How does dbus-daemon handle multiple services with same Name?

[Michael Biebl]

2015-06-21 20:58 GMT+02:00 Armin K. <krejzi at email.com>:
> On 06/21/2015 08:42 PM, Lennart Poettering wrote:
>> On Sun, 21.06.15 13:02, Armin K. (krejzi at email.com) wrote:
>>
>>> Hi,
>>>
>>> While trying kdbus with systemd, I have ran across a problem.
>>> Apparently it doesn't yet properly handle multiple D-Bus services
>>> with same name.
>>>
>>> I'm interested in how does D-Bus daemon handle this? Which one
>>> gets loaded and how it decides which one to load?
>>>
>>> An example:
>>>
>>> I have two services with Name=org.freedesktop.FileManager1
>>>
>>> One is GNOME's Nautilus, other is MATE's Caja File Manager.
>>> Which one would get loaded and why?
>>
>> Only one service can own a name at a time. If one already owns a name,
>> and another one tries to take possession of it as well, then one of
>> three things can happen:
>>
>>       a) the request to own the name is queued, and dbus-daemon passes
>>       it on automatically and atomically as soon as the old owner
>>       voluntarily gives it up.
>>
>>       b) the operation to request the name will simply fail (this mode
>>       of operation you get by passing DBUS_NAME_FLAG_DO_NOT_QUEUE when
>>       acquiring the name)
>>
>>       c) the name will be taken away forcible from the old owner (only
>>       if the new owner indicated DBUS_NAME_FLAG_REPLACE_EXISTING, and
>>       the old one DBUS_NAME_FLAG_ALLOW_REPLACEMENT).
>>
>> Either way, and no times there are two owners of a name at the same
>> time, neither in kdbus, nor in dbus-daemon.
>>
>> Lennart
>>
>
> I'm asking specifically because current dbus-daemon seems to
> kinda handle it, but kdbus will error out when it tries to
> create a symlink for bus-proxyd. I was asked on systemd
> bug tracker how dbus handles this, that's why I asked here.

What you are referring to is having files in
/usr/share/dbus-1/{system}-services/
which provide the same "Name".
It's not enforced, that the Name in the .service file is named after
the D-Bus name.

So you can have
 foo.service with Name=org.fdo.baz
and
 bar.service with Name=org.fdo.baz

I think your question is, assuming that neither foo or bar is running,
and there is an incoming request for org.fdo.baz, which one of the two
is started?




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