loop/fsm/fsm.go

package fsm

import (
	"errors"
	"fmt"
	"sync"
)

// ErrEventRejected is the error returned when the state machine cannot process
// an event in the state that it is in.
var (
	ErrEventRejected        = errors.New("event rejected")
	ErrWaitForStateTimedOut = errors.New(
		"timed out while waiting for event",
	)
	ErrInvalidContextType = errors.New("invalid context")
)

const (
	// Default represents the default state of the system.
	Default StateType = ""

	// NoOp represents a no-op event.
	NoOp EventType = "NoOp"

	// OnError can be used when an action returns a generic error.
	OnError EventType = "OnError"

	// ContextValidationFailed can be when the passed context if
	// not of the expected type.
	ContextValidationFailed EventType = "ContextValidationFailed"
)

// StateType represents an extensible state type in the state machine.
type StateType string

// EventType represents an extensible event type in the state machine.
type EventType string

// EventContext represents the context to be passed to the action
// implementation.
type EventContext interface{}

// Action represents the action to be executed in a given state.
type Action func(eventCtx EventContext) EventType

// Transitions represents a mapping of events and states.
type Transitions map[EventType]StateType

// State binds a state with an action and a set of events it can handle.
type State struct {
	// EntryFunc is a function that is called when the state is entered.
	EntryFunc func()
	// ExitFunc is a function that is called when the state is exited.
	ExitFunc func()
	// Action is the action to be executed in the state.
	Action Action
	// Transitions is a mapping of events and states.
	Transitions Transitions
}

// States represents a mapping of states and their implementations.
type States map[StateType]State

// Notification represents a notification sent to the state machine's
// notification channel.
type Notification struct {
	// PreviousState is the state the state machine was in before the event
	// was processed.
	PreviousState StateType
	// NextState is the state the state machine is in after the event was
	// processed.
	NextState StateType
	// Event is the event that was processed.
	Event EventType
}

// Observer is an interface that can be implemented by types that want to
// observe the state machine.
type Observer interface {
	Notify(Notification)
}

// StateMachine represents the state machine.
type StateMachine struct {
	// Context represents the state machine context.
	States States

	// ActionEntryFunc is a function that is called before an action is
	// executed.
	ActionEntryFunc func()

	// ActionExitFunc is a function that is called after an action is
	// executed.
	ActionExitFunc func()

	// mutex ensures that only 1 event is processed by the state machine at
	// any given time.
	mutex sync.Mutex

	// LastActionError is an error set by the last action executed.
	LastActionError error

	// previous represents the previous state.
	previous StateType

	// current represents the current state.
	current StateType

	// observers is a slice of observers that are notified when the state
	// machine transitions between states.
	observers []Observer

	// observerMutex ensures that observers are only added or removed
	// safely.
	observerMutex sync.Mutex
}

// NewStateMachine creates a new state machine.
func NewStateMachine(states States) *StateMachine {
	return &StateMachine{
		States:    states,
		observers: make([]Observer, 0),
	}
}

// getNextState returns the next state for the event given the machine's current
// state, or an error if the event can't be handled in the given state.
func (s *StateMachine) getNextState(event EventType) (State, error) {
	var (
		state State
		ok    bool
	)

	stateMap := s.States

	if state, ok = stateMap[s.current]; !ok {
		return State{}, NewErrConfigError("current state not found")
	}

	if state.Transitions == nil {
		return State{}, NewErrConfigError(
			"current state has no transitions",
		)
	}

	var next StateType
	if next, ok = state.Transitions[event]; !ok {
		return State{}, NewErrConfigError(
			"event not found in current transitions",
		)
	}

	// Identify the state definition for the next state.
	state, ok = stateMap[next]
	if !ok {
		return State{}, NewErrConfigError("next state not found")
	}

	if state.Action == nil {
		return State{}, NewErrConfigError("next state has no action")
	}

	// Transition over to the next state.
	s.previous = s.current
	s.current = next

	return state, nil
}

// SendEvent sends an event to the state machine. It returns an error if the
// event cannot be processed in the current state. Otherwise, it only returns
// nil if the event for the last action is a no-op.
func (s *StateMachine) SendEvent(event EventType, eventCtx EventContext) error {
	s.mutex.Lock()
	defer s.mutex.Unlock()

	if s.States == nil {
		return NewErrConfigError("state machine config is nil")
	}

	for {
		// Determine the next state for the event given the machine's
		// current state.
		state, err := s.getNextState(event)
		if err != nil {
			return ErrEventRejected
		}

		// Notify the state machine's observers.
		s.observerMutex.Lock()
		for _, observer := range s.observers {
			observer.Notify(Notification{
				PreviousState: s.previous,
				NextState:     s.current,
				Event:         event,
			})
		}
		s.observerMutex.Unlock()

		// Execute the state machines ActionEntryFunc.
		if s.ActionEntryFunc != nil {
			s.ActionEntryFunc()
		}

		// Execute the current state's entry function
		if state.EntryFunc != nil {
			state.EntryFunc()
		}

		// Execute the next state's action and loop over again if the
		// event returned is not a no-op.
		nextEvent := state.Action(eventCtx)

		// Execute the current state's exit function
		if state.ExitFunc != nil {
			state.ExitFunc()
		}

		// Execute the state machines ActionExitFunc.
		if s.ActionExitFunc != nil {
			s.ActionExitFunc()
		}

		// If the next event is a no-op, we're done.
		if nextEvent == NoOp {
			return nil
		}

		event = nextEvent
	}
}

// RegisterObserver registers an observer with the state machine.
func (s *StateMachine) RegisterObserver(observer Observer) {
	s.observerMutex.Lock()
	defer s.observerMutex.Unlock()

	if observer != nil {
		s.observers = append(s.observers, observer)
	}
}

// RemoveObserver removes an observer from the state machine. It returns true
// if the observer was removed, false otherwise.
func (s *StateMachine) RemoveObserver(observer Observer) bool {
	s.observerMutex.Lock()
	defer s.observerMutex.Unlock()

	for i, o := range s.observers {
		if o == observer {
			s.observers = append(
				s.observers[:i], s.observers[i+1:]...,
			)
			return true
		}
	}

	return false
}

// HandleError is a helper function that can be used by actions to handle
// errors.
func (s *StateMachine) HandleError(err error) EventType {
	log.Errorf("StateMachine error: %s", err)
	s.LastActionError = err
	return OnError
}

// NoOpAction is a no-op action that can be used by states that don't need to
// execute any action.
func NoOpAction(_ EventContext) EventType {
	return NoOp
}

// ErrConfigError is an error returned when the state machine is misconfigured.
type ErrConfigError error

// NewErrConfigError creates a new ErrConfigError.
func NewErrConfigError(msg string) ErrConfigError {
	return (ErrConfigError)(fmt.Errorf("config error: %s", msg))
}

// ErrWaitingForStateTimeout is an error returned when the state machine times
// out while waiting for a state.
type ErrWaitingForStateTimeout error

// NewErrWaitingForStateTimeout creates a new ErrWaitingForStateTimeout.
func NewErrWaitingForStateTimeout(expected,
	actual StateType) ErrWaitingForStateTimeout {

	return (ErrWaitingForStateTimeout)(fmt.Errorf(
		"waiting for state timeout: expected %s, actual: %s",
		expected, actual,
	))
}