[Telepathy] Requestotron use-cases, part 1 (requests, not including Tubes or FT)

[Simon McVittie]

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Failures and other exceptional cases
- ------------------------------------

_`req34`: failing to send a text message
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Romeo opens a text channel to Juliet to send a message, but Juliet's server
is down and Romeo's server signals failure. (This is mostly applicable to
decentralized protocols like XMPP and SIP.)

Current implementation::

    the message is sent
    SendError (and soon DeliveryReporting) report the failure
    the channel remains open

_`req35`: failing to make a call
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Romeo makes a VoIP call to Juliet, but Juliet's server crashes and failure
is signalled.

Current implementation::

    Juliet is removed from the Group interface, with an error for the reason
    the StreamedMedia channel closes

_`req36`: Cancelling outgoing call
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Juliet starts a VoIP call to Tybalt, but then thinks better of it and
cancels the call before the channel has actually been opened.

Current implementation (NMC 4.x)::

    UI calls mission_control_cancel_channel_request()

    if dispatching of the channel has already begun:
        cancellation succeeds
    else:
        cancellation fails
        the UI is asked to handle the channel

Problems:

* In NMC 4.x the UI cannot distinguish between the channel that it no longer
  cares about and should close/refuse (this use case), and a channel requested
  by another process but handled by it (req5_)
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