Re: [PATCH BlueZ 2/4] gdbus: Automatically register '/' path

Hi Lucas,

On Mon, Nov 19, 2012 at 3:18 PM, Lucas De Marchi
[off-list ref] wrote:

On Mon, Nov 19, 2012 at 10:49 AM, Luiz Augusto von Dentz
[off-list ref] wrote:

quoted

Hi Lucas,

On Mon, Nov 19, 2012 at 2:06 PM, Lucas De Marchi
[off-list ref] wrote:

quoted

Hi Luiz,

On Mon, Nov 19, 2012 at 6:46 AM, Luiz Augusto von Dentz
[off-list ref] wrote:

quoted

From: Luiz Augusto von Dentz <redacted>

This makes g_dbus_setup_bus to automatically register '/' path so
user application that don't export any interface on '/' will have it
registered for ObjectManager.

Note that it is now required to call g_dbus_close before exit.

I'm no really fan of this. Why do we need to register '/' now?  If we
are not going to support ObjectManager interfaces in subtrees, it
would be easier to just move the ObjectManager to the shortest path
registered rather than always registering '/'.

Maybe I should have made clear this in the description, if you look at
the spec it suggest that each sub-tree root should implement
ObjectManager, the current implementation does that but if you think
about it probably doesn't make sense to have sub-trees because that
would creates extra round trips that ObjectManager was meant to

HUmn... I think you are a bit confused now.

1 entire tree can be managed by a single ObjectManager, or multiple
ones if there are objects that are not interesting to everybody and
only a few types of clients would be interested in the objects changed
beyond that path.

Example:

/org/bluez
  - ObjectManager
/org/bluez/adapter/bla/bli
  - MyInterfaceWithLotsOfObjects

If you put only 1 ObjectManager it means you get called for each
change in /org/bluez/adapter/bla/bli even though most of the clients
will not be interested in that. Therefore the spec allows you to
attach another ObjectManager in /org/bluez/adapter/[...]  so the first
ObjectManager only send updates and "manage" the objects until the
path containing another ObjectManager.

I do think we don't really need that feature right now. I think we are
fine with a single ObjectManager in "/", but always registering "/"
IMO is not the right solution.

As far I know /org/bluez/adapter/bla/bli is a child of /org/bluez thus
you don't need another ObjectManager, anyway the point is that
multiple ObjectManagers for the same connection might not make sense
as it will create more round trips to discover the object tree e.g:
/org/bluez vs /org/somethingelse

There is also the problem of supporting changing the root, the spec
doesn't mention anything about that even though it says it can support
sub-trees thus the application could in theory register a new root
e.g. /org at any point which basically invalidates the entire
sub-trees which I don't think would be convenient for anyone watching
it.

quoted

prevent, also Im not completely sure now but I remember someone
mentioning the '/' is kind special and should always be available no
matter what, so by registering '/' directly on g_dbus_setup_bus we
guarantee we have it always and that no sub-tree is going to be
generated since '/' is the root no matter what path the user code
register.

Kind of true... what really needs to be treated are calls to
Introspectable interface on "/". This is used by tools like d-feet to
navigate the tree (looking for the "<node ...>" tags) -- I *think*
this is also used by some bindings like python's. If you look in other
projects around, they prefer using the project's namespace rather than
"/"  (examples: systemd, udisks, policykit, dconf).

So, if we register "/", but not an Introspectable interface in there,
it means we will break tools like d-feet. If we don't register it,
libdbus will handle the messages to Introspectable (this is why other
projects don't even bother about "/").

Yep, I assume it was more convenient to have in '/' because it would
basically work in any case, but we can default to something else it
just a matter to have this set directly when we do g_dbus_setup_bus so
it is available since the beginning.

--
Luiz Augusto von Dentz