syncthing/lib/sync/sync.go

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// Copyright (C) 2015 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.
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package sync
import (
"fmt"
"path/filepath"
"runtime"
"strconv"
"strings"
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"sync"
"sync/atomic"
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"time"
)
var timeNow = time.Now
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type Mutex interface {
Lock()
Unlock()
}
type RWMutex interface {
Mutex
RLock()
RUnlock()
}
type WaitGroup interface {
Add(int)
Done()
Wait()
}
func NewMutex() Mutex {
if debug {
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mutex := &loggedMutex{}
mutex.holder.Store(holder{})
return mutex
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}
return &sync.Mutex{}
}
func NewRWMutex() RWMutex {
if debug {
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mutex := &loggedRWMutex{
readHolders: make(map[int][]holder),
unlockers: make(chan holder, 1024),
}
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mutex.holder.Store(holder{})
return mutex
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}
return &sync.RWMutex{}
}
func NewWaitGroup() WaitGroup {
if debug {
return &loggedWaitGroup{}
}
return &sync.WaitGroup{}
}
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type holder struct {
at string
time time.Time
goid int
}
func (h holder) String() string {
if h.at == "" {
return "not held"
}
return fmt.Sprintf("at %s goid: %d for %s", h.at, h.goid, timeNow().Sub(h.time))
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}
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type loggedMutex struct {
sync.Mutex
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holder atomic.Value
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}
func (m *loggedMutex) Lock() {
m.Mutex.Lock()
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m.holder.Store(getHolder())
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}
func (m *loggedMutex) Unlock() {
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currentHolder := m.holder.Load().(holder)
duration := timeNow().Sub(currentHolder.time)
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if duration >= threshold {
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l.Debugf("Mutex held for %v. Locked at %s unlocked at %s", duration, currentHolder.at, getHolder().at)
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}
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m.holder.Store(holder{})
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m.Mutex.Unlock()
}
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func (m *loggedMutex) Holders() string {
return m.holder.Load().(holder).String()
}
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type loggedRWMutex struct {
sync.RWMutex
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holder atomic.Value
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readHolders map[int][]holder
readHoldersMut sync.Mutex
logUnlockers atomic.Bool
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unlockers chan holder
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}
func (m *loggedRWMutex) Lock() {
start := timeNow()
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m.logUnlockers.Store(true)
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m.RWMutex.Lock()
m.logUnlockers.Store(false)
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holder := getHolder()
m.holder.Store(holder)
duration := holder.time.Sub(start)
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if duration > threshold {
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var unlockerStrings []string
loop:
for {
select {
case holder := <-m.unlockers:
unlockerStrings = append(unlockerStrings, holder.String())
default:
break loop
}
}
l.Debugf("RWMutex took %v to lock. Locked at %s. RUnlockers while locking:\n%s", duration, holder.at, strings.Join(unlockerStrings, "\n"))
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}
}
func (m *loggedRWMutex) Unlock() {
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currentHolder := m.holder.Load().(holder)
duration := timeNow().Sub(currentHolder.time)
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if duration >= threshold {
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l.Debugf("RWMutex held for %v. Locked at %s unlocked at %s", duration, currentHolder.at, getHolder().at)
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}
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m.holder.Store(holder{})
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m.RWMutex.Unlock()
}
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func (m *loggedRWMutex) RLock() {
m.RWMutex.RLock()
holder := getHolder()
m.readHoldersMut.Lock()
m.readHolders[holder.goid] = append(m.readHolders[holder.goid], holder)
m.readHoldersMut.Unlock()
}
func (m *loggedRWMutex) RUnlock() {
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id := goid()
m.readHoldersMut.Lock()
current := m.readHolders[id]
if len(current) > 0 {
m.readHolders[id] = current[:len(current)-1]
}
m.readHoldersMut.Unlock()
if m.logUnlockers.Load() {
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holder := getHolder()
select {
case m.unlockers <- holder:
default:
l.Debugf("Dropped holder %s as channel full", holder)
}
}
m.RWMutex.RUnlock()
}
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func (m *loggedRWMutex) Holders() string {
output := m.holder.Load().(holder).String() + " (writer)"
m.readHoldersMut.Lock()
for _, holders := range m.readHolders {
for _, holder := range holders {
output += "\n" + holder.String() + " (reader)"
}
}
m.readHoldersMut.Unlock()
return output
}
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type loggedWaitGroup struct {
sync.WaitGroup
}
func (wg *loggedWaitGroup) Wait() {
start := timeNow()
wg.WaitGroup.Wait()
duration := timeNow().Sub(start)
if duration >= threshold {
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l.Debugf("WaitGroup took %v at %s", duration, getHolder())
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}
}
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func getHolder() holder {
_, file, line, _ := runtime.Caller(2)
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file = filepath.Join(filepath.Base(filepath.Dir(file)), filepath.Base(file))
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return holder{
at: fmt.Sprintf("%s:%d", file, line),
goid: goid(),
time: timeNow(),
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}
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}
func goid() int {
var buf [64]byte
n := runtime.Stack(buf[:], false)
idField := strings.Fields(strings.TrimPrefix(string(buf[:n]), "goroutine "))[0]
id, err := strconv.Atoi(idField)
if err != nil {
return -1
}
return id
}
// TimeoutCond is a variant on Cond. It has roughly the same semantics regarding 'L' - it must be held
// both when broadcasting and when calling TimeoutCondWaiter.Wait()
// Call Broadcast() to broadcast to all waiters on the TimeoutCond. Call SetupWait to create a
// TimeoutCondWaiter configured with the given timeout, which can then be used to listen for
// broadcasts.
type TimeoutCond struct {
L sync.Locker
ch chan struct{}
}
// TimeoutCondWaiter is a type allowing a consumer to wait on a TimeoutCond with a timeout. Wait() may be called multiple times,
// and will return true every time that the TimeoutCond is broadcast to. Once the configured timeout
// expires, Wait() will return false.
// Call Stop() to release resources once this TimeoutCondWaiter is no longer needed.
type TimeoutCondWaiter struct {
c *TimeoutCond
timer *time.Timer
}
func NewTimeoutCond(l sync.Locker) *TimeoutCond {
return &TimeoutCond{
L: l,
}
}
func (c *TimeoutCond) Broadcast() {
// ch.L must be locked when calling this function
if c.ch != nil {
close(c.ch)
c.ch = nil
}
}
func (c *TimeoutCond) SetupWait(timeout time.Duration) *TimeoutCondWaiter {
timer := time.NewTimer(timeout)
return &TimeoutCondWaiter{
c: c,
timer: timer,
}
}
func (w *TimeoutCondWaiter) Wait() bool {
// ch.L must be locked when calling this function
// Ensure that the channel exists, since we're going to be waiting on it
if w.c.ch == nil {
w.c.ch = make(chan struct{})
}
ch := w.c.ch
w.c.L.Unlock()
defer w.c.L.Lock()
select {
case <-w.timer.C:
return false
case <-ch:
return true
}
}
func (w *TimeoutCondWaiter) Stop() {
w.timer.Stop()
}