Re: [PATCH v2 08/11] dbus: Fix the kdbus message encoding

[Denis Kenzior <denkenz@gmail.com>]

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Hi Andrew,

On 04/12/2016 06:06 PM, Andrzej Zaborowski wrote:
> Hi Denis,
>
> On 13 April 2016 at 00:25, Denis Kenzior <denkenz@gmail.com> wrote:
>> On 04/12/2016 05:10 PM, Andrzej Zaborowski wrote:
>>> Hmm true.  But the body is now enclosed in a variant, so what buffer
>>> do you suggest to write that into and assign to item->ver.address?
>>>
>>
>> This is actually a nasty issue with kdbus.  As you say, variant must be a
>> single type, this is specified in DBus-1 Spec somewhere.  So if we are to
>> enclose it in a variant like kdbus wants, then it either needs to be:
>> 1. enclosed in '()'
>> 2. special cased.
>>
>> You're doing 1, however won't this result in the message signature being
>> different depending on the the transport being used?
>
> Why?  If we assume that the message's signature is the type in the
> variant witout the () I don't see any ambiguity there.
>

 From what I understood, the current setup will result in 
l_dbus_message_get_signature() returning different results, depending on 
transport.

So we probably need to some extra handling to make that extra set of 
'()' hidden.

> I didn't find a specific statement of that in the docs although
> https://wiki.gnome.org/Projects/GLib/GDBus/Version2 does say:
>
> "The body of the message is inside of the variant at the end. The body
> must be a tuple. "
>

That's not terribly specific.  How does the kdbus signature look like?

>>
>> Can we special case the building of this variant for gvariant based
>> messages?  Similar to how we're generating the signature on the fly in the
>> builder?
>
> Is it worth it though, just to save a memcpy?
>

If the memcpy is like 64k, then it might be worth it.

Can we generate the needed variant inside dbus_message_builder_finalize? 
or somewhere around that time?

And this whole thing is a great example of how not to design a 
marshaling format.

Regards,
-Denis