--- /dev/null
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package rate provides a rate limiter.
+package rate
+
+import (
+ "context"
+ "fmt"
+ "math"
+ "sync"
+ "time"
+)
+
+// Limit defines the maximum frequency of some events.
+// Limit is represented as number of events per second.
+// A zero Limit allows no events.
+type Limit float64
+
+// Inf is the infinite rate limit; it allows all events (even if burst is zero).
+const Inf = Limit(math.MaxFloat64)
+
+// Every converts a minimum time interval between events to a Limit.
+func Every(interval time.Duration) Limit {
+ if interval <= 0 {
+ return Inf
+ }
+ return 1 / Limit(interval.Seconds())
+}
+
+// A Limiter controls how frequently events are allowed to happen.
+// It implements a "token bucket" of size b, initially full and refilled
+// at rate r tokens per second.
+// Informally, in any large enough time interval, the Limiter limits the
+// rate to r tokens per second, with a maximum burst size of b events.
+// As a special case, if r == Inf (the infinite rate), b is ignored.
+// See https://en.wikipedia.org/wiki/Token_bucket for more about token buckets.
+//
+// The zero value is a valid Limiter, but it will reject all events.
+// Use NewLimiter to create non-zero Limiters.
+//
+// Limiter has three main methods, Allow, Reserve, and Wait.
+// Most callers should use Wait.
+//
+// Each of the three methods consumes a single token.
+// They differ in their behavior when no token is available.
+// If no token is available, Allow returns false.
+// If no token is available, Reserve returns a reservation for a future token
+// and the amount of time the caller must wait before using it.
+// If no token is available, Wait blocks until one can be obtained
+// or its associated context.Context is canceled.
+//
+// The methods AllowN, ReserveN, and WaitN consume n tokens.
+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
+}
+
+// Limit returns the maximum overall event rate.
+func (lim *Limiter) Limit() Limit {
+ lim.mu.Lock()
+ defer lim.mu.Unlock()
+ return lim.limit
+}
+
+// Burst returns the maximum burst size. Burst is the maximum number of tokens
+// that can be consumed in a single call to Allow, Reserve, or Wait, so higher
+// Burst values allow more events to happen at once.
+// A zero Burst allows no events, unless limit == Inf.
+func (lim *Limiter) Burst() int {
+ lim.mu.Lock()
+ defer lim.mu.Unlock()
+ return lim.burst
+}
+
+// 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,
+ }
+}
+
+// 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
+}
+
+// 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
+}
+
+// OK returns whether the limiter can provide the requested number of tokens
+// within the maximum wait time. If OK is false, Delay returns InfDuration, and
+// Cancel does nothing.
+func (r *Reservation) OK() bool {
+ return r.ok
+}
+
+// Delay is shorthand for DelayFrom(time.Now()).
+func (r *Reservation) Delay() time.Duration {
+ return r.DelayFrom(time.Now())
+}
+
+// InfDuration is the duration returned by Delay when a Reservation is not OK.
+const InfDuration = time.Duration(1<<63 - 1)
+
+// 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
+}
+
+// Cancel is shorthand for CancelAt(time.Now()).
+func (r *Reservation) Cancel() {
+ r.CancelAt(time.Now())
+ return
+}
+
+// CancelAt indicates that the reservation holder will not perform the reserved action
+// and reverses the effects of this Reservation on the rate limit as much as possible,
+// considering that other reservations may have already been made.
+func (r *Reservation) CancelAt(now time.Time) {
+ if !r.ok {
+ return
+ }
+
+ r.lim.mu.Lock()
+ defer r.lim.mu.Unlock()
+
+ if r.lim.limit == Inf || r.tokens == 0 || r.timeToAct.Before(now) {
+ return
+ }
+
+ // calculate tokens to restore
+ // The duration between lim.lastEvent and r.timeToAct tells us how many tokens were reserved
+ // after r was obtained. These tokens should not be restored.
