// Package ae provides tools to synchronize state between local and remote consul servers.
package ae

import (
	"errors"
	"log"
	"math"
	"sync"
	"time"

	"github.com/hashicorp/consul/lib"
)

// scaleThreshold is the number of nodes after which regular sync runs are
// spread out farther apart. The value should be a power of 2 since the
// scale function uses log2.
//
// When set to 128 nodes the delay between regular runs is doubled when the
// cluster is larger than 128 nodes. It doubles again when it passes 256
// nodes, and again at 512 nodes and so forth. At 8192 nodes, the delay
// factor is 8.
//
// If you update this, you may need to adjust the tuning of
// CoordinateUpdatePeriod and CoordinateUpdateMaxBatchSize.
const scaleThreshold = 128

// scaleFactor returns a factor by which the next sync run should be delayed to
// avoid saturation of the cluster. The larger the cluster grows the farther
// the sync runs should be spread apart.
//
// The current implementation uses a log2 scale which doubles the delay between
// runs every time the cluster doubles in size.
func scaleFactor(nodes int) int {
	if nodes <= scaleThreshold {
		return 1.0
	}
	return int(math.Ceil(math.Log2(float64(nodes))-math.Log2(float64(scaleThreshold))) + 1.0)
}

type State interface {
	SyncChanges() error
	SyncFull() error
}

// StateSyncer manages background synchronization of the given state.
//
// The state is synchronized on a regular basis or on demand when either
// the state has changed or a new Consul server has joined the cluster.
//
// The regular state sychronization provides a self-healing mechanism
// for the cluster which is also called anti-entropy.
type StateSyncer struct {
	// State contains the data that needs to be synchronized.
	State State

	// Interval is the time between two full sync runs.
	Interval time.Duration

	// ShutdownCh is closed when the application is shutting down.
	ShutdownCh chan struct{}

	// Logger is the logger.
	Logger *log.Logger

	// ClusterSize returns the number of members in the cluster to
	// allow staggering the sync runs based on cluster size.
	// This needs to be set before Run() is called.
	ClusterSize func() int

	// SyncFull allows triggering an immediate but staggered full sync
	// in a non-blocking way.
	SyncFull *Trigger

	// SyncChanges allows triggering an immediate partial sync
	// in a non-blocking way.
	SyncChanges *Trigger

	// paused stores whether sync runs are temporarily disabled.
	pauseLock sync.Mutex
	paused    int

	// stagger randomly picks a duration between 0s and the given duration.
	stagger func(time.Duration) time.Duration

	// serverUpInterval is the max time after which a full sync is
	// performed when a server has been added to the cluster.
	serverUpInterval time.Duration

	// retryFailInterval is the time after which a failed full sync is retried.
	retryFailInterval time.Duration
}

const (
	// serverUpIntv is the max time to wait before a sync is triggered
	// when a consul server has been added to the cluster.
	serverUpIntv = 3 * time.Second

	// retryFailIntv is the min time to wait before a failed sync is retried.
	retryFailIntv = 15 * time.Second
)

func NewStateSyncer(state State, intv time.Duration, shutdownCh chan struct{}, logger *log.Logger) *StateSyncer {
	s := &StateSyncer{
		State:             state,
		Interval:          intv,
		ShutdownCh:        shutdownCh,
		Logger:            logger,
		SyncFull:          NewTrigger(),
		SyncChanges:       NewTrigger(),
		serverUpInterval:  serverUpIntv,
		retryFailInterval: retryFailIntv,
	}
	s.stagger = func(d time.Duration) time.Duration {
		f := scaleFactor(s.ClusterSize())
		return lib.RandomStagger(time.Duration(f) * d)
	}
	return s
}

var errPaused = errors.New("paused")

// Run is the long running method to perform state synchronization
// between local and remote servers.
func (s *StateSyncer) Run() {
	if s.ClusterSize == nil {
		panic("ClusterSize not set")
	}

FullSync:
	for {
		// attempt a full sync
		err := s.ifNotPausedRun(s.State.SyncFull)
		if err != nil {
			if err != errPaused {
				s.Logger.Printf("[ERR] agent: failed to sync remote state: %v", err)
			}

			select {
			// trigger a full sync immediately.
			// this is usually called when a consul server was added to the cluster.
			// stagger the delay to avoid a thundering herd.
			case <-s.SyncFull.Notif():
				select {
				case <-time.After(s.stagger(s.serverUpInterval)):
					continue FullSync
				case <-s.ShutdownCh:
					return
				}

			// retry full sync after some time
			// todo(fs): why don't we use s.Interval here?
			case <-time.After(s.retryFailInterval + s.stagger(s.retryFailInterval)):
				continue FullSync

			case <-s.ShutdownCh:
				return
			}
		}

		// do partial syncs until it is time for a full sync again
		for {
			select {
			// trigger a full sync immediately
			// this is usually called when a consul server was added to the cluster.
			// stagger the delay to avoid a thundering herd.
			case <-s.SyncFull.Notif():
				select {
				case <-time.After(s.stagger(s.serverUpInterval)):
					continue FullSync
				case <-s.ShutdownCh:
					return
				}

			// time for a full sync again
			case <-time.After(s.Interval + s.stagger(s.Interval)):
				continue FullSync

			// do partial syncs on demand
			case <-s.SyncChanges.Notif():
				err := s.ifNotPausedRun(s.State.SyncChanges)
				if err != nil && err != errPaused {
					s.Logger.Printf("[ERR] agent: failed to sync changes: %v", err)
				}

			case <-s.ShutdownCh:
				return
			}
		}
	}
}

func (s *StateSyncer) ifNotPausedRun(f func() error) error {
	s.pauseLock.Lock()
	defer s.pauseLock.Unlock()
	if s.paused != 0 {
		return errPaused
	}
	return f()
}

// Pause temporarily disables sync runs.
func (s *StateSyncer) Pause() {
	s.pauseLock.Lock()
	s.paused++
	s.pauseLock.Unlock()
}

// Paused returns whether sync runs are temporarily disabled.
func (s *StateSyncer) Paused() bool {
	s.pauseLock.Lock()
	defer s.pauseLock.Unlock()
	return s.paused != 0
}

// Resume re-enables sync runs.
func (s *StateSyncer) Resume() {
	s.pauseLock.Lock()
	s.paused--
	if s.paused < 0 {
		panic("unbalanced pause/resume")
	}
	if s.paused == 0 {
		s.SyncChanges.Trigger()
	}
	s.pauseLock.Unlock()
}