[Bug 70013] New: TpDBusPropertiesMixin should support either async set, or overriding

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=70013

          Priority: medium
            Bug ID: 70013
          Assignee: telepathy-bugs at lists.freedesktop.org
           Summary: TpDBusPropertiesMixin should support either async set,
                    or overriding
        QA Contact: telepathy-bugs at lists.freedesktop.org
          Severity: enhancement
    Classification: Unclassified
                OS: All
          Reporter: simon.mcvittie at collabora.co.uk
          Hardware: Other
            Status: NEW
           Version: git master
         Component: tp-glib
           Product: Telepathy

At the moment, the D-Bus properties mixin imposes these restrictions on an
object:

* Get() must be synchronous
* Get() must always return something (perhaps a dummy value like 0 or "")
* Get() cannot fail
* Set() must be synchronous, or pretend to be

(but Set() *may* fail.)

Mission Control already has a mixture of async set methods that are not
implemented with TpDBusPropertiesMixin (Bug #32416), and async set methods that
pretend to succeed synchronously, but actually do async things.

If we want to use a write-only property for "my nickname" (Bug #14540) or "my
avatar" (Bug #55920), or make MC more honest about sync/async, then we need to
have an async Set method.

Two options:

* Have vfuncs for set_async, set_finish. The default implementation of
set_async would be "call the synchronous setter". I'm tempted to do them in
terms of GVariant, too.

* WONTFIX this, and have MC and CMs override Set() to do this instead:

  if (interface == COMPLICATED_1)
    ...
  else if (interface == COMPLICATED_2)
    ...
  else
    {
      if (tp_dbus_properties_mixin_set (self, ..., &error))
        dbus-glib: return successfully
      else
        dbus-glib: return with error
    }

My concern about that second option is that the properties mixin gives us a
nice data-driven multiplexer for all the interfaces, which becomes less useful.

For Bug #32416, MC also needs a version of tp_dbus_properties_mixin_set() that
will allow setting properties that aren't actually flagged as read/write, for
things like Parameters... unless we rework MC to pass a dict of initial
properties as a construct-time property of accounts (and duplicate most of
their setter logic, unfortunately) instead of constructing a blank account and
poking properties into it like it does now.

I have about half a branch for async set in TpDBusPropertiesMixin.

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