Chrome中CORS AJAX请求到本地WSGI服务器延迟过高

Question

Chrome中CORS AJAX请求到本地WSGI服务器延迟过高

7

设置

在本地运行两个不同端口的WSGI服务器。一个服务器返回一个包含JavaScript的HTML页面，使用jQuery进行跨域AJAX请求到另一个WSGI服务器。

origin_server.py 在 http://localhost:9010 提供HTML服务。

#!/usr/bin/env python

from wsgiref.simple_server import make_server


def origin_html(environ, start_response):
    status = '200 OK'
    response_headers = [('Content-Type', 'text/html')]
    start_response(status, response_headers)    

    f = open('./index.html', 'rb')
    return [f.read()]

httpd = make_server('localhost', 9010, origin_html)
httpd.serve_forever()

cors_server.py会提供javascript请求的跨域资源。

#!/usr/bin/env python

from wsgiref.simple_server import make_server
import json


def cors_json(environ, start_response):
    status = '200 OK'
    response_headers = [
        ('Content-Type', 'application/json'),
        ('Access-Control-Allow-Origin', '*')
    ]
    start_response(status, response_headers)

    return [json.dumps({'foo': 'bar'})]

httpd = make_server('localhost', 9011, cors_json)
httpd.serve_forever()

index.html是由origin_server.py提供的页面。

<!DOCTYPE html>
<html>
    <head>
        <script
            type="text/javascript"
            src="https://code.jquery.com/jquery-2.1.3.min.js"
            >
        </script>

        <script type="text/javascript">
            $(document).ready(function () { 
                console.log('Doing the thing.');
                $.get('http://localhost:9011').done(function (data) {
                    console.log('Got the thing', data);
                });
            });
        </script>
    </head>
    <body>
        Beep boop.
    </body>
</html>

在github上查看上述代码：https://github.com/thatjpk/local-wsgi-cors-ajax

参考版本信息：

2012年中期MacBook Pro，OS X 10.10
Python 2.7.6
wsgiref 0.1.2
Chrome 40.0.2214.93（为了测试禁用扩展和重置设置）
Firefox 35.0.1

问题

假设我打开两个终端，在一个终端中启动origin_server.py，在另一个终端中启动cors_server.py，然后打开浏览器选项卡并将其指向http://localhost:9010/。这会加载页面并执行跨域请求。

理想情况下，所有延迟都应该非常低，因为它全部是本地的。但是，我始终看到跨域请求的延迟超过十秒钟。奇怪的是，我只在Chrome的非无痕窗口中看到过这种情况。Firefox、curl、httpie和同一台机器上的Chrome无痕标签都可以在子10ms内完成跨域请求，而非无痕Chrome标签需要三个数量级的时间。

如果我从等式中去掉CORS，只需在Chrome地址栏中输入http://localhost:9011，json就会立即加载到视图中。这显然只发生在请求来自javascript时。

如果在Chrome中对cors_server.py进行另一个请求，而第一个请求处于“挂起”状态，则第二个请求将等待第一个请求完成。这意味着第一个请求在执行到cors_server.py处理程序之前会占用wsgi服务器很长时间。

当请求需要很长时间才能完成时，肯定有更多的事情要做。我使用tcpdump -i lo0 dst port 9011 or src port 9011收集了以下信息。

