Sdk.Extension.devenv

This is a little experiement of packaging a user-defined shell environment with CLI tools as a Flatpak SDK extension.
Naming is not very imaginative, and might change.
I didn't like the alternatives, connecting a shell from a Flatpak sandbox to a host shell or a Podman container shell, both sound self-defeating. There should be a good reason to break out of the Flatpak sandbox.
Shorty after I started playing with this, I figured out that there's no reason why I shouldn't re-use this with other containers, taking advantage of Flatpak's packaging tools to build and keep this updated, so for the most part, the packaged binaries are statically linked, which also avoids introducing shared libraries.
This is very opiniated and user-specific, so you probably don't want to use this as it is, but it will give you a good idea how to package your own custom environment.
It should be noted that a running Flatpak instance will lose a mounted extension when the latter is updated.
For that reason, and to support re-use of this with other containers, an alternative root path /var/lib/devenv will also be supported.

An example application, org.freedesktop.DevEnv, is used to create a Flatpak instance with this SDK extension enabled, but the extension is mainly intended to be used with Flatpak packaged IDEs.

How to build

Set up build cache

You would want to make sure that ccache is enable and being used by Flatpak Builder (check for ccache hits).
Essentially, it means adding the --cache option, setting CCACHE_DIR environment variable, if ccache dir is not in the default location.
Flatpak Builder will bind mount ccache dir to /run/ccache in the sandbox and will set CCACHE_DIR to this path.

To give access to sccache and golang build cache dirs, move them to CCACHE_DIR/sccache and CCACHE_DIR/gocache accordingly, and then link to them from XDG_CACHE_DIR/sccache and XDG_CACHE_DIR/go-build, so they will still be shared with toolchains running directly in the host environment.

Dependencies

You will need first to build and install the following SDK extensions:

• org.freedesktop.Sdk.Extension.musl
• org.freedesktop.Sdk.Extension.go-musl
• org.freedesktop.Sdk.Extension.rust-musl
• org.freedesktop.Sdk.Extension.sccache

Building the extension

In addition to the --ccache option, I suggest also adding --disable-updates to have rebuilds start faster, as there are a good number of git sources here.

Tips

• Set default shell

  • Set DEVENV_SHELL variable in the host environment or as a Flatpak override to one of the packaged shells: bash (default), foot, or zsh.

• Easily enable extension and enter shell

  • Set this variable in the host environment: DEVENV_EXEC=/usr/lib/sdk/devenv/bin/enable-devenv
  • Enter a sandbox: $ flatpak run --command=$DEVENV_EXEC --devel APPID
  • Note that the runtime branch of the application must match or be based on the same Freedesktop branch of the extension

• Automatically enable extension and enter shell

  • Add persist override: $ flatpak override --user --persist=. APPID
  • Enter the sandbox: $ flatpak run --devel APPID
  • Create .bashrc: $ echo 'source $DEVENV_EXEC' > .bashrc
  • Notes
    • As the previous, runtime branch must match
    • Only works with actual apps, not runtimes
    • The persist feature is not without flaws, and you should avoid setting it to .