Libproxy support?

[Paul van Tilburg <paul luon net>]

Hello all,

We recently inspected the proxy support of OSTree before deploying it to
several corporate environments.  Given that it might warrant some
discussion, we'd thought it might be better to discuss it here first,
before creating a Github ticket/pull request.

Our findings are that OSTree currently has the same level of support as
using a plain http_proxy environment combined with optional basic
authentication.  This means:  no support for automatic proxy discovery
(though a PAC URL), and no support for NTLM authentication.
In our other applications, libsoup use libproxy through GIO's
GProxyResolver to determine the proxy URL for the system.  (We have even
added a module to libproxy to read it from a custom central location,
since there doesn't seem to be one defined for Linux systems.) Also when
using the environment variable GIO_USE_PROXY_RESOLVER=libproxy, OSTree
didn't want to use libproxy configuration, and I couldn't find what
OSTree does differently here.

If libproxy were to work, we're left with two questions:
* where to indicate that proxying is enabled (without setting an URL)?
* where to leave the authentication credentials?

Also, where to indicate that the authentication type is not basic, but
NTLM?

Finally, how to handle this for the upcoming libcurl backend.  I've seen
that there recently some work on it: https://github.com/curl/curl/pull/977,
but it has not been accepted/merged yet.

Kind regards,
Paul

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