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

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

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.

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 "/").


Lucas De Marchi