Wait result of multiple goroutines - go

I am searching a way to execute asynchronously two functions in go which returns different results and errors, wait for them to finish and print both results. Also if one of function returned error I do not want to wait for another function, and just print the error.
For example, I have this functions:
func methodInt(error bool) (int, error) {
<-time.NewTimer(time.Millisecond * 100).C
if error {
return 0, errors.New("Some error")
} else {
return 1, nil
}
}
func methodString(error bool) (string, error) {
<-time.NewTimer(time.Millisecond * 120).C
if error {
return "", errors.New("Some error")
} else {
return "Some result", nil
}
}
Here https://play.golang.org/p/-8StYapmlg is how I implemented it, but it has too much code I think. It can be simplified by using interface{} but I don't want to go this way. I want something simpler as, for example, can be implemented in C# with async/await. Probably there is some library that simplifies such operation.
UPDATE: Thank for your responses! It is awesome how fast I got help! I like the usage of WaitGroup. It obviously makes the code more robust to changes, so I easily can add another async method without changing exact count of methods in the end. However, there is still so much code in comparison to same in C#. I know that in go I don't need to explicitly mark methods as async, making them actually to return tasks, but methods call looks much more simple, for example, consider this link actually catching exception is also needed
By the way, I found that in my task I actually don't need to know returning type of the functions I want to run async because it will be anyway marshaled to json, and now I just call multiple services in the endpoint layer of go-kit.

You should create two channels for errors and results, then first read errors if no erorrs then read the results, this sample should works for your use case:
package main
import (
"errors"
"sync"
)
func test(i int) (int, error) {
if i > 2 {
return 0, errors.New("test error")
}
return i + 5, nil
}
func test2(i int) (int, error) {
if i > 3 {
return 0, errors.New("test2 error")
}
return i + 7, nil
}
func main() {
results := make(chan int, 2)
errors := make(chan error, 2)
var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.Done()
result, err := test(3)
if err != nil {
errors <- err
return
}
results <- result
}()
wg.Add(1)
go func() {
defer wg.Done()
result, err := test2(3)
if err != nil {
errors <- err
return
}
results <- result
}()
// here we wait in other goroutine to all jobs done and close the channels
go func() {
wg.Wait()
close(results)
close(errors)
}()
for err := range errors {
// here error happend u could exit your caller function
println(err.Error())
return
}
for res := range results {
println("--------- ", res, " ------------")
}
}

I think here sync.WaitGroup can be used. It can waits for different and dynamic number of goroutines.

I have created a smaller, self-contained example of how you can have two go routines run asynchronously and wait for both to finish or quit the program if an error occurs (see below for an explanation):
package main
import (
"errors"
"fmt"
"math/rand"
"time"
)
func main() {
rand.Seed(time.Now().UnixNano())
// buffer the channel so the async go routines can exit right after sending
// their error
status := make(chan error, 2)
go func(c chan<- error) {
if rand.Intn(2) == 0 {
c <- errors.New("func 1 error")
} else {
fmt.Println("func 1 done")
c <- nil
}
}(status)
go func(c chan<- error) {
if rand.Intn(2) == 0 {
c <- errors.New("func 2 error")
} else {
fmt.Println("func 2 done")
c <- nil
}
}(status)
for i := 0; i < 2; i++ {
if err := <-status; err != nil {
fmt.Println("error encountered:", err)
break
}
}
}
What I do is create a channel that is used for synchronization of the two go routines. Writing to and reading from it blocks. The channel is used to pass the error value around, or nil if the function succeeds.
At the end I read one value per async go routine from the channel. This blocks until a value is received. If an error occurs, I exit the loop, thus quitting the program.
The functions either succeed or fail randomly.
I hope this gets you going on how to coordinate go routines, if not, let me know in the comments.
Note that if you run this in the Go Playground, the rand.Seed will do nothing, the playground always has the same "random" numbers, so the behavior will not change.

