common/mclock: add Alarm (#26333)

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.

common/mclock/alarm.go

@@ -0,0 +1,106 @@
+// Copyright 2022 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package mclock
+
+import (
+	"time"
+)
+
+// Alarm sends timed notifications on a channel. This is very similar to a regular timer,
+// but is easier to use in code that needs to re-schedule the same timer over and over.
+//
+// When scheduling an Alarm, the channel returned by C() will receive a value no later
+// than the scheduled time. An Alarm can be reused after it has fired and can also be
+// canceled by calling Stop.
+type Alarm struct {
+	ch       chan struct{}
+	clock    Clock
+	timer    Timer
+	deadline AbsTime
+}
+
+// NewAlarm creates an Alarm.
+func NewAlarm(clock Clock) *Alarm {
+	if clock == nil {
+		panic("nil clock")
+	}
+	return &Alarm{
+		ch:    make(chan struct{}, 1),
+		clock: clock,
+	}
+}
+
+// C returns the alarm notification channel. This channel remains identical for
+// the entire lifetime of the alarm, and is never closed.
+func (e *Alarm) C() <-chan struct{} {
+	return e.ch
+}
+
+// Stop cancels the alarm and drains the channel.
+// This method is not safe for concurrent use.
+func (e *Alarm) Stop() {
+	// Clear timer.
+	if e.timer != nil {
+		e.timer.Stop()
+	}
+	e.deadline = 0
+
+	// Drain the channel.
+	select {
+	case <-e.ch:
+	default:
+	}
+}
+
+// Schedule sets the alarm to fire no later than the given time. If the alarm was already
+// scheduled but has not fired yet, it may fire earlier than the newly-scheduled time.
+func (e *Alarm) Schedule(time AbsTime) {
+	now := e.clock.Now()
+	e.schedule(now, time)
+}
+
+func (e *Alarm) schedule(now, newDeadline AbsTime) {
+	if e.timer != nil {
+		if e.deadline > now && e.deadline <= newDeadline {
+			// Here, the current timer can be reused because it is already scheduled to
+			// occur earlier than the new deadline.
+			//
+			// The e.deadline > now part of the condition is important. If the old
+			// deadline lies in the past, we assume the timer has already fired and needs
+			// to be rescheduled.
+			return
+		}
+		e.timer.Stop()
+	}
+
+	// Set the timer.
+	d := time.Duration(0)
+	if newDeadline < now {
+		newDeadline = now
+	} else {
+		d = newDeadline.Sub(now)
+	}
+	e.timer = e.clock.AfterFunc(d, e.send)
+	e.deadline = newDeadline
+}
+
+func (e *Alarm) send() {
+	select {
+	case e.ch <- struct{}{}:
+	default:
+	}
+}

common/mclock/alarm_test.go

@@ -0,0 +1,116 @@
+// Copyright 2022 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package mclock
+
+import "testing"
+
+// This test checks basic functionality of Alarm.
+func TestAlarm(t *testing.T) {
+	clk := new(Simulated)
+	clk.Run(20)
+	a := NewAlarm(clk)
+
+	a.Schedule(clk.Now() + 10)
+	if recv(a.C()) {
+		t.Fatal("Alarm fired before scheduled deadline")
+	}
+	if ntimers := clk.ActiveTimers(); ntimers != 1 {
+		t.Fatal("clock has", ntimers, "active timers, want", 1)
+	}
+	clk.Run(5)
+	if recv(a.C()) {
+		t.Fatal("Alarm fired too early")
+	}
+
+	clk.Run(5)
+	if !recv(a.C()) {
+		t.Fatal("Alarm did not fire")
+	}
+	if recv(a.C()) {
+		t.Fatal("Alarm fired twice")
+	}
+	if ntimers := clk.ActiveTimers(); ntimers != 0 {
+		t.Fatal("clock has", ntimers, "active timers, want", 0)
+	}
+
+	a.Schedule(clk.Now() + 5)
+	if recv(a.C()) {
+		t.Fatal("Alarm fired before scheduled deadline when scheduling the second event")
+	}
+
+	clk.Run(5)
+	if !recv(a.C()) {
+		t.Fatal("Alarm did not fire when scheduling the second event")
+	}
+	if recv(a.C()) {
+		t.Fatal("Alarm fired twice when scheduling the second event")
+	}
+}
+
+// This test checks that scheduling an Alarm to an earlier time than the
+// one already scheduled works properly.
+func TestAlarmScheduleEarlier(t *testing.T) {
+	clk := new(Simulated)
+	clk.Run(20)
+	a := NewAlarm(clk)
+
+	a.Schedule(clk.Now() + 50)
+	clk.Run(5)
+	a.Schedule(clk.Now() + 1)
+	clk.Run(3)
+	if !recv(a.C()) {
+		t.Fatal("Alarm did not fire")
+	}
+}
+
+// This test checks that scheduling an Alarm to a later time than the
+// one already scheduled works properly.
+func TestAlarmScheduleLater(t *testing.T) {
+	clk := new(Simulated)
+	clk.Run(20)
+	a := NewAlarm(clk)
+
+	a.Schedule(clk.Now() + 50)
+	clk.Run(5)
+	a.Schedule(clk.Now() + 100)
+	clk.Run(50)
+	if !recv(a.C()) {
+		t.Fatal("Alarm did not fire")
+	}
+}
+
+// This test checks that scheduling an Alarm in the past makes it fire immediately.
+func TestAlarmNegative(t *testing.T) {
+	clk := new(Simulated)
+	clk.Run(50)
+	a := NewAlarm(clk)
+
+	a.Schedule(-1)
+	clk.Run(1) // needed to process timers
+	if !recv(a.C()) {
+		t.Fatal("Alarm did not fire for negative time")
+	}
+}
+
+func recv(ch <-chan struct{}) bool {
+	select {
+	case <-ch:
+		return true
+	default:
+		return false
+	}
+}

