[Telepathy] telepathy-glib: TpConnectionManager::protocols is awkward

[Olli Salli]

On Sun, Sep 28, 2008 at 4:10 PM, Murray Cumming <murrayc at murrayc.com> wrote:
> TpConnectionManager has a protocols struct field, containing an array of
> TpConnectionManagerProtocol, one for each protocol implemented by the
> connection manager:
> http://library.gnome.org/devel/telepathy-glib/unstable/telepathy-glib-connection-manager.html#TpConnectionManager-struct
>
> However, it seems that the connection manager must be "introspected"
> before this array is non-null. And this seems to be an asynchronous
> operation that happens at idle time. So I have some questions:
>
> 1. Surely this introspection just involves opening the shared library
> and calling a function? But, even with a little disk IO to open the
> library, this doesn't seem like something that would take a noticeable
> amount of time at any significant moment in an application. Why not just
> do it whenever necessary instead of making the API complicated?

No, Telepathy Connection Managers aren't shared libraries. They are
remote processes, exporting their services through D-Bus. A method
call to get the supported properties might take any time up to the
D-Bus method call timeout (if there even is any set). Consider eg. the
case of a simple not fully asynchronous connection manager, having its
mainloop stuck doing eg. a DNS resolve. (Idle, the IRC connection
manager, is one such example where this MIGHT happen in rare
circumstances). We don't want to freeze the application using the
library in this case.

Note that for some connection managers, for which there exists a
.manager file, no DBus call is actually necessary, just reading that
file is sufficient (and that can happen synchronously, sure). However,
for eg. Haze such a file can't be reliably provided, since the actual
set of supported protocols it supports depends on the underlying
version of libpurple. Therefore an asynchronous D-Bus introspection is
always needed.

>
> 1.1. It seems that we even need to trigger introspection to get the
> "cached" information. What is it about real introspection that takes
> even more time than getting cached data from a file?

Read above.

> --
> Murray Cumming
> murrayc at murrayc.com
> www.murrayc.com
> www.openismus.com
-- 

Br,
Olli Salli