Related

Idiomatic goroutine concurrency and error handling

In the below code block I am trying to run several routines and get results (Whether success or error) for all of them.
package main
import (
"fmt"
"sync"
)
func processBatch(num int, errChan chan<- error, resultChan chan<- int, wg *sync.WaitGroup) {
defer wg.Done()
if num == 3 {
resultChan <- 0
errChan <- fmt.Errorf("goroutine %d's error returned", num)
} else {
square := num * num
resultChan <- square
errChan <- nil
}
}
func main() {
var wg sync.WaitGroup
batches := [5]int{1, 2, 3, 4, 5}
resultChan := make(chan int)
errChan := make(chan error)
for i := range batches {
wg.Add(1)
go processBatch(batches[i], errChan, resultChan, &wg)
}
var results [5]int
var err [5]error
for i := range batches {
results[i] = <-resultChan
err[i] = <-errChan
}
wg.Wait()
close(resultChan)
close(errChan)
fmt.Println(results)
fmt.Println(err)
}
Playground: https://go.dev/play/p/zA-Py9gDjce
This code works and I get the result that I want i.e:
[25 1 4 0 16]
[<nil> <nil> <nil> goroutine 3's error returned <nil>]
I was wondering if there is a more idiomatic way to achieve this. I went through the errgroup package: https://pkg.go.dev/golang.org/x/sync/errgroup but wasn't able to find something that may help me here. Any suggestions are welcome.
Waitgroup is redundant in this code. Execution is perfectly synced with the loop that is waiting for the channel's result. Code is not moved forward until all functions finish their work and posted results are read from the channels.
Waitgroup is only necessary if your function needs to do any work AFTER results are posted to channels.
I also prefer a slightly different implementation. In a posted implementation, we are not sending both results and errors into the channels every time when the function is executed. Instead, we can send only the result for successful execution and send only an error when the code fails.
The advantage is simplified results/errors processing. We are getting slices of results and errors without nils.
In this example, the function returns a number, and we send its default value of 0 in case of error. It could be complicated to filter out not successful execution results from successful if zero could be legit function execution result.
Same with errors. To check if we have any errors, we can use simple code like if len(errs) != 0.
package main
import (
"fmt"
)
func processBatch(num int, errChan chan<- error, resultChan chan<- int) {
if num == 3 {
// no need to send result when it is meanenless
// resultChan <- 0
errChan <- fmt.Errorf("goroutine %d's error returned", num)
} else {
square := num * num
resultChan <- square
// no need to send errror when it is nil
// errChan <- nil
}
}
func main() {
batches := [5]int{1, 2, 3, 4, 5}
resultChan := make(chan int)
errChan := make(chan error)
for i := range batches {
go processBatch(batches[i], errChan, resultChan)
}
// use slices instead of arrays because legth varry now
var results []int
var errs []error
// every time function executes it sends singe piece of data to one of two channels
for range batches {
select {
case res := <-resultChan:
results = append(results, res)
case err := <-errChan:
errs = append(errs, err)
}
}
close(resultChan)
close(errChan)
fmt.Println(results)
fmt.Println(errs)
}
https://go.dev/play/p/SYmfl8iGxgD
[25 1 16 4]
[goroutine 3's error returned]
If you can use external packages, we can get benefits from some multierr package. For example, github.com/hashicorp/go-multierror.

Do goroutines with receiving channel as parameter stop, when the channel is closed?

