Migrates control to use subscription groups

- subscriptionGroups maintain a view of requested metrics, expanded metrics plugins and metric configuration per subscription group
- subscriptionGroups are added/removed when tasks are stopped/started (SubscribeDeps/UnsubscribeDeps)
- subscriptionGroups are processed on plugin load and unload events
  - processing a subscriptionGroup involves determing the resulting metrics from the requested metrics and subscribing/unsubscribing to plugins as necessary
- Removes the need for bound vs unbound subscriptions with respect to plugin pools

README.md

@@ -230,6 +230,7 @@ Documentation for Snap will be kept in this repository for now with an emphasis
 * [build and test](docs/BUILD_AND_TEST.md)
 * [REST API](docs/REST_API.md)
 * [tasks](docs/TASKS.md)
+* [plugin life cycle](docs/PLUGIN_LIFECYCLE.md)
 * [plugin signing](docs/PLUGIN_SIGNING.md)
 * [tribe](docs/TRIBE.md)
 

control/available_plugin.go

@@ -54,6 +54,7 @@ const (
 var (
 	ErrPoolNotFound = errors.New("plugin pool not found")
 	ErrBadKey       = errors.New("bad key")
+	ErrBadTaskId    = errors.New("bad task id")
 )
 
 // availablePlugin represents a plugin which is
@@ -538,6 +539,8 @@ func (ap *availablePlugins) findLatestPool(pType, name string) (strategy.Pool, s
 }
 
 func (ap *availablePlugins) getOrCreatePool(key string) (strategy.Pool, error) {
+	ap.Lock()
+	defer ap.Unlock()
 	var err error
 	pool, ok := ap.table[key]
 	if ok {

control/available_plugin_test.go

@@ -53,6 +53,7 @@ func TestAvailablePlugin(t *testing.T) {
 	Convey("Stop()", t, func() {
 		Convey("returns nil if plugin successfully stopped", func() {
 			r := newRunner()
+			r.SetEmitter(new(MockEmitter))
 			a := plugin.Arg{
 				PluginLogPath: "/tmp/snap-test-plugin-stop.log",
 			}