Fix not found error checks

[MusicDin]

Currently, when the provider retrieves LXD storage pool (or tries to delete it), it checks whether an error equals to No such object. In such case it detects that the storage pool does not exist and an error is not reported.

However, LXD returns different error in such cases (https://github.com/canonical/lxd/blob/7f46f71133411a66be20fce1a2b92de3923db705/lxd/db/storage_pools.go#L666) - Storage pool not found.

[MusicDin]

Since the error message has been changed only one year ago (canonical/lxd@ce7b3fb), maybe we should keep both options for now (for those who are still running older versions of LXD):

if err != nil && (err.Error() == "No such object" || err.Error() == "Storage pool not found") {
        // ...
}

@simondeziel @adamcstephens Any thoughts on that?

[simondeziel]

Thanks for thinking about backward compat! The commit you found was backported to 5.0 LTS (canonical/lxd@d442d20) but not to 4.0 LTS, so yeah, I think it's worth checking for both error strings.

Excuse my ignorance but isn't there a HTTP return code accompanying the error? I'd expect the old and new errors to both come with a 404. If that's available, that'd potentially be less brittle than trying to look at the error string itself.

[MusicDin]

You are right, we could check if the error is of type api.StatusError and extract the status code from it.

[MusicDin]

I have replaced the string comparison error checks with the status code error checks.

This approach works both for LXD version 4 and 5. However, LXD versions 3 and below do not implement StatusError, which is why I have created an utility function in errors.go (maybe not an appropriate place for that) that checks if the error message equals No such object or not found before trying to convert it into StatusError.

In addition, I have replaced most of the not found errors which follow the same approach.

[adamcstephens]

Thanks!