I am a newcomer to the go language. When I execute the code, I get the following error：
fatal error: concurrent map read and map write
func foo() {
var m = map[string]int{"a": 1}
var lock = sync.RWMutex{}
go Read(m, lock)
time.Sleep(1 * time.Second)
go Write(m, lock)
time.Sleep(1 * time.Minute)
}
func main() {
foo()
}
func Read(m map[string]int, lock sync.RWMutex) {
for {
read(m, lock)
}
}
func Write(m map[string]int, lock sync.RWMutex) {
for {
write(m, lock)
}
}
func read(m map[string]int, lock sync.RWMutex) {
lock.RLock()
defer lock.RUnlock()
_ = m["a"]
}
func write(m map[string]int, lock sync.RWMutex) {
lock.Lock()
defer lock.Unlock()
m["b"] = 2
}
anyone can tell me why?
You must pass a pointer to your sync.RWMutex value. Otherwise, you're making copies of the mutex every time you pass it to a new function, so no actual locking happens.
The go vet tool will detect this error for you. You should run go vet (and likely other linters) on all your code, to help catch such common errors. Related reading.
Related
I have the following code in Go using the semaphore library just as an example:
package main
import (
"fmt"
"context"
"time"
"golang.org/x/sync/semaphore"
)
// This protects the lockedVar variable
var lock *semaphore.Weighted
// Only one go routine should be able to access this at once
var lockedVar string
func acquireLock() {
err := lock.Acquire(context.TODO(), 1)
if err != nil {
panic(err)
}
}
func releaseLock() {
lock.Release(1)
}
func useLockedVar() {
acquireLock()
fmt.Printf("lockedVar used: %s\n", lockedVar)
releaseLock()
}
func causeDeadLock() {
acquireLock()
// calling this from a function that's already
// locked the lockedVar should cause a deadlock.
useLockedVar()
releaseLock()
}
func main() {
lock = semaphore.NewWeighted(1)
lockedVar = "this is the locked var"
// this is only on a separate goroutine so that the standard
// go "deadlock" message doesn't print out.
go causeDeadLock()
// Keep the primary goroutine active.
for true {
time.Sleep(time.Second)
}
}
Is there a way to get the acquireLock() function call to print a message after a timeout indicating that there is a potential deadlock but without unblocking the call? I would want the deadlock to persist, but a log message to be written in the event that a timeout is reached. So a TryAcquire isn't exactly what I want.
An example of what I want in psuedo code:
afterFiveSeconds := func() {
fmt.Printf("there is a potential deadlock\n")
}
lock.Acquire(context.TODO(), 1, afterFiveSeconds)
The lock.Acquire call in this example would call the afterFiveSeconds callback if the Acquire call blocked for more than 5 seconds, but it would not unblock the caller. It would continue to block.
I think I've found a solution to my problem.
func acquireLock() {
timeoutChan := make(chan bool)
go func() {
select {
case <-time.After(time.Second * time.Duration(5)):
fmt.Printf("potential deadlock while acquiring semaphore\n")
case <-timeoutChan:
break
}
}()
err := lock.Acquire(context.TODO(), 1)
close(timeoutChan)
if err != nil {
panic(err)
}
}
I am trying to pause and resume groutine. I understand I can sleep the run, but I am looking for is like a button "pause/resume" rather than a timer.
Here is my attempt. I am using the blocking feature of channel to pause, and select to switch what to execute based on channel value. However, the output is always Running in my case.
func main() {
ctx := wctx{}
go func(ctx wctx) {
for {
time.Sleep(1 * time.Second)
select {
case <-ctx.pause:
fmt.Print("Paused")
<-ctx.pause
case <-ctx.resume:
fmt.Print("Resumed")
default:
fmt.Print("Running \n")
}
}
}(ctx)
ctx.pause <- struct{}{}
ctx.resume <- struct{}{}
}
type wctx struct {
pause chan struct{}
resume chan struct{}
}
A select with multiple ready cases chooses one pseudo-randomly. So if the goroutine is "slow" to check those channels, you might send a value on both pause and resume (assuming they are buffered) so receiving from both channels could be ready, and resume could be chosen first, and in a later iteration the pause when the goroutine should not be paused anymore.