I have been reading "Building microservices with go" and the book introduces apache/go-resiliency/deadline package for handling timeouts.
deadline.go
// Package deadline implements the deadline (also known as "timeout") resiliency pattern for Go.
package deadline
import (
"errors"
"time"
)
// ErrTimedOut is the error returned from Run when the deadline expires.
var ErrTimedOut = errors.New("timed out waiting for function to finish")
// Deadline implements the deadline/timeout resiliency pattern.
type Deadline struct {
timeout time.Duration
}
// New constructs a new Deadline with the given timeout.
func New(timeout time.Duration) *Deadline {
return &Deadline{
timeout: timeout,
}
}
// Run runs the given function, passing it a stopper channel. If the deadline passes before
// the function finishes executing, Run returns ErrTimeOut to the caller and closes the stopper
// channel so that the work function can attempt to exit gracefully. It does not (and cannot)
// simply kill the running function, so if it doesn't respect the stopper channel then it may
// keep running after the deadline passes. If the function finishes before the deadline, then
// the return value of the function is returned from Run.
func (d *Deadline) Run(work func(<-chan struct{}) error) error {
result := make(chan error)
stopper := make(chan struct{})
go func() {
result <- work(stopper)
}()
select {
case ret := <-result:
return ret
case <-time.After(d.timeout):
close(stopper)
return ErrTimedOut
}
}
deadline_test.go
package deadline
import (
"errors"
"testing"
"time"
)
func takesFiveMillis(stopper <-chan struct{}) error {
time.Sleep(5 * time.Millisecond)
return nil
}
func takesTwentyMillis(stopper <-chan struct{}) error {
time.Sleep(20 * time.Millisecond)
return nil
}
func returnsError(stopper <-chan struct{}) error {
return errors.New("foo")
}
func TestDeadline(t *testing.T) {
dl := New(10 * time.Millisecond)
if err := dl.Run(takesFiveMillis); err != nil {
t.Error(err)
}
if err := dl.Run(takesTwentyMillis); err != ErrTimedOut {
t.Error(err)
}
if err := dl.Run(returnsError); err.Error() != "foo" {
t.Error(err)
}
done := make(chan struct{})
err := dl.Run(func(stopper <-chan struct{}) error {
<-stopper
close(done)
return nil
})
if err != ErrTimedOut {
t.Error(err)
}
<-done
}
func ExampleDeadline() {
dl := New(1 * time.Second)
err := dl.Run(func(stopper <-chan struct{}) error {
// do something possibly slow
// check stopper function and give up if timed out
return nil
})
switch err {
case ErrTimedOut:
// execution took too long, oops
default:
// some other error
}
}
1st question
// in deadline_test.go
if err := dl.Run(takesTwentyMillis); err != ErrTimedOut {
t.Error(err)
}
I have problem understanding the execution flow of above code. As far as I understand, because the takesTwentyMillis function sleeps longer than the set timeout duration of 10 milliseconds,
// in deadline.go
case <-time.After(d.timeout):
close(stopper)
return ErrTimedOut
time.After emits current time, and this case is selected. Then the stopper channel is closed and ErrTimeout is returned.
What I do not understand is, what closing the stopper channel does to the anonymous goroutine that might still be running
I think, when the stopper channel is closed, the below goroutine might still be running.
go func() {
result <- work(stopper)
}()
(Please correct me if I'm wrong here) I think after close(stopper), this goroutine will call takesTwentyMillis(=work function) with stopper channel as its parameter. And the function will proceed and sleep for 20 milliseconds and return nil to pass to result channel. And the main() ends here, right?
I do not see what is the point of closing the stopper channel here. The takesTwentyMillis function does not seem to use the channel within the function body anyway :(.
2nd question
// in deadline_test.go within TestDeadline()
done := make(chan struct{})
err := dl.Run(func(stopper <-chan struct{}) error {
<-stopper
close(done)
return nil
})
if err != ErrTimedOut {
t.Error(err)
}
<-done
This is the part I do not understand completely. I think when dl.Run is run, stopper channel is initialized. But because there is no value in the stopper channel, the function call will be blocked at <-stopper...but because I do not understand this code, I do not see why this code exists in the first place (i.e. what this code is trying to test, and how it is executed, etc).
3rd(additional) question regarding the 2nd question
So I understand that when Run function in the 2nd question triggers the stopper channel to close, the worker function gets the signal. And the worker closes the done channel and returns nil.
I used delve(=go debugger) to see this, and the gdb takes me to the goroutine in deadline.go after the line return nil.
err := dl.Run(func(stopper <-chan struct{}) error {
<-stopper
close(done)
--> return nil
})
After typing n for stepping over to the next line, delve takes me here
go func() {
--> result <- work(stopper)
}()
And the process kind of finishes here because when I type n again the command line prompts PASS and process exits. Why does the process finishes here? The work(stopper) seems to return nil, which should then be passed to result channel right? But this line does not seem to execute for some reason.
I know the main goroutine, which is the Run function, has already returned ErrTimedOut. So I guess it has something to do with this?
1st question
The use of the stopper channel is to signal the function e.g. takesTwentyMillis that it's deadline is reached and the caller no longer cares about its result. Usually this means that the worker function like takesTwentyMillis should check if the stopper channel is already closed so that it may cancel it's work. Still, checking for the stopper channel is the worker function's choice. It may or may not check the channel.
func takesTwentyMillis(stopper <-chan struct{}) error {
for i := 0; i < 20; i++ {
select {
case <-stopper:
// caller doesn't care anymore might as well stop working
return nil
case <-time.After(time.Second): // simulating work
}
}
// work is done
return nil
}
2nd question
This part of Deadline.Run() will close the stopper channel.
case <-time.After(d.timeout):
close(stopper)
Reading on a closed channel (<-stopper) will return a zero value for that channel immediately. I think it's just testing for a worker function that ultimately times-out.