+ restoreTokens := float64(r.tokens) - r.limit.tokensFromDuration(r.lim.lastEvent.Sub(r.timeToAct))
+ if restoreTokens <= 0 {
+ return
+ }
+ // advance time to now
+ now, _, tokens := r.lim.advance(now)
+ // calculate new number of tokens
+ tokens += restoreTokens
+ if burst := float64(r.lim.burst); tokens > burst {
+ tokens = burst
+ }
+ // update state
+ r.lim.last = now
+ r.lim.tokens = tokens
+ if r.timeToAct == r.lim.lastEvent {
+ prevEvent := r.timeToAct.Add(r.limit.durationFromTokens(float64(-r.tokens)))
+ if !prevEvent.Before(now) {
+ r.lim.lastEvent = prevEvent
+ }
+ }
+
+ return
+}
+
+// Reserve is shorthand for ReserveN(time.Now(), 1).
+func (lim *Limiter) Reserve() *Reservation {
+ return lim.ReserveN(time.Now(), 1)
+}
+
+// ReserveN returns a Reservation that indicates how long the caller must wait before n events happen.
+// The Limiter takes this Reservation into account when allowing future events.
+// The returned Reservation’s OK() method returns false if n exceeds the Limiter's burst size.
+// Usage example:
+// r := lim.ReserveN(time.Now(), 1)
+// if !r.OK() {
+// // Not allowed to act! Did you remember to set lim.burst to be > 0 ?
+// return
+// }
+// time.Sleep(r.Delay())
+// Act()
+// Use this method if you wish to wait and slow down in accordance with the rate limit without dropping events.
+// If you need to respect a deadline or cancel the delay, use Wait instead.
+// To drop or skip events exceeding rate limit, use Allow instead.
+func (lim *Limiter) ReserveN(now time.Time, n int) *Reservation {
+ r := lim.reserveN(now, n, InfDuration)
+ return &r
+}
+
+// Wait is shorthand for WaitN(ctx, 1).
+func (lim *Limiter) Wait(ctx context.Context) (err error) {
+ return lim.WaitN(ctx, 1)
+}
+
+// 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()
+ }
+}
+
+// SetLimit is shorthand for SetLimitAt(time.Now(), newLimit).
+func (lim *Limiter) SetLimit(newLimit Limit) {
+ lim.SetLimitAt(time.Now(), newLimit)
+}
+
+// SetLimitAt sets a new Limit for the limiter. The new Limit, and Burst, may be violated
+// or underutilized by those which reserved (using Reserve or Wait) but did not yet act
+// before SetLimitAt was called.
+func (lim *Limiter) SetLimitAt(now time.Time, newLimit Limit) {
+ lim.mu.Lock()
+ defer lim.mu.Unlock()
+
+ now, _, tokens := lim.advance(now)
+
+ lim.last = now
+ lim.tokens = tokens
+ lim.limit = newLimit
+}
+
+// SetBurst is shorthand for SetBurstAt(time.Now(), newBurst).
+func (lim *Limiter) SetBurst(newBurst int) {
+ lim.SetBurstAt(time.Now(), newBurst)
+}
+
+// SetBurstAt sets a new burst size for the limiter.
+func (lim *Limiter) SetBurstAt(now time.Time, newBurst int) {
+ lim.mu.Lock()
+ defer lim.mu.Unlock()
+
+ now, _, tokens := lim.advance(now)
+
+ lim.last = now
+ lim.tokens = tokens
+ lim.burst = newBurst
+}
+
+// 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
+}
+
+// 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
+ }
+
+ // Avoid making delta overflow below when last is very old.
+ maxElapsed := lim.limit.durationFromTokens(float64(lim.burst) - lim.tokens)
+ elapsed := now.Sub(last)
+ if elapsed > maxElapsed {
+ elapsed = maxElapsed
+ }
+
+ // Calculate the new number of tokens, due to time that passed.
+ delta := lim.limit.tokensFromDuration(elapsed)
+ tokens := lim.tokens + delta
+ if burst := float64(lim.burst); tokens > burst {
+ tokens = burst
+ }
+
+ return now, last, tokens
+}
+
+// durationFromTokens is a unit conversion function from the number of tokens to the duration
+// of time it takes to accumulate them at a rate of limit tokens per second.
+func (limit Limit) durationFromTokens(tokens float64) time.Duration {
+ seconds := tokens / float64(limit)
+ return time.Nanosecond * time.Duration(1e9*seconds)
+}
+
+// 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 {
+ // Split the integer and fractional parts ourself to minimize rounding errors.
+ // See golang.org/issues/34861.
+ sec := float64(d/time.Second) * float64(limit)
+ nsec := float64(d%time.Second) * float64(limit)
+ return sec + nsec/1e9
+}
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