Invalid calculation for max header size when headers are sent in at least two chunks

[Skrethel]

Waitress incorrectly calculates header_bytes_received when headers are sent in more than one chunk. Issue is related to following code in parser.py

            if index >= 0:
                # If the headers have ended, and we also have part of the body
                # message in data we still want to validate we aren't going
                # over our limit for received headers.
                self.header_bytes_received += index
                consumed = datalen - (len(s) - index)
            else:
                self.header_bytes_received += datalen
                consumed = datalen

Length of data from chunk first to n - 1 (n is last) is added to header_bytes_received and when finally end of headers is is found in last chunk the header_bytes_received is incremented by size of all headers. This means that length of chunks from 1 to n - 1 is counted twice in header_bytes_received.

I think that if end of headers is found, header_bytes_received should be set to position of end of headers (index).

To reproduce this issue following code can be used.
Server:

import waitress


def application(environ, start_response):
    start_response(
        "200 OK", [("Content-Type", "text/plain")]
    )
    yield b"OK\r\n"


if __name__ == "__main__":
    waitress.serve(
        application,
        max_request_header_size = 200,
    )

Client:

import socket
import time

header_data = b'1' * 100
s = socket.create_connection(('127.0.0.1', 8080), timeout = 60)

to_send = (
    b"GET /long_header HTTP/1.0\r\n"
    b"x-header:  " + header_data + b"\r\n"
)

to_send2 = (
    b"Content-Length: 0\r\n"
    b"\r\n"
)

s.send(to_send)
time.sleep(2)
s.send(to_send2)
print("Headers size: ", len(to_send) + len(to_send2))

with s.makefile('rb', 0) as fp:
    data = fp.read()
    print(data.decode())
    assert data.splitlines()[0] == b"HTTP/1.0 200 OK"

s.close()

Currently this code produces 431 HTTP error when it should work because sent headers are smaller than 200 (161 bytes)

HTTP/1.0 431 Request Header Fields Too Large
Connection: close
Content-Length: 91
Content-Type: text/plain
Date: Tue, 22 Mar 2022 12:01:39 GMT
Server: waitress

Request Header Fields Too Large

exceeds max_header of 200

(generated by waitress)