Golang, proper way to restart a routine that panicked

I have the following sample code. I want to maintain 4 goroutines running at all times. They have the possibility of panicking. In the case of the panic, I have a recover where I restart the goroutine.
The way I implemented works but I am not sure whether its the correct and proper way to do this. Any thoughts
package main
import (
"fmt"
"time"
)
var gVar string
var pCount int
func pinger(c chan int) {
for i := 0; ; i++ {
fmt.Println("adding ", i)
c <- i
}
}
func printer(id int, c chan int) {
defer func() {
if err := recover(); err != nil {
fmt.Println("HERE", id)
fmt.Println(err)
pCount++
if pCount == 5 {
panic("TOO MANY PANICS")
} else {
go printer(id, c)
}
}
}()
for {
msg := <-c
fmt.Println(id, "- ping", msg, gVar)
if msg%5 == 0 {
panic("PANIC")
}
time.Sleep(time.Second * 1)
}
}
func main() {
var c chan int = make(chan int, 2)
gVar = "Preflight"
pCount = 0
go pinger(c)
go printer(1, c)
go printer(2, c)
go printer(3, c)
go printer(4, c)
var input string
fmt.Scanln(&input)
}
You can extract the recover logic in a function such as:
func recoverer(maxPanics, id int, f func()) {
defer func() {
if err := recover(); err != nil {
fmt.Println("HERE", id)
fmt.Println(err)
if maxPanics == 0 {
panic("TOO MANY PANICS")
} else {
go recoverer(maxPanics-1, id, f)
}
}
}()
f()
}
And then use it like:
go recoverer(5, 1, func() { printer(1, c) })
Like Zan Lynx's answer, I'd like to share another way to do it (although it's pretty much similar to OP's way.) I used an additional buffered channel ch. When a goroutine panics, the recovery function inside the goroutine send its identity i to ch. In for loop at the bottom of main(), it detects which goroutine's in panic and whether to restart by receiving values from ch.
Run in Go Playground
package main
import (
"fmt"
"time"
)
func main() {
var pCount int
ch := make(chan int, 5)
f := func(i int) {
defer func() {
if err := recover(); err != nil {
ch <- i
}
}()
fmt.Printf("goroutine f(%v) started\n", i)
time.Sleep(1000 * time.Millisecond)
panic("goroutine in panic")
}
go f(1)
go f(2)
go f(3)
go f(4)
for {
i := <-ch
pCount++
if pCount >= 5 {
fmt.Println("Too many panics")
break
}
fmt.Printf("Detected goroutine f(%v) panic, will restart\n", i)
f(i)
}
}
Oh, I am not saying that the following is more correct than your way. It is just another way to do it.
Create another function, call it printerRecover or something like it, and do your defer / recover in there. Then in printer just loop on calling printerRecover. Add in function return values to check if you need the goroutine to exit for some reason.
The way you implemented is correct. Just for me the approach to maintain exactly 4 routines running at all times looks not much go_way, either handling routine's ID, either spawning in defer which may leads unpredictable stack due to closure. I don't think you can efficiently balance resource this way. Why don't you like to simple spawn worker when it needed
func main() {
...
go func(tasks chan int){ //multiplexer
for {
task = <-tasks //when needed
go printer(task) //just spawns handler
}
}(ch)
...
}
and let runtime do its job? This way things are done in stdlib listeners/servers and them known to be efficient enough. goroutines are very lightweight to spawn and runtime is quite smart to balance load. Sure you must to recover either way. It is my very personal opinion.

