Support a group plugin that is a wrapper on top of managed instance groups

[dgageot]

This would help better scale a group of cattle.

[FrenchBen]

Out of curiosity, what's the advantage of doing something like:

type plugin struct {
	API func() (gcloud.API, error)

	flavorPlugins group_plugin.FlavorPluginLookup
	groups        map[group.ID]settings
	lock          sync.Mutex
}

// NewGCEGroupPlugin creates a new GCE group plugin for a given project
// and zone.
func NewGCEGroupPlugin(project, zone string, flavorPlugins group_plugin.FlavorPluginLookup) group.Plugin {
	_, err := gcloud.New(project, zone)
	if err != nil {
		log.Fatal(err)
	}

	return &plugin{
		API: func() (gcloud.API, error) {
			return gcloud.New(project, zone)
		},
		flavorPlugins: flavorPlugins,
		groups:        map[group.ID]settings{},
	}
}

and creating an actual client later on, instead of doing something like this:

type plugin struct {
	API gcloud.API

	flavorPlugins group_plugin.FlavorPluginLookup
	groups        map[group.ID]settings
	lock          sync.Mutex
}

// NewGCEGroupPlugin creates a new GCE group plugin for a given project
// and zone.
func NewGCEGroupPlugin(project, zone string, flavorPlugins group_plugin.FlavorPluginLookup) group.Plugin {
	api, err := gcloud.New(project, zone)
	if err != nil {
		log.Fatal(err)
	}

	return &plugin{
		API: api,
		flavorPlugins: flavorPlugins,
		groups:        map[group.ID]settings{},
	}
}

[dgageot]

@FrenchBen we cannot keep a gcloud API for the whole lifetime of this plugin. So API is a factory that creates a new gcloud.API each time. In NewGCEGroupPlugin, we create create an gcloud.API instance to check that everything is well configured.

What I can do to improve the code is move this mechanism to the gcloud package.

[dgageot]

@FrenchBen Actually, not reusing a compute.Service for too long might be cargo cult on my side. Will run some long running tests without it