9e6a1c3834
Alarm is a timer utility that simplifies code where a timer needs to be rescheduled over and over. Doing this can be tricky with time.Timer or time.AfterFunc because the channel requires draining in some cases. Alarm is optimized for use cases where items are tracked in a heap according to their expiry time, and a goroutine with a for/select loop wants to be woken up whenever the next item expires. In this application, the timer needs to be rescheduled when an item is added or removed from the heap. Using a timer naively, these updates will always require synchronization with the global runtime timer datastructure to update the timer using Reset. Alarm avoids this by tracking the next expiry time and only modifies the timer if it would need to fire earlier than already scheduled. As an example use, I have converted p2p.dialScheduler to use Alarm instead of AfterFunc.
107 lines
2.9 KiB
Go
107 lines
2.9 KiB
Go
// Copyright 2022 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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package mclock
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import (
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"time"
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)
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// Alarm sends timed notifications on a channel. This is very similar to a regular timer,
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// but is easier to use in code that needs to re-schedule the same timer over and over.
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//
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// When scheduling an Alarm, the channel returned by C() will receive a value no later
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// than the scheduled time. An Alarm can be reused after it has fired and can also be
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// canceled by calling Stop.
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type Alarm struct {
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ch chan struct{}
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clock Clock
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timer Timer
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deadline AbsTime
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}
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// NewAlarm creates an Alarm.
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func NewAlarm(clock Clock) *Alarm {
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if clock == nil {
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panic("nil clock")
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}
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return &Alarm{
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ch: make(chan struct{}, 1),
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clock: clock,
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}
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}
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// C returns the alarm notification channel. This channel remains identical for
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// the entire lifetime of the alarm, and is never closed.
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func (e *Alarm) C() <-chan struct{} {
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return e.ch
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}
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// Stop cancels the alarm and drains the channel.
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// This method is not safe for concurrent use.
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func (e *Alarm) Stop() {
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// Clear timer.
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if e.timer != nil {
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e.timer.Stop()
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}
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e.deadline = 0
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// Drain the channel.
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select {
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case <-e.ch:
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default:
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}
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}
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// Schedule sets the alarm to fire no later than the given time. If the alarm was already
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// scheduled but has not fired yet, it may fire earlier than the newly-scheduled time.
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func (e *Alarm) Schedule(time AbsTime) {
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now := e.clock.Now()
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e.schedule(now, time)
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}
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func (e *Alarm) schedule(now, newDeadline AbsTime) {
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if e.timer != nil {
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if e.deadline > now && e.deadline <= newDeadline {
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// Here, the current timer can be reused because it is already scheduled to
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// occur earlier than the new deadline.
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//
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// The e.deadline > now part of the condition is important. If the old
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// deadline lies in the past, we assume the timer has already fired and needs
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// to be rescheduled.
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return
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}
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e.timer.Stop()
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}
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// Set the timer.
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d := time.Duration(0)
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if newDeadline < now {
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newDeadline = now
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} else {
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d = newDeadline.Sub(now)
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}
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e.timer = e.clock.AfterFunc(d, e.send)
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e.deadline = newDeadline
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}
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func (e *Alarm) send() {
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select {
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case e.ch <- struct{}{}:
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default:
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}
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}
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