这是一个来自httpie的请求（http get localhost:9011），它可以快速完成。

17:33:57.176028 IP localhost.64723 > localhost.9011: Flags [S], seq 2244357563, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910553864 ecr 0,sackOK,eol], length 0
17:33:57.176089 IP localhost.9011 > localhost.64723: Flags [S.], seq 1494086712, ack 2244357564, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910553864 ecr 910553864,sackOK,eol], length 0
17:33:57.176103 IP localhost.64723 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176113 IP localhost.9011 > localhost.64723: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176145 IP localhost.64723 > localhost.9011: Flags [P.], seq 1:136, ack 1, win 12759, options [nop,nop,TS val 910553864 ecr 910553864], length 135
17:33:57.176158 IP localhost.9011 > localhost.64723: Flags [.], ack 136, win 12755, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176760 IP localhost.9011 > localhost.64723: Flags [P.], seq 1:18, ack 136, win 12755, options [nop,nop,TS val 910553864 ecr 910553864], length 17
17:33:57.176781 IP localhost.64723 > localhost.9011: Flags [.], ack 18, win 12758, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176811 IP localhost.9011 > localhost.64723: Flags [P.], seq 18:55, ack 136, win 12755, options [nop,nop,TS val 910553864 ecr 910553864], length 37
17:33:57.176823 IP localhost.64723 > localhost.9011: Flags [.], ack 55, win 12757, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176834 IP localhost.9011 > localhost.64723: Flags [P.], seq 55:92, ack 136, win 12755, options [nop,nop,TS val 910553864 ecr 910553864], length 37
17:33:57.176844 IP localhost.64723 > localhost.9011: Flags [.], ack 92, win 12756, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176857 IP localhost.9011 > localhost.64723: Flags [P.], seq 92:178, ack 136, win 12755, options [nop,nop,TS val 910553864 ecr 910553864], length 86
17:33:57.176866 IP localhost.64723 > localhost.9011: Flags [.], ack 178, win 12753, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176878 IP localhost.9011 > localhost.64723: Flags [P.], seq 178:192, ack 136, win 12755, options [nop,nop,TS val 910553864 ecr 910553864], length 14
17:33:57.176886 IP localhost.64723 > localhost.9011: Flags [.], ack 192, win 12753, options [nop,nop,TS val 910553864 ecr 910553864], length 0
17:33:57.176961 IP localhost.9011 > localhost.64723: Flags [F.], seq 192, ack 136, win 12755, options [nop,nop,TS val 910553865 ecr 910553864], length 0
17:33:57.176975 IP localhost.64723 > localhost.9011: Flags [.], ack 193, win 12753, options [nop,nop,TS val 910553865 ecr 910553865], length 0
17:33:57.176980 IP localhost.9011 > localhost.64723: Flags [.], ack 136, win 12755, options [nop,nop,TS val 910553865 ecr 910553865], length 0
17:33:57.182034 IP localhost.64723 > localhost.9011: Flags [F.], seq 136, ack 193, win 12753, options [nop,nop,TS val 910553870 ecr 910553865], length 0
17:33:57.182095 IP localhost.9011 > localhost.64723: Flags [.], ack 137, win 12755, options [nop,nop,TS val 910553870 ecr 910553870], length 0

这是 Firefox 发出的一个快速完成的请求。

18:12:57.416741 IP localhost.65089 > localhost.9011: Flags [S], seq 2202457533, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 912890306 ecr 0,sackOK,eol], length 0
18:12:57.416794 IP localhost.9011 > localhost.65089: Flags [S.], seq 1807873905, ack 2202457534, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 912890306 ecr 912890306,sackOK,eol], length 0
18:12:57.416806 IP localhost.65089 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.416818 IP localhost.9011 > localhost.65089: Flags [.], ack 1, win 12759, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.416883 IP localhost.65089 > localhost.9011: Flags [P.], seq 1:346, ack 1, win 12759, options [nop,nop,TS val 912890306 ecr 912890306], length 345
18:12:57.416899 IP localhost.9011 > localhost.65089: Flags [.], ack 346, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417478 IP localhost.9011 > localhost.65089: Flags [P.], seq 1:18, ack 346, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 17
18:12:57.417492 IP localhost.65089 > localhost.9011: Flags [.], ack 18, win 12758, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417516 IP localhost.9011 > localhost.65089: Flags [P.], seq 18:55, ack 346, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 37
18:12:57.417538 IP localhost.65089 > localhost.9011: Flags [.], ack 55, win 12757, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417548 IP localhost.9011 > localhost.65089: Flags [P.], seq 55:92, ack 346, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 37
18:12:57.417555 IP localhost.65089 > localhost.9011: Flags [.], ack 92, win 12756, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417562 IP localhost.9011 > localhost.65089: Flags [P.], seq 92:192, ack 346, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 100
18:12:57.417569 IP localhost.65089 > localhost.9011: Flags [.], ack 192, win 12753, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417618 IP localhost.9011 > localhost.65089: Flags [F.], seq 192, ack 346, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417629 IP localhost.65089 > localhost.9011: Flags [.], ack 193, win 12753, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417634 IP localhost.9011 > localhost.65089: Flags [.], ack 346, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417638 IP localhost.65089 > localhost.9011: Flags [F.], seq 346, ack 193, win 12753, options [nop,nop,TS val 912890306 ecr 912890306], length 0
18:12:57.417647 IP localhost.9011 > localhost.65089: Flags [.], ack 347, win 12748, options [nop,nop,TS val 912890306 ecr 912890306], length 0

