type StateMachine struct {
	planner  Planner
	eventsIn chan Event

	name      interface{}
	st        *statestore.StoredState
	stateType reflect.Type

	stageDone chan struct{}
	closing   chan struct{}
	closed    chan struct{}

	busy int32
}

# planner
// Planner processes in queue events
// It returns:
// 1. a handler of type -- func(ctx Context, st <T>) (func(*<T>), error), where <T> is the typeOf(User) param
// 2. the number of events processed
// 3. an error if occured

# 状态机变化图
	// Now decide what to do next

	/*

				      UndefinedSectorState (start)
				       v                     |
				*<- WaitDeals <-> AddPiece   |
				|   |   /--------------------/
				|   v   v
				*<- Packing <- incoming committed capacity
				|   |
				|   v
				|   GetTicket
				|   |   ^
				|   v   |
				*<- PreCommit1 <--> SealPreCommit1Failed
				|   |       ^          ^^
				|   |       *----------++----\
				|   v       v          ||    |
				*<- PreCommit2 --------++--> SealPreCommit2Failed
				|   |                  ||
				|   v          /-------/|
				*   PreCommitting <-----+---> PreCommitFailed
				|   |                   |     ^
				|   v                   |     |
				*<- WaitSeed -----------+-----/
				|   |||  ^              |
				|   |||  \--------*-----/
				|   |||           |
				|   vvv      v----+----> ComputeProofFailed
				*<- Committing    |
				|   |        ^--> CommitFailed
				|   v             ^
			        |   SubmitCommit  |
		        	|   |             |
		        	|   v             |
				*<- CommitWait ---/
				|   |
				|   v
				|   FinalizeSector <--> FinalizeFailed
				|   |
				|   v
				*<- Proving
				|
				v
				FailedUnrecoverable

	*/

go func() {
				defer log.Debugw("leaving critical zone and resetting atomic var to zero", "len(pending)", len(pendingEvents))


				if nextStep != nil {
					res := reflect.ValueOf(nextStep).Call([]reflect.Value{reflect.ValueOf(ctx), reflect.ValueOf(ustate).Elem()})

					if res[0].Interface() != nil {
						log.Errorf("executing step: %+v", res[0].Interface().(error)) // TODO: propagate top level
						return
					}
				}

				atomic.StoreInt32(&fsm.busy, 0)
				fsm.stageDone <- struct{}{}
			}()

# lotus/extern/storage-sealing/fsm.go：19
func (m *Sealing) Plan(events []statemachine.Event, user interface{}) (interface{}, uint64, error) {
	next, processed, err := m.plan(events, user.(*SectorInfo))
	if err != nil || next == nil {
		return nil, processed, err
	}

	return func(ctx statemachine.Context, si SectorInfo) error {
		err := next(ctx, si)
		if err != nil {
			log.Errorf("unhandled sector error (%d): %+v", si.SectorNumber, err)
			return nil
		}

		return nil
	}, processed, nil // TODO: This processed event count is not very correct
}

# lotus/extern/storage-sealing/fsm.go：274
func (m *Sealing) plan(events []statemachine.Event, state *SectorInfo) (func(statemachine.Context, SectorInfo) error, uint64, error) {
	...

	switch state.State {
	// Happy path
	case Empty:
		fallthrough
	case WaitDeals:
		return m.handleWaitDeals, processed, nil
	case AddPiece:
		return m.handleAddPiece, processed, nil
	case Packing:
		return m.handlePacking, processed, nil
	case GetTicket:
		return m.handleGetTicket, processed, nil
	case PreCommit1:
		return m.handlePreCommit1, processed, nil
	case PreCommit2:
		return m.handlePreCommit2, processed, nil
	case PreCommitting:
		return m.handlePreCommitting, processed, nil
	case SubmitPreCommitBatch:
		return m.handleSubmitPreCommitBatch, processed, nil
	case PreCommitBatchWait:
		fallthrough
	case PreCommitWait:
		return m.handlePreCommitWait, processed, nil
	case WaitSeed:
		return m.handleWaitSeed, processed, nil
	case Committing:
		return m.handleCommitting, processed, nil
	case SubmitCommit:
		return m.handleSubmitCommit, processed, nil
	case SubmitCommitAggregate:
		return m.handleSubmitCommitAggregate, processed, nil
	case CommitAggregateWait:
		fallthrough
	case CommitWait:
		return m.handleCommitWait, processed, nil
	case CommitFinalize:
		fallthrough
	case FinalizeSector:
		return m.handleFinalizeSector, processed, nil

	// Handled failure modes
	case AddPieceFailed:
		return m.handleAddPieceFailed, processed, nil
	case SealPreCommit1Failed:
		return m.handleSealPrecommit1Failed, processed, nil
	case SealPreCommit2Failed:
		return m.handleSealPrecommit2Failed, processed, nil
	case PreCommitFailed:
		return m.handlePreCommitFailed, processed, nil
	case ComputeProofFailed:
		return m.handleComputeProofFailed, processed, nil
	case CommitFailed:
		return m.handleCommitFailed, processed, nil
	case CommitFinalizeFailed:
		fallthrough
	case FinalizeFailed:
		return m.handleFinalizeFailed, processed, nil
	case PackingFailed: // DEPRECATED: remove this for the next reset
		state.State = DealsExpired
		fallthrough
	case DealsExpired:
		return m.handleDealsExpired, processed, nil
	case RecoverDealIDs:
		return wrapCtx(m.HandleRecoverDealIDs), processed, nil

	// Post-seal
	case Proving:
		return m.handleProvingSector, processed, nil
	case Terminating:
		return m.handleTerminating, processed, nil
	case TerminateWait:
		return m.handleTerminateWait, processed, nil
	case TerminateFinality:
		return m.handleTerminateFinality, processed, nil
	case TerminateFailed:
		return m.handleTerminateFailed, processed, nil
	case Removing:
		return m.handleRemoving, processed, nil
	case Removed:
		return nil, processed, nil

	case RemoveFailed:
		return m.handleRemoveFailed, processed, nil

		// Faults
	case Faulty:
		return m.handleFaulty, processed, nil
	case FaultReported:
		return m.handleFaultReported, processed, nil

	// Fatal errors
	case UndefinedSectorState:
		log.Error("sector update with undefined state!")
	case FailedUnrecoverable:
		log.Errorf("sector %d failed unrecoverably", state.SectorNumber)
	default:
		log.Errorf("unexpected sector update state: %s", state.State)
	}

	return nil, processed, nil
}