counter-example for the duscission in golang/go#16100

discussion_go_issue_16100.go

package main

import (
	"context"
	"log"
	"net/http"
	"sync/atomic"
	"time"
)

var counter int64

func handler(w http.ResponseWriter, r *http.Request) {
	log.Print("Request received.")
	defer log.Print("Request done.")
	ctx := r.Context() // If we generate error consider context.WithCancel
	publisherChan := streamPublisher(ctx)
	resultChan := make(chan []byte)
	go writeResult(resultChan, w)
	for value := range publisherChan {
		select {
		case resultChan <- value:

		case <-r.Context().Done(): // Client has closed the socket.
			log.Print("Request is Done.")
			close(resultChan) // Close the result channel to exit the writer.
			return
		case <-time.After(1 * time.Second): // The socket is not writeable in given timeout, quit
			log.Print("Output channel is not writable for 1 second. Close and exit.")
			close(resultChan) // Close the result channel to exit the writer.
			return
		}
	}
}

func writeResult(resultChan chan []byte, w http.ResponseWriter) {
	atomic.AddInt64(&counter, 1)
	defer atomic.AddInt64(&counter, -1)

	for r := range resultChan {
		log.Printf("writing %s", r)
		w.Write(r)
	}
	log.Print("Write Result is done.")
}

func streamPublisher(ctx context.Context) chan []byte {
	dst := make(chan []byte)

	randomContent := []byte("Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum.")

	go func() { // publisher of sequential numbers
		defer log.Print("Exit publisher routine.")
		for {
			select {
			case <-ctx.Done(): // the request is closed.
				log.Print("publisher detected ctx.Done.")
				close(dst)
				return
			case dst <- randomContent:
				// log.Print("writing")
			}
		}
	}()
	return dst
}

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("/", handler)
	s := &http.Server{
		Addr:        ":8080",
		Handler:     mux,
		ReadTimeout: 10 * time.Second,
		// WriteTimeout:   10 * time.Second,
		MaxHeaderBytes: 1 << 20,
	}

	go func() {
		for ; ; <-time.Tick(2 * time.Second) {
			log.Printf("#writeResult = %d", counter)
		}
	}()

	log.Fatal(s.ListenAndServe())
}

Ok on Linux neither variant works. I guess it will be stuck on the slow consumer until one chunk of data is consumed and the buffers have room for one more.

It is OS dependent behavior.

PS Confirmed. Linux waits until some data gets confirmed and the next write cycles comes around. This is pretty bad.

PSS out of curiosity I will check Windows. I guess this is the underlying implementation of the IO which evidently is not good on Linux. It will be interesting to see http/2 where application layer may handle this

I does not work on Windows.

So it is simply luck I tested on MacOS first.