[h2load] add support for requests until reconnect via -q

[jdamato-fsly]

Greetings!

This PR should not be merged as is (because it has at least one bug), but highlights a feature I'd love to see in h2load.

I would love to tell h2load to force clients to reconnect to the server after a certain number of requests have been issued. I would like to do this so that I can benchmark TLS handshakes after a certain number of requests have been issued.

This PR attempts to do this only for http1.1 and it creates a bug where the last request is never issued to the server and is always marked as a failure.

In order to test this PR, I created a simple python web server:

#!/usr/bin/env python
import SimpleHTTPServer
import SocketServer

class MyHTTPRequestHandler(SimpleHTTPServer.SimpleHTTPRequestHandler):
    protocol_version = 'HTTP/1.1'

    def end_headers(self):
        self.send_my_headers()
        SimpleHTTPServer.SimpleHTTPRequestHandler.end_headers(self)

    def send_my_headers(self):
    	self.send_header("Cache-Control", "max-age=3800")

if __name__ == '__main__':
    SocketServer.TCPServer.allow_reuse_address = True
    httpd = SocketServer.TCPServer(("", 10000), MyHTTPRequestHandler)
    httpd.serve_forever()

which I ran with strace -tf -etrace=accept python test.py (so that I could more easily track the number of reconnects).

and then issued requests against it, like this:

./src/h2load --h1 -n 8 -c 1 -t 1 -q 4 http://127.0.0.1:10000/

This results in the server showing the following:

11:33:32 accept(3, {sa_family=AF_INET, sin_port=htons(43572), sin_addr=inet_addr("127.0.0.1")}, [16]) = 4
127.0.0.1 - - [21/Jun/2021 11:33:32] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [21/Jun/2021 11:33:32] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [21/Jun/2021 11:33:32] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [21/Jun/2021 11:33:32] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [21/Jun/2021 11:33:32] "GET / HTTP/1.1" 200 -
11:33:32 accept(3, {sa_family=AF_INET, sin_port=htons(43574), sin_addr=inet_addr("127.0.0.1")}, [16]) = 4
127.0.0.1 - - [21/Jun/2021 11:33:32] "GET / HTTP/1.1" 200 -
127.0.0.1 - - [21/Jun/2021 11:33:32] "GET / HTTP/1.1" 200 -

(This is off by 1 because the requests are counted in the msg_complete cb, it would probably be better to count them in submit_request or similar)

and h2load reports:

requests: 8 total, 8 started, 7 done, 7 succeeded, 1 failed, 1 errored, 0 timeout
status codes: 7 2xx, 0 3xx, 0 4xx, 0 5xx
traffic: 1.67KB (1709) total, 1.17KB (1197) headers (space savings 0.00%), 476B (476) data

As you can see, the 8th request is never issued and is always marked as a failure -- as such, this PR should not be merged.

Is this a feature that h2load would potentially benefit from? If so, perhaps I can get this into a better state fit for merging. Any suggestions on this approach are appreciated 😄