p2p/dial.go

@@ -118,8 +118,7 @@ type dialScheduler struct {
 
 	// The dial history keeps recently dialed nodes. Members of history are not dialed.
 	history      expHeap
-	historyTimer     mclock.Timer
-	historyTimerTime mclock.AbsTime
+	historyTimer *mclock.Alarm
 
 	// for logStats
 	lastStatsLog     mclock.AbsTime
@@ -160,8 +159,10 @@ func (cfg dialConfig) withDefaults() dialConfig {
 }
 
 func newDialScheduler(config dialConfig, it enode.Iterator, setupFunc dialSetupFunc) *dialScheduler {
+	cfg := config.withDefaults()
 	d := &dialScheduler{
-		dialConfig:  config.withDefaults(),
+		dialConfig:   cfg,
+		historyTimer: mclock.NewAlarm(cfg.clock),
 		setupFunc:    setupFunc,
 		dialing:      make(map[enode.ID]*dialTask),
 		static:       make(map[enode.ID]*dialTask),
@@ -223,7 +224,6 @@ func (d *dialScheduler) peerRemoved(c *conn) {
 func (d *dialScheduler) loop(it enode.Iterator) {
 	var (
 		nodesCh chan *enode.Node
-		historyExp = make(chan struct{}, 1)
 	)
 
 loop:
@@ -236,7 +236,7 @@ loop:
 		} else {
 			nodesCh = nil
 		}
-		d.rearmHistoryTimer(historyExp)
+		d.rearmHistoryTimer()
 		d.logStats()
 
 		select {
@@ -297,7 +297,7 @@ loop:
 				}
 			}
 
-		case <-historyExp:
+		case <-d.historyTimer.C():
 			d.expireHistory()
 
 		case <-d.ctx.Done():
@@ -306,7 +306,7 @@ loop:
 		}
 	}
 
-	d.stopHistoryTimer(historyExp)
+	d.historyTimer.Stop()
 	for range d.dialing {
 		<-d.doneCh
 	}
@@ -343,28 +343,15 @@ func (d *dialScheduler) logStats() {
 
 // rearmHistoryTimer configures d.historyTimer to fire when the
 // next item in d.history expires.
-func (d *dialScheduler) rearmHistoryTimer(ch chan struct{}) {
-	if len(d.history) == 0 || d.historyTimerTime == d.history.nextExpiry() {
+func (d *dialScheduler) rearmHistoryTimer() {
+	if len(d.history) == 0 {
 		return
 	}
-	d.stopHistoryTimer(ch)
-	d.historyTimerTime = d.history.nextExpiry()
-	timeout := time.Duration(d.historyTimerTime - d.clock.Now())
-	d.historyTimer = d.clock.AfterFunc(timeout, func() { ch <- struct{}{} })
-}
-
-// stopHistoryTimer stops the timer and drains the channel it sends on.
-func (d *dialScheduler) stopHistoryTimer(ch chan struct{}) {
-	if d.historyTimer != nil && !d.historyTimer.Stop() {
-		<-ch
-	}
+	d.historyTimer.Schedule(d.history.nextExpiry())
 }
 
 // expireHistory removes expired items from d.history.
 func (d *dialScheduler) expireHistory() {
-	d.historyTimer.Stop()
-	d.historyTimer = nil
-	d.historyTimerTime = 0
 	d.history.expire(d.clock.Now(), func(hkey string) {
 		var id enode.ID
 		copy(id[:], hkey)