snap restart lxd is causing systemd units that were not previously running to be started

[Gabriel Mougard]

An unintended behavior in the `snap restart lxd` command has been observed, resulting in the undesired creation of an `lxdbr0` interface and a modification to the default profile. This issue arises even when a different existing bridge is specified during `lxd init`. The precise steps to reproduce the problem are as follows:

1. Install LXD using `snap install lxd`.

2. Run `lxd init` and specify an existing bridge (i.e., `lxdbr1`) instead of creating a new one.

3. Confirm there is no `lxdbr0` using `lxc network list` and check that the default profile is configured as intended using `lxc profile show default`.

4. Restart LXD using `snap restart lxd`.

5.Run `lxc network list` and `lxc profile show default` again. At this point, a `lxdbr0` interface exists, and the default profile has been altered to use it.

It is important to note that this issue is not triggered when using the alternative commands `snap stop lxd` followed by `snap start lxd`, nor when using `sudo systemctl reload snap.lxd.daemon`. It appears to be specific to `snap restart lxd`, suggesting a potential bug within the `snap restart` command itself.

As mentioned in the original gh discussion thread, the problem seems related to the `snap restart` command starting sub-units, like the `lxd-user` process, even if they weren't previously running. The undesired side-effect of this behavior is an API request being sent to LXD upon `snap restart lxd` which creates the `lxdbr0` network, even if this is not desired or requested by the user.

A current workaround for this issue is to have an interface named `lxdbr0` present, either by leaving the unwanted one that LXD creates, or by naming an unmanaged interface as `lxdbr0`. Without this, the `snap restart lxd` command will create `lxdbr0` and change the default profile.

---

Here is the link to the github thread of the original issue : https://github.com/canonical/lxd/issues/11906

Thanks

[Thomas Parrott (tomparrott)]

The description above is a side effect of the apparent behaviour of `snap restart lxd`.

Example:

On a fresh ubuntu:22.04 VM:

```
systemctl | grep snap.lxd
  snap-lxd-25112.mount loaded active mounted Mount unit for lxd, revision 25112
  snap.lxd.daemon.unix.socket loaded active listening Socket unix for snap application lxd.daemon
  snap.lxd.user-daemon.unix.socket loaded active listening Socket unix for snap application lxd.user-daemon
```

Note that the sockets are in a listening state for lxd.daemon and lxd.user-daemon.

Then if we do `snap stop lxd` followed by `snap start lxd` we can see the systemd units are restored to the same state as before (listening).

```
root@v1:~# snap stop lxd
Stopped.
root@v1:~# systemctl | grep snap.lxd
  snap-lxd-25112.mount loaded active mounted Mount unit for lxd, revision 25112
```

```
root@v1:~# systemctl | grep snap.lxd
  snap-lxd-25112.mount loaded active mounted Mount unit for lxd, revision 25112
root@v1:~# snap start lxd
Started.
root@v1:~# systemctl | grep snap.lxd
  snap-lxd-25112.mount loaded active mounted Mount unit for lxd, revision 25112
  snap.lxd.daemon.unix.socket loaded active listening Socket unix for snap application lxd.daemon
  snap.lxd.user-daemon.unix.socket loaded active listening Socket unix for snap application lxd.user-daemon
```

But if we now do `snap restart lxd` (which to my mind should be equivalent of doing `snap stop lxd` and then`snap start lxd`) we end up with something quite different:

```
root@v1:~# snap restart lxd
Restarted.
root@v1:~# systemctl | grep snap.lxd
  snap-lxd-25112.mount loaded active mounted Mount unit for lxd, revision 25112
  var-snap-lxd-common-ns-mntns.mount loaded active mounted /var/snap/lxd/common/ns/mntns
  var-snap-lxd-common-ns-shmounts.mount loaded active mounted /var/snap/lxd/common/ns/shmounts
  var-snap-lxd-common-ns.mount loaded active mounted /var/snap/lxd/common/ns
  snap.lxd.daemon.service loaded active running Service for snap application lxd.daemon
  snap.lxd.user-daemon.service loaded active running Service for snap applicati...

[Thomas Parrott (tomparrott)]

Hi there,

Is there any update on this issue?

Thanks
Tom