context cancel does not exit

Issue

Expected: To be done after approx. 2 seconds

Actual: Runs indefinitely.

Don’t understand what could be causing it to run indefinitely.

package main

import (
    "context"
    "fmt"
    "time"
)

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    for i := range generator(ctx) {
        select {
        case <-time.After(2 * time.Second):
            cancel()
            return
        default:
            fmt.Println(i)
        }
    }
}

func generator(ctx context.Context) <-chan int {
    ch := make(chan int)

    go func() {
        count := 0
        for {
            select {
            case <-ctx.Done():
                return
            case ch <- count:
                count++
            }
        }
    }()

    return ch
}

Solution

The main issue is that your channel returned from generator(ctx) emits values almost as fast as you can read them.

The channel created by time.After(2 * time.Second) is discarded almost immediately, and you create a new timeout channel every iteration through the generator.

If you make one small change; create the timeout channel outside the loop, and then put it in the select clause you’ll see it begin to work.

timeout := time.After(2 * time.Second)
for i := range generator(ctx) {
    select {
    case <-timeout:
        cancel()
        return
    default:
        fmt.Println(i)
    }
}

https://play.golang.org/p/zb3wn5FJuK

Answered By – John Weldon

Answer Checked By – Gilberto Lyons (GoLangFix Admin)

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