Fix detection of suitable architecture for conversion when LXD is clustered

[MusicDin]

Thanks to @escabo for reporting the issue where conversion fails if LXD is clustered and supports API extension instance_import_conversion.

Not sure how this slipped through for such a long time. Seems I have messed something up when juggling with commit order on initial implementation, as I remember configuring this.

Anyway, the issue is detection of suitable architecture, where source type was not matched with any case resulting in an error:

Error: Unknown instance source type "conversion"

[tomponline]

As a separate PR please can we have constants created for these values so we can see where they are referenced in the code.

[tomponline]

@MusicDin @escabo why was architecture not populated btw?

[tomponline]

@MusicDin once merged please can you prepare a cherry-pick against lxd-pkg-snap latest-candidate branch to include this commit in 6.2

[MusicDin]

Architecture was populated, but source.Type=conversion did not match any case in the function, reaching the end, where an error Error: Unknown instance source type "conversion" is returned.

[tomponline]

Architecture was populated, but source.Type=conversion did not match any case in the function, reaching the end, where an error Error: Unknown instance source type "conversion" is returned.

why was clustering relevant?

[MusicDin]

why was clustering relevant?

When target server is not provided the architecture is used to decide upon suitable cluster member to run the new instance.

lxd/lxd/instances_post.go

Lines 1227 to 1259 in 6d29cd6

	if s.ServerClustered && !clusterNotification && targetMemberInfo == nil {
	architectures, err := instance.SuitableArchitectures(ctx, s, tx, targetProjectName, sourceInst, sourceImageRef, req)
	if err != nil {
	return err
	}
	// If no architectures have been ascertained from the source then use the default
	// architecture from project or global config if available.
	if len(architectures) < 1 {
	defaultArch := targetProject.Config["images.default_architecture"]
	if defaultArch == "" {
	defaultArch = s.GlobalConfig.ImagesDefaultArchitecture()
	}
	if defaultArch != "" {
	defaultArchID, err := osarch.ArchitectureId(defaultArch)
	if err != nil {
	return err
	}
	architectures = append(architectures, defaultArchID)
	} else {
	architectures = nil // Don't exclude candidate members based on architecture.
	}
	}
	clusterGroupsAllowed := limits.GetRestrictedClusterGroups(targetProject)
	candidateMembers, err = tx.GetCandidateMembers(ctx, allMembers, architectures, targetGroupName, clusterGroupsAllowed, s.GlobalConfig.OfflineThreshold())
	if err != nil {
	return err
	}
	}