For this you should use a "state" variable synchronized by a mutex. Something like this:
const (
StateRunning = iota
StatePaused
)
type wctx struct {
mu sync.Mutex
state int
}
func (w *wctx) SetState(state int) {
w.mu.Lock()
defer w.mu.Unlock()
w.state = state
}
func (w *wctx) State() int {
w.mu.Lock()
defer w.mu.Unlock()
return w.state
}
Testing it:
ctx := &wctx{}
go func(ctx *wctx) {
for {
time.Sleep(1 * time.Millisecond)
switch state := ctx.State(); state {
case StatePaused:
fmt.Println("Paused")
default:
fmt.Println("Running")
}
}
}(ctx)
time.Sleep(3 * time.Millisecond)
ctx.SetState(StatePaused)
time.Sleep(3 * time.Millisecond)
ctx.SetState(StateRunning)
time.Sleep(2 * time.Millisecond)
Output (try it on the Go Playground):
Running
Running
Running
Paused
Paused
Paused
Running
Running
You need to initialize your channels, remember that reads from nil channels always blocks.
A select with a default case never blocks.
Here is a modified version of your program, that fixes the above mentioned issues:
package main
import (
"fmt"
"time"
)
func main() {
ctx := wctx{
pause: make(chan struct{}),
resume: make(chan struct{}),
}
go func(ctx wctx) {
for {
select {
case <-ctx.pause:
fmt.Println("Paused")
case <-ctx.resume:
fmt.Println("Resumed")
}
fmt.Println("Running")
time.Sleep(time.Second)
}
}(ctx)
ctx.pause <- struct{}{}
ctx.resume <- struct{}{}
}
type wctx struct {
pause chan struct{}
resume chan struct{}
}
I'm new to go and trying to make two methods run at the same time periodically for as long at the application is running. I've managed to come up with the following but the for true part does not feel right as this is blocking.
Would channels be a better way todo this? Any pointers in the right direction would be helpful.
func main() {
t1 := schedule(ping, time.Second)
t2 := schedule(ping, 2*time.Second)
for true {
time.Sleep(1 * time.Second)
}
t1.Stop()
t2.Stop()
}
func schedule(f func(interval time.Duration), interval time.Duration) *time.Ticker {
ticker := time.NewTicker(interval)
go func() {
for range ticker.C {
f(interval)
}
}()
return ticker
}
func ping(interval time.Duration) {
log.Println("ping ", interval)
}
To prevent the application from exiting, the main goroutine must block.
Use select {} to block the main goroutine.
Because the tickers run for the duration of the application, there's no need to stop the tickers.
func main() {
schedule(ping, time.Second)
schedule(ping, 2*time.Second)
select {}
}
Following problem:
I have a function that only should allow one caller to execute.
If someone tries to call the function and it is already busy the second caller should immediatly return with an error.
I tried the following:
1. Use a mutex
Would be pretty easy. But the problem is, you cannot check if a mutex is locked. You can only block on it. Therefore it does not work
2. Wait on a channel
var canExec = make(chan bool, 1)
func init() {
canExec <- true
}
func onlyOne() error {
select {
case <-canExec:
default:
return errors.New("already busy")
}
defer func() {
fmt.Println("done")
canExec <- true
}()
// do stuff
}
What I don't like here:
looks really messi
if easy to mistakenly block on the channel / mistakenly write to the channel
3. Mixture of mutex and shared state
var open = true
var myMutex *sync.Mutex
func canExec() bool {
myMutex.Lock()
defer myMutex.Unlock()
if open {
open = false
return true
}
return false
}
func endExec() {
myMutex.Lock()
defer myMutex.Unlock()
open = true
}
func onlyOne() error {
if !canExec() {
return errors.New("busy")
}
defer endExec()
// do stuff
return nil
}
I don't like this either. Using a shard variable with mutex is not that nice.
Any other idea?
I'll throw my preference out there - use the atomic package.
var (
locker uint32
errLocked = errors.New("Locked out buddy")
)
func OneAtATime(d time.Duration) error {
if !atomic.CompareAndSwapUint32(&locker, 0, 1) { // <-----------------------------
return errLocked // All logic in these |
} // four lines |
defer atomic.StoreUint32(&locker, 0) // <-----------------------------
// logic here, but we will sleep
time.Sleep(d)
return nil
}
The idea is pretty simple. Set the initial value to 0 (0 value of uint32). The first thing you do in the function is check if the value of locker is currently 0 and if so it changes it to 1. It does all of this atomically. If it fails simply return an error (or however else you like to handle a locked state). If successful, you immediately defer replacing the value (now 1) with 0. You don't have to use defer obviously, but failing to set the value back to 0 before returning would leave you in a state where the function could no longer be run.
