الإلغاء وخطوط المعالجة المحدودة

package main

import (
	"context"
	"fmt"
)

func produce(ctx context.Context) (<-chan int, <-chan struct{}) {
	output := make(chan int)
	done := make(chan struct{})
	go func() {
		defer close(done)
		defer close(output)
		select {
		case output <- 7:
		case <-ctx.Done():
			return
		}
	}()
	return output, done
}

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	cancel()
	output, done := produce(ctx)
	<-done
	_, ok := <-output
	fmt.Println("closed without value", !ok)
}

closed without value true

لا يتاح مستقبل قبل done، فينهي الإلغاء المنتج ويغلق الخرج.

package main

import (
	"context"
	"fmt"
	"sync"
)

func main() {
	ctx, cancel := context.WithCancel(context.Background())
	defer cancel()
	jobs := make(chan int)
	results := make(chan int)
	producerDone := make(chan struct{})
	go func() {
		defer close(producerDone)
		defer close(jobs)
		for n := 1; n <= 4; n++ {
			select {
			case jobs <- n:
			case <-ctx.Done():
				return
			}
		}
	}()
	var workers sync.WaitGroup
	workers.Add(2)
	for i := 0; i < 2; i++ {
		go func() {
			defer workers.Done()
			for n := range jobs {
				select {
				case results <- n * n:
				case <-ctx.Done():
					return
				}
			}
		}()
	}
	coordinatorDone := make(chan struct{})
	go func() { defer close(coordinatorDone); workers.Wait(); close(results) }()
	total := 0
	for result := range results {
		total += result
	}
	<-producerDone
	<-coordinatorDone
	fmt.Println("squares", total)
}

squares 30

تجمع المربعات الأربعة إلى ثلاثين وتُلاحظ أعمار المنتج والعاملين والمنسق.