feat(overlord): allow adding external managers

internals/daemon/daemon.go

@@ -217,6 +217,10 @@ func userFromRequest(state interface{}, r *http.Request) (*UserState, error) {
 	return nil, nil
 }
 
+func (d *Daemon) Overlord() *overlord.Overlord {
+	return d.overlord
+}
+
 func (c *Command) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	st := c.d.state
 	st.Lock()

internals/overlord/overlord.go

@@ -64,7 +64,6 @@ type Overlord struct {
 	pruneTicker *time.Ticker
 
 	// managers
-	inited     bool
 	runner     *state.TaskRunner
 	serviceMgr *servstate.ServiceManager
 	commandMgr *cmdstate.CommandManager
@@ -78,7 +77,6 @@ func New(pebbleDir string, restartHandler restart.Handler, serviceOutput io.Writ
 	o := &Overlord{
 		pebbleDir: pebbleDir,
 		loopTomb:  new(tomb.Tomb),
-		inited:    true,
 	}
 
 	if !filepath.IsAbs(pebbleDir) {
@@ -130,8 +128,7 @@ func New(pebbleDir string, restartHandler restart.Handler, serviceOutput io.Writ
 	// Tell service manager about check failures.
 	o.checkMgr.NotifyCheckFailed(o.serviceMgr.CheckFailed)
 
-	// the shared task runner should be added last!
-	o.stateEng.AddManager(o.runner)
+	o.stateEng.SetTaskRunner(o.runner)
 
 	return o, nil
 }
@@ -427,20 +424,14 @@ func Fake() *Overlord {
 func FakeWithState(handleRestart func(restart.RestartType)) *Overlord {
 	o := &Overlord{
 		loopTomb: new(tomb.Tomb),
-		inited:   false,
 	}
 	s := state.New(fakeBackend{o: o})
 	o.stateEng = NewStateEngine(s)
 	o.runner = state.NewTaskRunner(s)
 	return o
 }
 
-// AddManager adds a manager to a fake overlord. It cannot be used for
-// a normally initialized overlord those are already fully populated.
 func (o *Overlord) AddManager(mgr StateManager) {
-	if o.inited {
-		panic("internal error: cannot add managers to a fully initialized Overlord")
-	}
 	o.addManager(mgr)
 }
 

internals/overlord/stateengine.go

@@ -56,8 +56,24 @@ type StateEngine struct {
 	state   *state.State
 	stopped bool
 	// managers in use
-	mgrLock  sync.Mutex
-	managers []StateManager
+	mgrLock    sync.Mutex
+	managers   []StateManager
+	taskRunner StateManager
+}
+
+// orderedManagers returns all the managers added to this engine.
+//
+// The managers are ordered by their precedence when executing Ensure, Stop
+// and Wait: the task runner is the last item in the result.
+func (se *StateEngine) orderedManagers() []StateManager {
+	result := make([]StateManager, 0, len(se.managers)+1)
+	for _, m := range se.managers {
+		result = append(result, m)
+	}
+	if se.taskRunner != nil {
+		result = append(result, se.taskRunner)
+	}
+	return result
 }
 
 // NewStateEngine returns a new state engine.
@@ -95,7 +111,7 @@ func (se *StateEngine) Ensure() error {
 		return fmt.Errorf("state engine already stopped")
 	}
 	var errs []error
-	for _, m := range se.managers {
+	for _, m := range se.orderedManagers() {
 		err := m.Ensure()
 		if err != nil {
 			logger.Noticef("state ensure error: %v", err)
@@ -115,14 +131,20 @@ func (se *StateEngine) AddManager(m StateManager) {
 	se.managers = append(se.managers, m)
 }
 
+func (se *StateEngine) SetTaskRunner(taskRunner StateManager) {
+	se.mgrLock.Lock()
+	defer se.mgrLock.Unlock()
+	se.taskRunner = taskRunner
+}
+
 // Wait waits for all managers current activities.
 func (se *StateEngine) Wait() {
 	se.mgrLock.Lock()
 	defer se.mgrLock.Unlock()
 	if se.stopped {
 		return
 	}
-	for _, m := range se.managers {
+	for _, m := range se.orderedManagers() {
 		if waiter, ok := m.(StateWaiter); ok {
 			waiter.Wait()
 		}
@@ -136,7 +158,7 @@ func (se *StateEngine) Stop() {
 	if se.stopped {
 		return
 	}
-	for _, m := range se.managers {
+	for _, m := range se.orderedManagers() {
 		if stopper, ok := m.(StateStopper); ok {
 			stopper.Stop()
 		}