Detect new available names in bus

[Thomas Kluyver]

>From my reading of the spec it makes sense that NameOwnerChanged is only triggered when an application actually runs, connects to D-Bus and claims the name. 'Owner' seems to refer to a D-Bus connection (see e.g. the description of GetNameOwner). So registering a name for activation is not sufficient to 'own' it. On my system, NameHasOwner is True only for names with an active connection, though this is different from what Sergio said in his original message.

I think it's a reasonable suggestion that there should be a signal when the set of activatable names changes. To be clear, though, I have no power in changing the D-Bus spec.

My only concern is that this would require a particular kind of implementation. At present, the bus *could* opt to search through the files for activatable names only when it gets a request with a name not already claimed (or when ListActivatableNames is called). Specifying that it must send a signal means it has to constantly monitor those files (either with something like inotify, or regular polling). But if the reference implementation is already doing that, maybe this isn't important.

Best wishes,
Thomas

On Tue, 15 Feb 2022, at 13:13, Sergio Costas wrote:
> Hi:
>
> El 15/2/22 a las 13:58, Ralf Habacker escribió:
>> Am 15.02.22 um 11:35 schrieb Sergio Costas:
>>> Which confirms that the dbus daemon is already monitoring those 
>>> folders (both with inotify for linux and kqueue for BSD), so adding 
>>> that dbus signal sould be quite straightforward...
>>>
>>> So... How much complex is to submit a proposal for modifying the 
>>> specification...?
>>>
>>
>> Have you checked 
>> https://dbus.freedesktop.org/doc/dbus-specification.html to see if 
>> there is already a signal that meets your requirements?
>>
>> A quick look shows me that these may be usable here:
>>
>> org.freedesktop.DBus.NameOwnerChanged - This signal indicates that the 
>> owner of a name has changed. It's also the signal to use to detect the 
>> appearance of new names on the bus.
>>
>> org.freedesktop.DBus.NameLost - This signal is sent to a specific 
>> application when it loses ownership of a name.
>>
>> org.freedesktop.DBus.NameAcquired - This signal is sent to a specific 
>> application when it gains ownership of a name. 
>
> As I commented in a previous message, I already checked all those. 
> NameLost and NameAcquired seems to be emitted only to the current owner 
> of the name, to notify that it has successfully acquired a name or if 
> some other service "stole" it. I connected to them but don't receive any 
> signal, no matter that I install or uninstall the files, launch or kill 
> de daemon...
>
> About NameOwnerChanged, it only notifies when a daemon actually acquires 
> a name, but that requires it to be launched. This means that an 
> activatable service won't emit that unless any of their methods is 
> called, thus loading the daemon. Again, that's not what I need. What I 
> need is to detect when the .service file is added or removed.
>
> -- 
>
> Nos leemos
> 		         RASTER    (Linux user #228804)
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