[pulseaudio-discuss] [PATCH 1/3] core: add generic message interface

[Georg Chini]

On 06.08.2017 07:26, Tanu Kaskinen wrote:
> On Sat, 2017-08-05 at 21:32 +0200, Georg Chini wrote:
>> On 05.08.2017 13:37, Tanu Kaskinen wrote:
>>> On Fri, 2017-08-04 at 15:37 +0200, Georg Chini wrote:
>>>> This patch adds a new feature to the core which allows to exchange
>>>> messages between objects. An object can register/unregister a message
>>>> handler with pa_core_message_handler_{register, unregister}() while
>>>> any other object can send a message to the handler using the
>>>> pa_core_send_message() function. A message has 5 arguments (apart
>>>> from passing the core):
>>>>
>>>> recipient: The name of the message handler that will receive the message
>>>> message: message command
>>>> message_parameters: A string containing additional parameters
>>>> message_data: void pointer to some parameter structure, can be used
>>>>                 as alternative to message_parameters
>>>> response: Pointer to a response string that will be filled by the
>>>>             message handler. The caller is responsible to free the string.
>>>>
>>>> The patch is a precondition for the following patches that also allow
>>>> clients to send messages to pulseaudio objects.
>>>>
>>>> Because not every message handler should be visible to clients, a flag
>>>> was added to the handler structure which allows to mark a handler as
>>>> public or private.
>>>>
>>>> There is no restriction on object names, except that a handler name
>>>> always starts with a "/". The intention is to use a path-like syntax,
>>>> for example /core/sink_1 for a sink or /name/instances/index for modules.
>>>> The exact naming convention still needs to be agreed.
>>>>
>>>> Message groups are also implemented, so that a handler can subscribe
>>>> to a message group using pa_core_message_handler_group_[un]subscribe()
>>>> to receive messages sent to the group. To distinguish group and handler
>>>> names, group names lack the leading "/". Some of the code used to
>>>> implement the message groups was adapted from hook-list.c. Message
>>>> groups are created/deleted implicitely on subscription/unsubscription.
>>>>
>>>> The messaging interface can serve as a full replacement for the current
>>>> hook system with several advantages:
>>>> - no need to change header files when a new handler/group is implemented
>>>> - slightly simpler registration interface
>>>> - multi-purpose message handlers that can handle multiple events
>>>> - mesage handlers may also be accessible from the client side
>>> We agree that it's good to allow clients to send messages to modules.
>>> Unfortunately, in this patch you're assuming that we'll also replace
>>> hooks with the same system. Can we please keep things simple and do one
>>> change at a time? I'm not enthusiastic about replacing hooks, and I'd
>>> rather move on with the client message passing before getting consensus
>>> on the hook stuff.
>>>
>>> For reference, here's a list of unnecessary (from pure client message
>>> passing point of view) things I can gather from the commit message:
>>>
>>> - void pointer argument in message handlers
>>> - public/private flag
>>> - message groups (signals would seem like a better fit for the purpose
>>> anyway)
>>>
>> Possibly I am only using the wrong words. Let me put it like that:
>> My intention is not to replace the hooks but to incorporate them
>> into a more general concept. I even used a lot of the code and
>> put it in the new message context. The functionality of the hooks
>> is a subset of the messages concept and is fully preserved.
>> So by replacing hooks with messages, nothing would be lost
>> (at least not intentionally), only the interface would change.
>> Take a look at the second patch of the series for an example.
>>
>> The only disadvantage I can think of is that there is one more
>> lookup step required compared to the hooks when finding
>> the right handler.
> For starters, the conversion has to be implemented, reviewed, and the
> new interface has to be learned by everyone. Even if I thought that the
> message system didn't have any API design problems, it's not clear that
> the conversion would be worth the effort. I would probably be ok with
> some new API that allows using one callback for multiple hooks, but I
> think you'll need to get an ok from Arun too. I won't elaborate on the
> API design issues now, because I don't think it's good to block the
> client message passing feature.

Why would it be urgent to get the feature implemented? Nobody is
pressing us and we are designing a new API. In my opinion, most
of the thinking should go into the design phase to get it right from
the start. It's not a good idea to come up with something quick,
only to have more work patching it up later.

I think you are reacting too strong (nearly only) to my statement, that
it would be possible to replace the hooks. All our discussion has been
centered around that topic up to now, while this is not at all the main
intention of the patch. It is just mentioned as one possibility in the
commit message and I can leave out this part if you prefer. You are
right, for the hooks themselves there is no obvious reason why they
should be replaced. The mechanism is working, long established and
seems sufficient for what we are doing with it. I also have no objections
to it. So let us assume that there will be no replacement and come back
to what the patch actually does.

Our main disagreement seems to be that you want a very simple and
client-only way to pass messages, while my goal is to design a versatile
message interface that can be used internally and externally. If it was
only about clients sending messages to modules, my first approach
would have been fully sufficient. It can even be done with the extension
interface today.

I wonder if you are at all willing to accept something more general or
are completely focused on the client-only approach. I hoped, that the
features and the two use cases (internal: implementation of signals,
external: message handler to list handlers) could convince you that
the new messaging system is a definite win for pulseaudio even if it
partly duplicates existing functionality.
My current implementation might have drawbacks and problems
that I do not see, but I am willing to put more work into it until those
potential issues are solved. I already pointed out multiple advantages
over existing mechanisms and your categoric denial currently seems
only based on the hook discussion and therefore a bit unfounded to
me.

As a side note: Message groups would also make sense from a client
perspective. If a client could create a group (not implemented, but could
be easily done), multiple instances of the same object could be controlled
by one group. Consider an application implementing something like a
"precision" combine sink by using loopbacks from a null-sink to multiple
real sinks. Then the loopbacks could be added to some my-loopbacks
group and controlled via that group.