Stopping running function using context timeout in Golang

Question

I want to utilize context in golang to be used for cancellation when timeout reached.

The code:

package main

import "fmt"
import "time"
import "context"

func F(ctx context.Context) error {
  ctx, cancel := context.WithTimeout(ctx,3*time.Second)
  defer cancel()
  for i:=0;i<10;i++ {
    time.Sleep(1 * time.Second)
    fmt.Println("No: ",i)
  }
  select {
    case <-ctx.Done():
      fmt.Println("TIME OUT")
      cancel()
      return ctx.Err()
    default:
      fmt.Println("ALL DONE")
      return nil
  }
}

func main() {
  ctx := context.Background()
  err := F(ctx)
  if err != nil {
    fmt.Println(err)
  }else {
    fmt.Println("Success")
  }
}

Expectation: code above should stop running the loop at counter 2, because the timeout is 3 second and looping run 1 second each. So I expect someting like this:

No:  0
No:  1
No:  2
TIME OUT
context deadline exceeded

Actual: What actually happen is the loop keep running until finish even though the context meet timeout and the select listener catch that on <-ctx.Done(). This code prints this:

No:  0
No:  1
No:  2
No:  3
No:  4
No:  5
No:  6
No:  7
No:  8
No:  9
TIME OUT
context deadline exceeded

How to stop the function execution after timeout meet?

icza · Accepted Answer · 2021-07-04 07:05:24Z

9

context.Context can only relay the message that timeout or cancellation happened. It does not have the power to actually stop any goroutines (for details, see cancel a blocking operation in Go). The goroutine itself is responsible for checking the timeout and cancellation, and abort early.

You have a loop which unconditionally iterates 10 times and prints something. And you only check the timeout after the loop.

You have to move the context checking into the loop:

func F(ctx context.Context) error {
    ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
    defer cancel()
    for i := 0; i < 10; i++ {
        select {
        case <-ctx.Done():
            fmt.Println("TIME OUT")
            return ctx.Err()
        default:
            time.Sleep(1 * time.Second)
            fmt.Println("No: ", i)
        }
    }
    fmt.Println("ALL DONE")
    return nil
}

With this change, output will be (try it on the Go Playground):

No:  0
No:  1
No:  2
No:  3
TIME OUT
context deadline exceeded

Note: whether you see "No: 3" printed may or may not happen, as any iteration takes 1 second, and timeout is 3 seconds = 3 * iteration delay, so whether timeout happens first or the 4th iteration begins first is "racy". If you decrease the timeout to like 2900 ms, "No: 3" will not be printed.

edited Jul 4, 2021 at 7:05

answered Apr 5, 2020 at 11:57

icza

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4 Comments

colm.anseo Over a year ago

I know the Sleep was probably illustrative in the OP, but if one is in a polling loop that requires a wait, it's better to add a <-time.After(...) to the select block. That way the cancellation will be instant, instead of (worse case) 1 second late.

Jake Muller Over a year ago

Looks like it depends on the compiler/version or the machine where it compiled or running. I tried running it in repl.it using ctx.Done() it prints until No: 2 consistently. When I tried it on golang playground using ctx.Done() it always print No: 3, but if I used time.After it consistently prints until No: 2.

Sif Over a year ago

Is the cancel() call inside the case <-ctx.Done(): block necessary? Won't the defer after the context initialization call it anyways?

icza Over a year ago

@Sif Yes, you're right, it's not needed. I left it there because I copied the select code form the question, but removed it to avoid confusion.

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