After you do those 4 lines of setup, you do whatever you would normally.
https://play.golang.org/p/riryVJM4Qf
You can make things a little nicer if desired by using named values for your states.
const (
stateUnlocked uint32 = iota
stateLocked
)
var (
locker = stateUnlocked
errLocked = errors.New("Locked out buddy")
)
func OneAtATime(d time.Duration) error {
if !atomic.CompareAndSwapUint32(&locker, stateUnlocked, stateLocked) {
return errLocked
}
defer atomic.StoreUint32(&locker, stateUnlocked)
// logic here, but we will sleep
time.Sleep(d)
return nil
}
You can use a semaphore for this (go get golang.org/x/sync/semaphore)
package main
import (
"errors"
"fmt"
"sync"
"time"
"golang.org/x/sync/semaphore"
)
var sem = semaphore.NewWeighted(1)
func main() {
var wg sync.WaitGroup
for i := 0; i < 10; i++ {
wg.Add(1)
go func() {
defer wg.Done()
if err := onlyOne(); err != nil {
fmt.Println(err)
}
}()
time.Sleep(time.Second)
}
wg.Wait()
}
func onlyOne() error {
if !sem.TryAcquire(1) {
return errors.New("busy")
}
defer sem.Release(1)
fmt.Println("working")
time.Sleep(5 * time.Second)
return nil
}
You could use standard channel approach with select statement.
var (
ch = make(chan bool)
)
func main() {
i := 0
wg := sync.WaitGroup{}
for i < 100 {
i++
wg.Add(1)
go func() {
defer wg.Done()
err := onlyOne()
if err != nil {
fmt.Println("Error: ", err)
} else {
fmt.Println("Ok")
}
}()
go func() {
ch <- true
}()
}
wg.Wait()
}
func onlyOne() error {
select {
case <-ch:
// do stuff
return nil
default:
return errors.New("Busy")
}
}
Do you want a function to be executed exactly once or once at given time? In former case take a look at https://golang.org/pkg/sync/#Once.
If you want once at a time solution:
package main
import (
"fmt"
"sync"
"time"
)
// OnceAtATime protects function from being executed simultaneously.
// Example:
// func myFunc() { time.Sleep(10*time.Second) }
// func main() {
// once := OnceAtATime{}
// once.Do(myFunc)
// once.Do(myFunc) // not executed
// }
type OnceAtATime struct {
m sync.Mutex
executed bool
}
func (o *OnceAtATime) Do(f func()) {
o.m.Lock()
if o.executed {
o.m.Unlock()
return
}
o.executed = true
o.m.Unlock()
f()
o.m.Lock()
o.executed = false
o.m.Unlock()
}
// Proof of concept
func f(m int, done chan<- struct{}) {
for i := 0; i < 10; i++ {
fmt.Printf("%d: %d\n", m, i)
time.Sleep(250 * time.Millisecond)
}
close(done)
}
func main() {
done := make(chan struct{})
once := OnceAtATime{}
go once.Do(func() { f(1, done) })
go once.Do(func() { f(2, done) })
<-done
done = make(chan struct{})
go once.Do(func() { f(3, done) })
<-done
}
https://play.golang.org/p/nZcEcWAgKp
But the problem is, you cannot check if a mutex is locked. You can only block on it. Therefore it does not work
With possible Go 1.18 (Q1 2022), you will be able to test if a mutex is locked... without blocking on it.
See (as mentioned by Go 101) the issue 45435 from Tye McQueen :
sync: add Mutex.TryLock
This is followed by CL 319769, with the caveat:
Use of these functions is almost (but not) always a bad idea.
Very rarely they are necessary, and third-party implementations (using a mutex and an atomic word, say) cannot integrate as well with the race detector as implementations in package sync itself.
The objections (since retracted) were:
Locks are for protecting invariants.
If the lock is held by someone else, there is nothing you can say about the invariant.
TryLock encourages imprecise thinking about locks; it encourages making assumptions about the invariants that may or may not be true.
That ends up being its own source of races.
Thinking more about this, there is one important benefit to building TryLock into Mutex, compared to a wrapper:
failed TryLock calls wouldn't create spurious happens-before edges to confuse the race detector.
And:
A channel-based implementation is possible, but performs poorly in comparison.
There's a reason we have sync.Mutex rather than just using channel for locking.
