Example of context with message passing

context_in_struct.go

package main

import "fmt"
import "golang.org/x/net/context"

// A message processes parameter and returns the result on responseChan.
// ctx is places in a struct, but this is ok to do.
type message struct {
	responseChan chan<- int
	parameter    string
	ctx          context.Context
}

func ProcessMessages(work <-chan message) {
	for job := range work {
		select {
		// If the context is finished, don't bother processing the
		// message.
		case <-job.ctx.Done():
			continue
		default:
		}
		// Assume this takes a long time to calculate
		hardToCalculate := len(job.parameter)
		select {
		case <-job.ctx.Done():
		case job.responseChan <- hardToCalculate:
		}
	}
}

func newRequest(ctx context.Context, input string, q chan<- message) {
	r := make(chan int)
	select {
	// If the context finishes before we can send msg onto q,
	// exit early
	case <-ctx.Done():
		fmt.Println("Context ended before q could see message")
		return
	case q <- message{
		responseChan: r,
		parameter:    input,
		// We are placing a context in a struct.  This is ok since it
		// is only stored as a passed message and we want q to know
		// when it can discard this message
		ctx: ctx,
	}:
	}

	select {
	case out := <-r:
		fmt.Printf("The len of %s is %d\n", input, out)
	// If the context finishes before we could get the result, exit early
	case <-ctx.Done():
		fmt.Println("Context ended before q could process message")
	}
}

func main() {
	q := make(chan message)
	go ProcessMessages(q)
	ctx := context.Background()
	newRequest(ctx, "hi", q)
	newRequest(ctx, "hello", q)
	close(q)
}