Idiomatic goroutine termination and error handling

I have a simple concurrency use case in go, and I cannot figure out an elegant solution to my problem.
I want to write a method fetchAll that queries an unspecified number of resources from remote servers in parallel. If any of the fetches fails, I want to return that first error immediately.
My initial implementation leaks goroutines:
package main
import (
"fmt"
"math/rand"
"sync"
"time"
)
func fetchAll() error {
wg := sync.WaitGroup{}
errs := make(chan error)
leaks := make(map[int]struct{})
defer fmt.Println("these goroutines leaked:", leaks)
// run all the http requests in parallel
for i := 0; i < 4; i++ {
leaks[i] = struct{}{}
wg.Add(1)
go func(i int) {
defer wg.Done()
defer delete(leaks, i)
// pretend this does an http request and returns an error
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
errs <- fmt.Errorf("goroutine %d's error returned", i)
}(i)
}
// wait until all the fetches are done and close the error
// channel so the loop below terminates
go func() {
wg.Wait()
close(errs)
}()
// return the first error
for err := range errs {
if err != nil {
return err
}
}
return nil
}
func main() {
fmt.Println(fetchAll())
}
Playground: https://play.golang.org/p/Be93J514R5
I know from reading https://blog.golang.org/pipelines that I can create a signal channel to cleanup the other threads. Alternatively, I could probably use context to accomplish it. But it seems like such a simple use case should have a simpler solution that I'm missing.
Using Error Group makes this even simpler. This automatically waits for all the supplied Go Routines to complete successfully, or cancels all those remaining in the case of any one routine returning an error (in which case that error is the one bubble back up to the caller).
package main
import (
"context"
"fmt"
"math/rand"
"time"
"golang.org/x/sync/errgroup"
)
func fetchAll(ctx context.Context) error {
errs, ctx := errgroup.WithContext(ctx)
// run all the http requests in parallel
for i := 0; i < 4; i++ {
errs.Go(func() error {
// pretend this does an http request and returns an error
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
return fmt.Errorf("error in go routine, bailing")
})
}
// Wait for completion and return the first error (if any)
return errs.Wait()
}
func main() {
fmt.Println(fetchAll(context.Background()))
}
All but one of your goroutines are leaked, because they're still waiting to send to the errs channel - you never finish the for-range that empties it. You're also leaking the goroutine who's job is to close the errs channel, because the waitgroup is never finished.
(Also, as Andy pointed out, deleting from map is not thread-safe, so that'd need protection from a mutex.)
However, I don't think maps, mutexes, waitgroups, contexts etc. are even necessary here. I'd rewrite the whole thing to just use basic channel operations, something like the following:
package main
import (
"fmt"
"math/rand"
"time"
)
func fetchAll() error {
var N = 4
quit := make(chan bool)
errc := make(chan error)
done := make(chan error)
for i := 0; i < N; i++ {
go func(i int) {
// dummy fetch
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
err := error(nil)
if rand.Intn(2) == 0 {
err = fmt.Errorf("goroutine %d's error returned", i)
}
ch := done // we'll send to done if nil error and to errc otherwise
if err != nil {
ch = errc
}
select {
case ch <- err:
return
case <-quit:
return
}
}(i)
}
count := 0
for {
select {
case err := <-errc:
close(quit)
return err
case <-done:
count++
if count == N {
return nil // got all N signals, so there was no error
}
}
}
}
func main() {
rand.Seed(time.Now().UnixNano())
fmt.Println(fetchAll())
}
Playground link: https://play.golang.org/p/mxGhSYYkOb
EDIT: There indeed was a silly mistake, thanks for pointing it out. I fixed the code above (I think...). I also added some randomness for added Realismâ„¢.
Also, I'd like to stress that there really are multiple ways to approach this problem, and my solution is but one way. Ultimately it comes down to personal taste, but in general, you want to strive towards "idiomatic" code - and towards a style that feels natural and easy to understand for you.
Here's a more complete example using errgroup suggested by joth. It shows processing successful data, and will exit on the first error.
https://play.golang.org/p/rU1v-Mp2ijo
package main
import (
"context"
"fmt"
"golang.org/x/sync/errgroup"
"math/rand"
"time"
)
func fetchAll() error {
g, ctx := errgroup.WithContext(context.Background())
results := make(chan int)
for i := 0; i < 4; i++ {
current := i
g.Go(func() error {
// Simulate delay with random errors.
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
if rand.Intn(2) == 0 {
return fmt.Errorf("goroutine %d's error returned", current)
}
// Pass processed data to channel, or receive a context completion.
select {
case results <- current:
return nil
// Close out if another error occurs.
case <-ctx.Done():
return ctx.Err()
}
})
}
// Elegant way to close out the channel when the first error occurs or
// when processing is successful.
go func() {
g.Wait()
close(results)
}()
for result := range results {
fmt.Println("processed", result)
}
// Wait for all fetches to complete.
return g.Wait()
}
func main() {
fmt.Println(fetchAll())
}
As long as each goroutine completes, you won't leak anything. You should create the error channel as buffered with the buffer size equal to the number of goroutines so that the send operations on the channel won't block. Each goroutine should always send something on the channel when it finishes, whether it succeeds or fails. The loop at the bottom can then just iterate for the number of goroutines and return if it gets a non-nil error. You don't need the WaitGroup or the other goroutine that closes the channel.
I think the reason it appears that goroutines are leaking is that you return when you get the first error, so some of them are still running.
By the way, maps are not goroutine safe. If you share a map among goroutines and some of them are making changes to the map, you need to protect it with a mutex.
This answer includes the ability to get the responses back into doneData -
package main
import (
"fmt"
"math/rand"
"os"
"strconv"
)
var doneData []string // responses
func fetchAll(n int, doneCh chan bool, errCh chan error) {
partialDoneCh := make(chan string)
for i := 0; i < n; i++ {
go func(i int) {
if r := rand.Intn(100); r != 0 && r%10 == 0 {
// simulate an error
errCh <- fmt.Errorf("e33or for reqno=" + strconv.Itoa(r))
} else {
partialDoneCh <- strconv.Itoa(i)
}
}(i)
}
// mutation of doneData
for d := range partialDoneCh {
doneData = append(doneData, d)
if len(doneData) == n {
close(partialDoneCh)
doneCh <- true
}
}
}
func main() {
// rand.Seed(1)
var n int
var e error
if len(os.Args) > 1 {
if n, e = strconv.Atoi(os.Args[1]); e != nil {
panic(e)
}
} else {
n = 5
}
doneCh := make(chan bool)
errCh := make(chan error)
go fetchAll(n, doneCh, errCh)
fmt.Println("main: end")
select {
case <-doneCh:
fmt.Println("success:", doneData)
case e := <-errCh:
fmt.Println("failure:", e, doneData)
}
}
Execute using go run filename.go 50 where N=50 i.e amount of parallelism

