Building snaps within an isolated environment, such as LXD, has several advantages over a native build:
- containers keep your host system clear of your new snap’s dependencies
- your host system can’t expose conflicting libraries to your new snap build
Read on to discover how to build snaps with LXD.
Install LXD
Recent non-desktop versions of Ubuntu install LXD by default - you can check whether it is installed with the following command:
$ lxd version
3.7
The easiest way to add LXD to your system is via its snap:
$ sudo snap install lxd
Now initialise LXD with the following command, accepting all the default options unless you have specific requirements:
$ sudo lxd init
ⓘ If the system you are installing LXD onto is using a network with a
10.x.x.xsubnet then network create may fail. Step through the following to resolve this.
Manually create a network on a 10.x.x.x subnet
If you try to run lxd init on a system that is connected to a network with a 10.x.x.x subnet, then the final step of the Iinit* may fail with the following error:
Error: Failed to create network 'lxdbr0': Failed to automatically find an unused IPv4 subnet, manual configuration required
To resolve this issue, you must manually create the network with the following command:
$ sudo lxc network create lxdbr0 ipv4.address=10.0.3.1/24 ipv4.nat=true
You can then re-run lxd init. When you are prompted to create a new network bridge you must respond no.
Would you like to create a new network bridge (yes/no) [default=yes]? no
Group permissions
If you want to build snaps as a non-root user, which is advised, then you need to add your user account to the lxd group:
$ sudo usermod -g lxd ${USER}
You now need to either restart your session, reboot your computer, or use newgrp to acquire the new group assignment:
$ newgrp lxd
ⓘ The newgrp command will start a new sub-shell (shell within a shell) with the new
lxdgroup assigned.
Building
ⓘ If snapcraft is not yet installed, enter
sudo snap install snapcraft --classic.
To build your project using lxd run:
$ cd $HOME/my-snap-project
$ snapcraft cleanbuild
If the build fails, add the --debug option to the snapcraft cleanbuild command. This will drop you into a shell running within the LXD environment that just attempted to build your snap, allowing you to examine where the issue may be.
Experimental: persistent containers
When building with LXD, the old container is discarded. This means that LXD needs to build a new environment, each time you build your snap. This is acceptable with small snaps, or for a final build, but can be problematic during development.
For example, during development, you might normally use:
$ snapcraft
$ snap try prime
The ‘try’ method allows you to directly edit files in a snap’s prime directory without having to re-install or re build after each change. However, when using cleanbuild, this feature is not supported.
But there is an experimental feature that uses LXD in a different way, allowing you to retain intermediate files, and the container:
$ export SNAPCRAFT_BUILD_ENVIRONMENT=lxd
$ snapcraft
If you get the following error:
Failed to start container: the local folder could not be mounted into the container.
Add a line containing root:1000:1 to the following files:
/etc/subuid/etc/subgid
You may need to log out and back in again before reattempting the build. If successful, you can now try the snap, make changes, and rebuild quickly:
$ snap try prime
The environment variable SNAPCRAFT_BUILD_ENVIRONMENT instructs snapcraft to use experimental LXD integration to build the snap in a persistent LXD container. This container remains after the build has finished, and retains intermediate build files to speed up rebuilds.
