Add support for pipelined SASL handshakes

[refi64]

sd-bus sends the BEGIN command in at the same time as the AUTH command,
assuming that the authentication will succeed. This ends up confusing
xdg-dbus-proxy, as it assumes that BEGIN is not sent at the same time as
any other messages. As a result, the server's authentication
replies end up interpreted as standard D-Bus messages, failing parsing
and resulting in the client being unable to connect.

This changes the code to keep track of the number of authentication
commands pipelined with BEGIN, then counts the responses from the server
to know when the authentication phase of the connection has actually
completed.

Fixes #21 (finally!)

Signed-off-by: Ryan Gonzalez [email protected]

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META NOTES: This PR is built on top of #20, which is required to be able to move through the pipelined messages in the first place.

[doraskayo]

I tested this MR and it seemed to work well. It allowed me to run an app that relied upon sd-bus for its DBus communication inside the Flatpak sandbox.

It seems like this is fixing a relatively basic functionality which many apps potentially require, but it hasn't gotten any attention yet. Getting this reviewed and merged should be made a priority in my opinion.

[alexlarsson]

This doesn't seem right to me.

During the AUTH_WAITING_FOR_BEGIN we pass through buffers from the client to the bus as is, but keep all the from-bus data around until we get the BEGIN. Then we count how many responses we got before the BEGIN, and let through that many lines of data in the WAITING_FOR_BACKLOG phase. However, we may already have sent some buffers during the first phase, so we will not get that many sent lines.

This really needs to correctly handle lines sent from the client during the WAITING_FOR_BEGIN phase too.