating it to Flathub to be distributed.

In order to test this easily, there’s also an appropriate Makefile target:

# Build the Flatpak bundle for release
flatpak-release: target/release/gDiceRoller
	mkdir -p flatpak
	flatpak-builder flatpak data/codes.nora.gDiceRoller.json

For either the release or the development version, it’s possible to test the build as documented here .

Flathub

AppData

The final purpose of packaging the application into a Flatpak is to distribute it, and Flathub is the place to be for distribution. In order to be permitted onto Flatpak, an application must provide:

  • Desktop files
  • Application icons
  • An AppData file

gDiceRoller has everything but an AppData file, so far. You can find the AppData specification from FreeDesktop . The AppData file for gDiceRoller is at gdiceroller/data/codes.nora.gDiceRoller.appdata.xml , and the Makefile installs it to $prefix/share/metainfo/ .

It’s an XML document, and is kind of long, so I’ve excerpted the relevant sections.

<id>codes.nora.gDiceRoller</id>
<name>Dice Roller</name>
<project_license>GPL-3.0</project_license>
<metadata_license>CC0-1.0</metadata_license>
<developer_name>Leonora Tindall</developer_name>
<summary>Roll dice of many different shapes and sizes in all possible combinations.</summary>
<url type="homepage">https://gitlab.gnome.org/NoraCodes/gDiceRoller/</url>
<update_contact>nora@nora.codes</update_contact>

This section specifies the crucial information about the app - the unique ID, the display name, my name, a short summary, the project’s URL, and the relevent licenses. metadata_license is the license for the info within the AppData file and should almost always be CC0 or similar. update_contact should be an e-mail at which someone can be reached who can actually change things in the package, if it needs to be updated. This is very important if someone other than the main development team is maintaining the package.

<description>
    <p>
    Roll dice of many different shapes and sizes in all possible combinations.

    gDiceRoller provides several kinds of dice, from a d4 to a d100, which can be
    rolled with a simple button press. In addition, users can enter their own
    dice with any number of sides, and perform arbitrary arithemtic on their
    results.
    </p>
</description>
<categories>
    <category>Game</category>
    <category>GTK</category>
</categories>
<screenshots>
    <screenshot type="default">
    <image type="source">https://gitlab.gnome.org/NoraCodes/gDiceRoller/raw/master/data/screenshot.png</image>
    </screenshot>
</screenshots>
<releases>
    <release version="1.1.3" date="2019-06-28"/>
    <release version="1.1.2" date="2019-06-28"/>
    <release version="1.1.1" date="2019-06-28"/>
    <release version="1.1" date="2019-06-27"/>
</releases>

This section provides some more metadata about the application. description can contain HTML which most application stores, like Flathub, will render. screenshots will be shown in header, usually below the application’s name and above the description.

categories is important, because it places the application in view of users in the relevant categories in app stores.

The releases array contains release elements. Here, there’s nothing within them, but they can contain sub- descriptions which will be rendered, usually below the main description. Use them to document changes in your application.

<launchable type="desktop-id">codes.nora.gDiceRoller.desktop</launchable>
<provides>
    <binary>codes.nora.gDiceRoller</binary>
</provides>

Finally, a few housekeeping tags. These set the actual desktop file that launches the application, and the binary it provides, so that app stores can figure out conflicts.

Publishing

With the AppData file finished, we can move on to actually publishing the project to Flathub. This is a pretty simple process:

  1. Fork the Flathub repository ( https://github.com/flathub/flathub )
  2. Clone the fork
  3. Create a new branch with the name of the app
  4. Add and commit the production manifest
  5. Make a pull request

For example, here ’s the PR I made for this application.

Once that process is finished, the Flathub staff will review your application. Assuming they like it, they’ll create a new repository and make it available on a page like this , and after a while, it’ll show up in desktop app stores.

Congratulations!

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