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Understanding how cancellation works
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| package main | |
| import ( | |
| "context" | |
| "fmt" | |
| "sync" | |
| "time" | |
| ) | |
| // simply runs a program to showcase how to properly leverage | |
| // context cancellations to abort goroutines timely. | |
| func main() { | |
| ctx, cancel := context.WithCancel(context.Background()) | |
| // ALWAYS defer the cancel. Even if you plan on cancelling manually at some | |
| // point, it's safer to defer a cancel because when you invoke a WithX function, | |
| // WithCancel in this case, a new goroutine is spawned in the background to | |
| // propagate the cancellation to children contexts as well as keeping track | |
| // of them. Were you to forget cancelling, that would end up being a leak. | |
| defer cancel() | |
| var wg sync.WaitGroup | |
| // this goroutine thinks it's listening to canceled context, but it isn't. | |
| wg.Add(1) | |
| go func() { | |
| defer wg.Done() | |
| fmt.Println("starting work 1!") | |
| // at the time this line is reached, the context hasn't been cancelled yet, | |
| // so the program continues and doesn't abort. | |
| if ctx.Err() != nil { | |
| fmt.Printf("aborting work 1: %s!\n", ctx.Err().Error()) | |
| return | |
| } | |
| time.Sleep(time.Millisecond * 5) | |
| fmt.Println("done work 1!") | |
| }() | |
| // this goroutine actually is listening to cancelled contexts and will abort timely. | |
| wg.Add(1) | |
| go func() { | |
| defer wg.Done() | |
| fmt.Println("starting work 2!") | |
| select { | |
| case <-ctx.Done(): | |
| fmt.Printf("aborting work 2: %s!\n", ctx.Err().Error()) | |
| return | |
| case <-time.After(time.Millisecond * 5): | |
| fmt.Println("done work 2!") | |
| return | |
| } | |
| }() | |
| // cancel emits an empty struct through the "done" channel child contexts have, | |
| // so if any code is listening to ctx.Done(), they'll get the message. | |
| time.Sleep(time.Millisecond) | |
| cancel() | |
| wg.Wait() | |
| fmt.Println("finished!") | |
| } |
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