Bug#216951: Apt-get update pipelines abusively

[Juliusz Chroboczek <jch@pps.jussieu.fr>]

Package: apt
Version: 0.5.14
Severity: minor

Apt-get update uses HTTP/1.1 pipeling by default, which is pretty
good.  However, it doesn't check whether the server supports
pipelining, which breaks certain proxies (notably WWWOFFLE and
Junkbuster).

In order to reproduce that, run ``nc -l -p 1234'' in one window, and
``http_proxy=http://localhost:1234 apt-get update'' in another.
You'll see a bunch of requests straight away, which is incorrect in
the case of an HTTP/1.0 proxy.

The recommended approach (RFC 2616 8.1.2) is to do a single request
first, and then start pipelining if the proxy or server gave an
HTTP/1.1 (or later) persistent reply.  Whether to pipeline when the
server gave an HTTP/1.0 persistent reply is up to you, old Apache will
work, but old IIS will break.

Please do not disable pipelining altogether, as this would signi-
ficantly slow apt-get down.

Thanks,

                                        Juliusz Chroboczek