Passing parameters to function closure

I'm trying to understand the difference in Go between creating an anonymous function which takes a parameter, versus having that function act as a closure. Here is an example of the difference.
With parameter:
func main() {
done := make(chan bool, 1)
go func(c chan bool) {
time.Sleep(50 * time.Millisecond)
c <- true
}(done)
<-done
}
As closure:
func main() {
done := make(chan bool, 1)
go func() {
time.Sleep(50 * time.Millisecond)
done <- true
}()
<-done
}
My question is, when is the first form better than the second? Would you ever use a parameter for this kind of thing? The only time I can see the first form being useful is when returning a func(x, y) from another function.
The difference between using a closure vs using a function parameter has to do with sharing the same variable vs getting a copy of the value. Consider these two examples below.
In the Closure all function calls will use the value stored in i. This value will most likely already reach 3 before any of the goroutines has had time to print it's value.
In the Parameter example each function call will get passed a copy of the value of i when the call was made, thus giving us the result we more likely wanted:
Closure:
for i := 0; i < 3; i++ {
go func() {
fmt.Println(i)
}()
}
Result:
3
3
3
Parameter:
for i := 0; i < 3; i++ {
go func(v int) {
fmt.Println(v)
}(i)
}
Result:
0
1
2
Playground: http://play.golang.org/p/T5rHrIKrQv
When to use parameters
Definitely the first form is preferred if you plan to change the value of the variable which you don't want to observe in the function.
This is the typical case when the anonymous function is inside a for loop and you intend to use the loop's variables, for example:
for i := 0; i < 10; i++ {
go func(i int) {
fmt.Println(i)
}(i)
}
Without passing the variable i you might observe printing 10 ten times. With passing i, you will observe numbers printed from 0 to 9.
When not to use parameters
If you don't want to change the value of the variable, it is cheaper not to pass it and thus not create another copy of it. This is especially true for large structs. Although if you later alter the code and modify the variable, you may easily forget to check its effect on the closure and get unexpected results.
Also there might be cases when you do want to observe changes made to "outer" variables, such as:
func GetRes(name string) (Res, error) {
res, err := somepack.OpenRes(name)
if err != nil {
return nil, err
}
closeres := true
defer func() {
if closeres {
res.Close()
}
}()
// Do other stuff
if err = otherStuff(); err != nil {
return nil, err // res will be closed
}
// Everything went well, return res, but
// res must not be closed, it will be the responsibility of the caller
closeres = false
return res, nil // res will not be closed
}
In this case the GetRes() is to open some resource. But before returning it other things have to be done which might also fail. If those fail, res must be closed and not returned. If everything goes well, res must not be closed and returned.
This is a example of parameter from net/Listen
package main
import (
"io"
"log"
"net"
)
func main() {
// Listen on TCP port 2000 on all available unicast and
// anycast IP addresses of the local system.
l, err := net.Listen("tcp", ":2000")
if err != nil {
log.Fatal(err)
}
defer l.Close()
for {
// Wait for a connection.
conn, err := l.Accept()
if err != nil {
log.Fatal(err)
}
// Handle the connection in a new goroutine.
// The loop then returns to accepting, so that
// multiple connections may be served concurrently.
go func(c net.Conn) {
// Echo all incoming data.
io.Copy(c, c)
// Shut down the connection.
c.Close()
}(conn)
}
}

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