golang源码分析：time/rate

   r := rate.Every(100 * time.Millisecond)
   limit := rate.NewLimiter(r, 20)
   for {
       if limit.AllowN(time.Now(), 8) {
           log.Info("log:event happen")
       } else {
           log.Info("log:event not allow")
       }

   }

rate.go
rate_test.go

// NewLimiter returns a new Limiter that allows events up to rate r and permits
// bursts of at most b tokens.
func NewLimiter(r Limit, b int) *Limiter {
  return &Limiter{
    limit: r,
    burst: b,
  }
}

type Limiter struct {
  mu     sync.Mutex
  limit  Limit
  burst  int
  tokens float64
  // last is the last time the limiter's tokens field was updated
  last time.Time
  // lastEvent is the latest time of a rate-limited event (past or future)
  lastEvent time.Time
}

func Every(interval time.Duration) Limit {
  if interval <= 0 {
    return Inf
  }
  return 1 / Limit(interval.Seconds())
}

// Allow is shorthand for AllowN(time.Now(), 1).
func (lim *Limiter) Allow() bool {
  return lim.AllowN(time.Now(), 1)
}

// AllowN reports whether n events may happen at time now.
// Use this method if you intend to drop / skip events that exceed the rate limit.
// Otherwise use Reserve or Wait.
func (lim *Limiter) AllowN(now time.Time, n int) bool {
  return lim.reserveN(now, n, 0).ok
}

// reserveN is a helper method for AllowN, ReserveN, and WaitN.
// maxFutureReserve specifies the maximum reservation wait duration allowed.
// reserveN returns Reservation, not *Reservation, to avoid allocation in AllowN and WaitN.
func (lim *Limiter) reserveN(now time.Time, n int, maxFutureReserve time.Duration) Reservation {
  lim.mu.Lock()

  if lim.limit == Inf {
    lim.mu.Unlock()
    return Reservation{
      ok:        true,
      lim:       lim,
      tokens:    n,
      timeToAct: now,
    }
  }

  now, last, tokens := lim.advance(now)

  // Calculate the remaining number of tokens resulting from the request.
  tokens -= float64(n)

  // Calculate the wait duration
  var waitDuration time.Duration
  if tokens < 0 {
    waitDuration = lim.limit.durationFromTokens(-tokens)
  }

  // Decide result
  ok := n <= lim.burst && waitDuration <= maxFutureReserve

  // Prepare reservation
  r := Reservation{
    ok:    ok,
    lim:   lim,
    limit: lim.limit,
  }
  if ok {
    r.tokens = n
    r.timeToAct = now.Add(waitDuration)
  }

  // Update state
  if ok {
    lim.last = now
    lim.tokens = tokens
    lim.lastEvent = r.timeToAct
  } else {
    lim.last = last
  }

  lim.mu.Unlock()
  return r
}

// A Reservation holds information about events that are permitted by a Limiter to happen after a delay.
// A Reservation may be canceled, which may enable the Limiter to permit additional events.
type Reservation struct {
  ok        bool
  lim       *Limiter
  tokens    int
  timeToAct time.Time
  // This is the Limit at reservation time, it can change later.
  limit Limit
}

// advance calculates and returns an updated state for lim resulting from the passage of time.
// lim is not changed.
// advance requires that lim.mu is held.
func (lim *Limiter) advance(now time.Time) (newNow time.Time, newLast time.Time, newTokens float64) {
  last := lim.last
  if now.Before(last) {
    last = now
  }

  // Calculate the new number of tokens, due to time that passed.
  elapsed := now.Sub(last)
  delta := lim.limit.tokensFromDuration(elapsed)
  tokens := lim.tokens + delta
  if burst := float64(lim.burst); tokens > burst {
    tokens = burst
  }
  return now, last, tokens
}

 delta := lim.limit.tokensFromDuration(elapsed)

type Limit float64
// tokensFromDuration is a unit conversion function from a time duration to the number of tokens
// which could be accumulated during that duration at a rate of limit tokens per second.
func (limit Limit) tokensFromDuration(d time.Duration) float64 {
  return d.Seconds() * float64(limit)
}

ok := n <= lim.burst && waitDuration <= maxFutureReserve

func (lim *Limiter) Allow() bool
func (lim *Limiter) AllowN(now time.Time, n int) bool

func (lim *Limiter) Wait(ctx context.Context) (err error)
func (lim *Limiter) WaitN(ctx context.Context, n int) (err error)

// WaitN blocks until lim permits n events to happen.
// It returns an error if n exceeds the Limiter's burst size, the Context is
// canceled, or the expected wait time exceeds the Context's Deadline.
// The burst limit is ignored if the rate limit is Inf.
func (lim *Limiter) WaitN(ctx context.Context, n int) (err error) {
  lim.mu.Lock()
  burst := lim.burst
  limit := lim.limit
  lim.mu.Unlock()

  if n > burst && limit != Inf {
    return fmt.Errorf("rate: Wait(n=%d) exceeds limiter's burst %d", n, burst)
  }
  // Check if ctx is already cancelled
  select {
  case <-ctx.Done():
    return ctx.Err()
  default:
  }
  // Determine wait limit
  now := time.Now()
  waitLimit := InfDuration
  if deadline, ok := ctx.Deadline(); ok {
    waitLimit = deadline.Sub(now)
  }
  // Reserve
  r := lim.reserveN(now, n, waitLimit)
  if !r.ok {
    return fmt.Errorf("rate: Wait(n=%d) would exceed context deadline", n)
  }
  // Wait if necessary
  delay := r.DelayFrom(now)
  if delay == 0 {
    return nil
  }
  t := time.NewTimer(delay)
  defer t.Stop()
  select {
  case <-t.C:
    // We can proceed.
    return nil
  case <-ctx.Done():
    // Context was canceled before we could proceed.  Cancel the
    // reservation, which may permit other events to proceed sooner.
    r.Cancel()
    return ctx.Err()
  }
}

// DelayFrom returns the duration for which the reservation holder must wait
// before taking the reserved action.  Zero duration means act immediately.
// InfDuration means the limiter cannot grant the tokens requested in this
// Reservation within the maximum wait time.
func (r *Reservation) DelayFrom(now time.Time) time.Duration {
  if !r.ok {
    return InfDuration
  }
  delay := r.timeToAct.Sub(now)
  if delay < 0 {
    return 0
  }
  return delay
}

func (lim *Limiter) Reserve() *Reservation
func (lim *Limiter) ReserveN(now time.Time, n int) *Reservation

func (lim *Limiter) ReserveN(now time.Time, n int) *Reservation {
  r := lim.reserveN(now, n, InfDuration)
  return &r
}