这是一个来自Chrome隐身标签页的请求，速度很快。

17:28:10.096905 IP6 localhost.64691 > localhost.9011: Flags [S], seq 2861078667, win 65535, options [mss 16324,nop,wscale 5,nop,nop,TS val 910207638 ecr 0,sackOK,eol], length 0
17:28:10.096926 IP6 localhost.9011 > localhost.64691: Flags [R.], seq 0, ack 2861078668, win 0, length 0
17:28:10.097010 IP localhost.64692 > localhost.9011: Flags [S], seq 3615577823, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910207638 ecr 0,sackOK,eol], length 0
17:28:10.097056 IP localhost.9011 > localhost.64692: Flags [S.], seq 3533709271, ack 3615577824, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910207638 ecr 910207638,sackOK,eol], length 0
17:28:10.097066 IP localhost.64692 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910207638 ecr 910207638], length 0
17:28:10.097075 IP localhost.9011 > localhost.64692: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910207638 ecr 910207638], length 0
17:28:10.097274 IP localhost.64692 > localhost.9011: Flags [P.], seq 1:373, ack 1, win 12759, options [nop,nop,TS val 910207638 ecr 910207638], length 372
17:28:10.097293 IP localhost.9011 > localhost.64692: Flags [.], ack 373, win 12747, options [nop,nop,TS val 910207638 ecr 910207638], length 0
17:28:10.098108 IP localhost.9011 > localhost.64692: Flags [P.], seq 1:18, ack 373, win 12747, options [nop,nop,TS val 910207639 ecr 910207638], length 17
17:28:10.098131 IP localhost.64692 > localhost.9011: Flags [.], ack 18, win 12758, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.098166 IP localhost.9011 > localhost.64692: Flags [P.], seq 18:55, ack 373, win 12747, options [nop,nop,TS val 910207639 ecr 910207639], length 37
17:28:10.098184 IP localhost.64692 > localhost.9011: Flags [.], ack 55, win 12757, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.098212 IP localhost.9011 > localhost.64692: Flags [P.], seq 55:92, ack 373, win 12747, options [nop,nop,TS val 910207639 ecr 910207639], length 37
17:28:10.098232 IP localhost.64692 > localhost.9011: Flags [.], ack 92, win 12756, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.098246 IP localhost.9011 > localhost.64692: Flags [P.], seq 92:178, ack 373, win 12747, options [nop,nop,TS val 910207639 ecr 910207639], length 86
17:28:10.098267 IP localhost.64692 > localhost.9011: Flags [.], ack 178, win 12753, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.098277 IP localhost.9011 > localhost.64692: Flags [P.], seq 178:192, ack 373, win 12747, options [nop,nop,TS val 910207639 ecr 910207639], length 14
17:28:10.098283 IP localhost.64692 > localhost.9011: Flags [.], ack 192, win 12753, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.098352 IP localhost.9011 > localhost.64692: Flags [F.], seq 192, ack 373, win 12747, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.098369 IP localhost.64692 > localhost.9011: Flags [.], ack 193, win 12753, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.098380 IP localhost.9011 > localhost.64692: Flags [.], ack 373, win 12747, options [nop,nop,TS val 910207639 ecr 910207639], length 0
17:28:10.099183 IP localhost.64692 > localhost.9011: Flags [F.], seq 373, ack 193, win 12753, options [nop,nop,TS val 910207640 ecr 910207639], length 0
17:28:10.099217 IP localhost.9011 > localhost.64692: Flags [.], ack 374, win 12747, options [nop,nop,TS val 910207640 ecr 910207640], length 0

