Skip Unchanged Downloads with ETag Requests

package main

import (
	"fmt"
	"io"
	"net/http"
	"net/http/httptest"
)

var payload = []byte("configuration-v1")

func fetchConditional(
	client *http.Client,
	url string,
	etag string,
) (body []byte, newETag string, notModified bool, err error) {
	request, err := http.NewRequest(http.MethodGet, url, nil)
	if err != nil {
		return nil, "", false, err
	}
	if etag != "" {
		request.Header.Set("If-None-Match", etag)
	}

	response, err := client.Do(request)
	if err != nil {
		return nil, "", false, err
	}
	defer response.Body.Close()

	if response.StatusCode == http.StatusNotModified {
		return nil, etag, true, nil
	}
	if response.StatusCode != http.StatusOK {
		return nil, "", false, fmt.Errorf("unexpected status %d", response.StatusCode)
	}
	body, err = io.ReadAll(io.LimitReader(response.Body, 1<<20))
	if err != nil {
		return nil, "", false, err
	}
	return body, response.Header.Get("ETag"), false, nil
}

func main() {
	requests := 0
	server := httptest.NewServer(http.HandlerFunc(func(writer http.ResponseWriter, request *http.Request) {
		requests++
		writer.Header().Set("ETag", "\"v1\"")
		if request.Header.Get("If-None-Match") == "\"v1\"" {
			writer.WriteHeader(http.StatusNotModified)
			return
		}
		_, _ = writer.Write(payload)
	}))
	defer server.Close()

	client := server.Client()
	body, etag, notModified, err := fetchConditional(client, server.URL, "")
	if err != nil || notModified || string(body) != string(payload) || etag == "" {
		panic("first request should download the payload")
	}
	fmt.Printf("first: body=%q etag=%s\n", body, etag)

	body, _, notModified, err = fetchConditional(client, server.URL, etag)
	if err != nil || !notModified || len(body) != 0 || requests != 2 {
		panic("second request should return 304 without a body")
	}
	fmt.Printf("second: notModified=%v requests=%d\n", notModified, requests)
}