I came up with the following generic solution for that:
Works for me, or do you see any problem with that?
import (
"sync"
)
const (
ONLYONECALLER_LOCK = "onlyonecaller"
ANOTHER_LOCK = "onlyonecaller"
)
var locks = map[string]bool{}
var mutex = &sync.Mutex{}
func Lock(lock string) bool {
mutex.Lock()
defer mutex.Unlock()
locked, ok := locks[lock]
if !ok {
locks[lock] = true
return true
}
if locked {
return false
}
locks[lock] = true
return true
}
func IsLocked(lock string) bool {
mutex.Lock()
defer mutex.Unlock()
locked, ok := locks[lock]
if !ok {
return false
}
return locked
}
func Unlock(lock string) {
mutex.Lock()
defer mutex.Unlock()
locked, ok := locks[lock]
if !ok {
return
}
if !locked {
return
}
locks[lock] = false
}
see: https://play.golang.org/p/vUUsHcT3L-
How about this package: https://github.com/viney-shih/go-lock . It use channel and semaphore (golang.org/x/sync/semaphore) to solve your problem.
go-lock implements TryLock, TryLockWithTimeout and TryLockWithContext functions in addition to Lock and Unlock. It provides flexibility to control the resources.
Examples:
package main
import (
"fmt"
"time"
"context"
lock "github.com/viney-shih/go-lock"
)
func main() {
casMut := lock.NewCASMutex()
casMut.Lock()
defer casMut.Unlock()
// TryLock without blocking
fmt.Println("Return", casMut.TryLock()) // Return false
// TryLockWithTimeout without blocking
fmt.Println("Return", casMut.TryLockWithTimeout(50*time.Millisecond)) // Return false
// TryLockWithContext without blocking
ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)
defer cancel()
fmt.Println("Return", casMut.TryLockWithContext(ctx)) // Return false
// Output:
// Return false
// Return false
// Return false
}
Lets keep it simple:
package main
import (
"fmt"
"time"
"golang.org/x/sync/semaphore"
)
var sem *semaphore.NewWeighted(1)
func init() {
sem = emaphore.NewWeighted(1)
}
func doSomething() {
if !sem.TryAcquire(1) {
return errors.New("I'm busy")
}
defer sem.Release(1)
fmt.Println("I'm doing my work right now, then I'll take a nap")
time.Sleep(10)
}
func main() {
go func() {
doSomething()
}()
}
I tried to implement a locking version of reading/writing from a map in golang, but it doesn't return the desired result.
package main
import (
"sync"
"fmt"
)
var m = map[int]string{}
var lock = sync.RWMutex{}
func StoreUrl(id int, url string) {
for {
lock.Lock()
defer lock.Unlock()
m[id] = url
}
}
func LoadUrl(id int, ch chan string) {
for {
lock.RLock()
defer lock.RUnlock()
r := m[id]
ch <- r
}
}
func main() {
go StoreUrl(125, "www.google.com")
chb := make(chan string)
go LoadUrl(125, chb);
C := <-chb
fmt.Println("Result:", C)
}
The output is:
Result:
Meaning the value is not returned via the channel, which I don't get. Without the locking/goroutines it seems to work fine. What did I do wrong?
The code can also be found here:
https://play.golang.org/p/-WmRcMty5B
Infinite loops without sleep or some kind of IO are always bad idea.
In your code if you put a print statement at the start of StoreUrl, you will find that it never gets printed i.e the go routine was never started, the go call is setting putting the info about this new go routine in some run queue of the go scheduler but the scheduler hasn't ran yet to schedule that task. How do you run the scheduler? Do sleep/IO/channel reading/writing.
Another problem is that your infinite loop is taking lock and trying to take the lock again, which will cause it to deadlock. Defer only run after function exit and that function will never exit because of infinite loop.
Below is modified code that uses sleep to make sure every execution thread gets time to do its job.
package main
import (
"sync"
"fmt"
"time"
)
var m = map[int]string{}
var lock = sync.RWMutex{}
func StoreUrl(id int, url string) {
for {
lock.Lock()
m[id] = url
lock.Unlock()
time.Sleep(1)
}
}
func LoadUrl(id int, ch chan string) {
for {
lock.RLock()
r := m[id]
lock.RUnlock()
ch <- r
}
}
func main() {
go StoreUrl(125, "www.google.com")
time.Sleep(1)
chb := make(chan string)
go LoadUrl(125, chb);
C := <-chb
fmt.Println("Result:", C)
}
Edit: As #Jaun mentioned in the comment, you can also use runtime.Gosched() instead of sleep.
Usage of defer incorrect, defer execute at end of function, not for statement.
func StoreUrl(id int, url string) {
for {
func() {
lock.Lock()
defer lock.Unlock()
m[id] = url
}()
}
}
or
func StoreUrl(id int, url string) {
for {
lock.Lock()
m[id] = url
lock.Unlock()
}
}
We can't control the order of go routine, so add time.Sleep() to control the order.
code here:
https://play.golang.org/p/Bu8Lo46SA2