这是一个来自常规Chrome标签页的请求，耗时约18秒。

17:28:27.147218 IP6 localhost.64699 > localhost.9011: Flags [S], seq 1481186887, win 65535, options [mss 16324,nop,wscale 5,nop,nop,TS val 910224643 ecr 0,sackOK,eol], length 0
17:28:27.147230 IP6 localhost.9011 > localhost.64699: Flags [R.], seq 0, ack 1481186888, win 0, length 0
17:28:27.147277 IP6 localhost.64700 > localhost.9011: Flags [S], seq 1928824698, win 65535, options [mss 16324,nop,wscale 5,nop,nop,TS val 910224643 ecr 0,sackOK,eol], length 0
17:28:27.147287 IP6 localhost.9011 > localhost.64700: Flags [R.], seq 0, ack 1928824699, win 0, length 0
17:28:27.147331 IP6 localhost.64701 > localhost.9011: Flags [S], seq 63463797, win 65535, options [mss 16324,nop,wscale 5,nop,nop,TS val 910224643 ecr 0,sackOK,eol], length 0
17:28:27.147340 IP6 localhost.9011 > localhost.64701: Flags [R.], seq 0, ack 63463798, win 0, length 0
17:28:27.147578 IP localhost.64704 > localhost.9011: Flags [S], seq 2185396531, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224643 ecr 0,sackOK,eol], length 0
17:28:27.147602 IP localhost.64705 > localhost.9011: Flags [S], seq 2307844059, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224643 ecr 0,sackOK,eol], length 0
17:28:27.147617 IP localhost.9011 > localhost.64704: Flags [S.], seq 4199967470, ack 2185396532, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224643 ecr 910224643,sackOK,eol], length 0
17:28:27.147627 IP localhost.64706 > localhost.9011: Flags [S], seq 324888549, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224643 ecr 0,sackOK,eol], length 0
17:28:27.147638 IP localhost.9011 > localhost.64705: Flags [S.], seq 1097272416, ack 2307844060, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224643 ecr 910224643,sackOK,eol], length 0
17:28:27.147643 IP localhost.64704 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224643 ecr 910224643], length 0
17:28:27.147665 IP localhost.9011 > localhost.64706: Flags [S.], seq 837656804, ack 324888550, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224643 ecr 910224643,sackOK,eol], length 0
17:28:27.147669 IP localhost.64705 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224643 ecr 910224643], length 0
17:28:27.147675 IP localhost.9011 > localhost.64704: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224643 ecr 910224643], length 0
17:28:27.147681 IP localhost.64706 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224643 ecr 910224643], length 0
17:28:27.147686 IP localhost.9011 > localhost.64705: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224643 ecr 910224643], length 0
17:28:27.147691 IP localhost.9011 > localhost.64706: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224643 ecr 910224643], length 0
17:28:27.243730 IP6 localhost.64708 > localhost.9011: Flags [S], seq 4213763930, win 65535, options [mss 16324,nop,wscale 5,nop,nop,TS val 910224733 ecr 0,sackOK,eol], length 0
17:28:27.243748 IP6 localhost.9011 > localhost.64708: Flags [R.], seq 0, ack 4213763931, win 0, length 0
17:28:27.243855 IP localhost.64709 > localhost.9011: Flags [S], seq 1721904940, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224733 ecr 0,sackOK,eol], length 0
17:28:27.243910 IP localhost.9011 > localhost.64709: Flags [S.], seq 2521348026, ack 1721904941, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 910224733 ecr 910224733,sackOK,eol], length 0
17:28:27.243925 IP localhost.64709 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224733 ecr 910224733], length 0
17:28:27.243934 IP localhost.9011 > localhost.64709: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910224733 ecr 910224733], length 0
17:28:27.244093 IP localhost.64709 > localhost.9011: Flags [P.], seq 1:390, ack 1, win 12759, options [nop,nop,TS val 910224733 ecr 910224733], length 389
17:28:27.244108 IP localhost.9011 > localhost.64709: Flags [.], ack 390, win 12747, options [nop,nop,TS val 910224733 ecr 910224733], length 0
17:28:45.693569 IP localhost.64704 > localhost.9011: Flags [F.], seq 1, ack 1, win 12759, options [nop,nop,TS val 910243145 ecr 910224643], length 0
17:28:45.693596 IP localhost.64705 > localhost.9011: Flags [F.], seq 1, ack 1, win 12759, options [nop,nop,TS val 910243145 ecr 910224643], length 0
17:28:45.693605 IP localhost.64706 > localhost.9011: Flags [F.], seq 1, ack 1, win 12759, options [nop,nop,TS val 910243145 ecr 910224643], length 0
17:28:45.693627 IP localhost.9011 > localhost.64704: Flags [.], ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693636 IP localhost.9011 > localhost.64705: Flags [.], ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693645 IP localhost.9011 > localhost.64706: Flags [.], ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693655 IP localhost.64704 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693661 IP localhost.64705 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693666 IP localhost.64706 > localhost.9011: Flags [.], ack 1, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693737 IP localhost.9011 > localhost.64704: Flags [F.], seq 1, ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693766 IP localhost.64704 > localhost.9011: Flags [.], ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693862 IP localhost.9011 > localhost.64705: Flags [F.], seq 1, ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693909 IP localhost.64705 > localhost.9011: Flags [.], ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693935 IP localhost.9011 > localhost.64706: Flags [F.], seq 1, ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.693949 IP localhost.64706 > localhost.9011: Flags [.], ack 2, win 12759, options [nop,nop,TS val 910243145 ecr 910243145], length 0
17:28:45.694861 IP localhost.9011 > localhost.64709: Flags [P.], seq 1:18, ack 390, win 12747, options [nop,nop,TS val 910243146 ecr 910224733], length 17
17:28:45.694886 IP localhost.64709 > localhost.9011: Flags [.], ack 18, win 12758, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.694937 IP localhost.9011 > localhost.64709: Flags [P.], seq 18:55, ack 390, win 12747, options [nop,nop,TS val 910243146 ecr 910243146], length 37
17:28:45.694958 IP localhost.64709 > localhost.9011: Flags [.], ack 55, win 12757, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.694971 IP localhost.9011 > localhost.64709: Flags [P.], seq 55:92, ack 390, win 12747, options [nop,nop,TS val 910243146 ecr 910243146], length 37
17:28:45.694982 IP localhost.64709 > localhost.9011: Flags [.], ack 92, win 12756, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.694997 IP localhost.9011 > localhost.64709: Flags [P.], seq 92:178, ack 390, win 12747, options [nop,nop,TS val 910243146 ecr 910243146], length 86
17:28:45.695009 IP localhost.64709 > localhost.9011: Flags [.], ack 178, win 12753, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.695016 IP localhost.9011 > localhost.64709: Flags [P.], seq 178:192, ack 390, win 12747, options [nop,nop,TS val 910243146 ecr 910243146], length 14
17:28:45.695020 IP localhost.64709 > localhost.9011: Flags [.], ack 192, win 12753, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.695087 IP localhost.9011 > localhost.64709: Flags [F.], seq 192, ack 390, win 12747, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.695111 IP localhost.64709 > localhost.9011: Flags [.], ack 193, win 12753, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.695120 IP localhost.9011 > localhost.64709: Flags [.], ack 390, win 12747, options [nop,nop,TS val 910243146 ecr 910243146], length 0
17:28:45.696046 IP localhost.64709 > localhost.9011: Flags [F.], seq 390, ack 193, win 12753, options [nop,nop,TS val 910243147 ecr 910243146], length 0
17:28:45.696069 IP localhost.9011 > localhost.64709: Flags [.], ack 391, win 12747, options [nop,nop,TS val 910243147 ecr 910243147], length 0

