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87b4bbb94f
Add Value, which measures the rate at which an event occurs, exponentially weighted towards recent activity. It is guaranteed to occupy O(1) memory, operate in O(1) runtime, and is safe for concurrent use. Signed-off-by: Joe Tsai <joetsai@digital-static.net>
91 lines
2.4 KiB
Go
91 lines
2.4 KiB
Go
// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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// This is a modified, simplified version of code from golang.org/x/time/rate.
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// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package rate provides a rate limiter.
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package rate
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import (
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"sync"
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"time"
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"tailscale.com/tstime/mono"
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)
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// Limit defines the maximum frequency of some events.
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// Limit is represented as number of events per second.
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// A zero Limit is invalid.
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type Limit float64
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// Every converts a minimum time interval between events to a Limit.
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func Every(interval time.Duration) Limit {
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if interval <= 0 {
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panic("invalid interval")
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}
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return 1 / Limit(interval.Seconds())
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}
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// A Limiter controls how frequently events are allowed to happen.
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// It implements a [token bucket] of a particular size b,
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// initially full and refilled at rate r tokens per second.
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// Informally, in any large enough time interval,
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// the Limiter limits the rate to r tokens per second,
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// with a maximum burst size of b events.
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// Use NewLimiter to create non-zero Limiters.
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//
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// [token bucket]: https://en.wikipedia.org/wiki/Token_bucket
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type Limiter struct {
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limit Limit
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burst float64
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mu sync.Mutex // protects following fields
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tokens float64 // number of tokens currently in bucket
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last mono.Time // the last time the limiter's tokens field was updated
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}
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// NewLimiter returns a new Limiter that allows events up to rate r and permits
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// bursts of at most b tokens.
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func NewLimiter(r Limit, b int) *Limiter {
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if b < 1 {
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panic("bad burst, must be at least 1")
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}
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return &Limiter{limit: r, burst: float64(b)}
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}
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// Allow reports whether an event may happen now.
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func (lim *Limiter) Allow() bool {
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return lim.allow(mono.Now())
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}
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func (lim *Limiter) allow(now mono.Time) bool {
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lim.mu.Lock()
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defer lim.mu.Unlock()
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// If time has moved backwards, look around awkwardly and pretend nothing happened.
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if now.Before(lim.last) {
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lim.last = now
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}
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// Calculate the new number of tokens available due to the passage of time.
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elapsed := now.Sub(lim.last)
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tokens := lim.tokens + float64(lim.limit)*elapsed.Seconds()
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if tokens > lim.burst {
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tokens = lim.burst
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}
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// Consume a token.
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tokens--
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// Update state.
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ok := tokens >= 0
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if ok {
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lim.last = now
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lim.tokens = tokens
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}
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return ok
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}
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