看起来Chrome首先尝试IPv6，这是wsgiref不喜欢的。因此，服务器发送的连接重置是有意义的。

在非隐身模式Chrome的情况下，客户端似乎会进行多次连接尝试。服务器会重置前几次尝试，然后接受一些尝试，然后仅使用其中一个连接，客户端才会实际发送请求。我假设由于wsgiref.simple_server是单线程的，前面的连接必须超时，才能处理将进行实际请求的连接；因此会出现延迟。

有趣的是，即使在隐身模式下，Chrome也仍然会遇到重置，但只重试一次，然后按预期工作。Firefox和httpie都只建立一个平稳工作的连接。

问题

- 为什么Chrome要为一个请求打开这么多连接？ - Chrome的隐身模式有什么特别之处，导致它在这里表现得更好吗？ - 这是……一个错误吗？（无论是Chrome还是wsgiref？）

- jpk

2个回答

0

我相信你也在从服务器读取元数据。检查一下你所发出的请求的日志。

- Ashik Basheer

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- jpk · Accepted Answer

简而言之，Chrome进行了过度优化。这是因为Chrome采用了一种优化方法来加速页面加载和API交互。Chrome有一个名为“预测网络操作以提高页面加载性能”的设置（在高级设置的隐私中）。启用此设置后，即使客户端代码尚未达到执行点以告诉浏览器发出HTTP请求，Chrome也会预先打开TCP连接到托管页面资源的服务器。在这种情况下，Chrome看到我将向CORS服务器发出AJAX请求，并准备好多次向该服务器打开TCP连接以便于以后可能发出的其他HTTP请求（即使我的Javascript只会向服务器发出一个请求）。它显然会在实际执行Javascript发出一个HTTP请求之前打开这些额外的连接。这意味着对于单线程服务器，在服务请求的连接之前必须等待那些额外的连接超时。这就是延迟的原因。

“预测网络操作以提高页面加载性能”设置默认启用，并且在隐身模式下无效（可能会向实际上不会收到请求的服务器泄露用户行为）。禁用它可以防止打开这些额外的连接，并且在这种情况下，Chrome的行为与Firefox相同。

这让我想知道Chrome在全球生产服务器上打开了多少TCP连接，这些连接要么没有使用，要么在发送请求之前保持了过长的时间。未使用的连接甚至不会出现在访问日志中，因为没有发出HTTP请求，因此很难评估Chrome用户创建了多少额外负载。在我看来，这种行为是有害的。首先，因为它试图提高客户端性能（这是值得钦佩的），但是却以服务器运营者为代价（这是不好的）。其次，因为（显然被禁用于隐身模式下）具有负面的隐私影响。