[Jack-Devel] Cannot callback the server in notification thread!

[Tim]

On 8/9/19 12:01 PM, Ethan Funk wrote:
> Yes, what you describe is what I am implementing. I have also called it 
> "persistence" in my application. Normally, when all audio ports are 
> disconnected, my application quits. But I have an command line option to 
> keep it running and have it "persist," not quit, try to reconnect.

I took it one step further and the 'persistence' is stored in the
  song file so that the next time the song is loaded, MusE does not
  complain that this or that is missing and will be removed, instead
  it simply chugs along happily and when the device reappears,
  everything works as before.

I have slowly adopted a policy of "nothing is removed from a song
  behind the user's back unless they specifically say so, even if it
  is ultimately missing some component, such as a port is missing
  or a plugin is missing on the system".
Thus, persistent... everything! across sessions is the goal.

> I just implemented a simple queue of jack_port_ids. Callbacks add their 
> relevant port_ids into the queue, with my main thread checking the queue 
> periodically, to further check the queued ports for re-connection. This 
> was not as hard as I though it would be, and works well so far. Just 
> queueing the port id is a lot simpler that what you are doing in MusE, 
> but it is sufficient for my application.

Yes, as you might see, I actually store a few callback types in our
  callback queue and I must 'review' the queue when something happens
  to carefully determine what exactly took place, for example
  did a port disconnection happen before an unregistration or
  was it an unregistration alone, etc.
It was tricky. Those port ID functions sure help though.

> One remaining question: Am I safe assuming that jack2 will call my 
> callbacks one at a time from a single thread, or do I need to make my 
> port id queue writing function thread safe, just in case jack2 calls my 
> callbacks concurrently?

Don't quote me, I might be getting this wrong, if I recall correctly ;-)
Jack 1 issues all callbacks in the realtime audio thread.
Jack 2 issues some important callbacks, such as the Sync callback,
  in the realtime audio thread, but most other callbacks are
  issued in ONE separate thread.
Therefore I don't think it could call them concurrently,
  but I'm not an expert here...
I think our queue is multi-writer anyway so I probably might not
  have noticed if they were concurrent, but I don't think I
  noticed anything unusual.

Tim.

> 
> Ethan...
> 
> On Fri, 2019-08-09 at 01:52 -0400, Tim wrote:
>> On 8/8/19 10:20 PM, Ethan Funk wrote:
>>> In an application I am writing, I am getting an error message out stderr
>>> (see email subject), and a connection failure return result, when I try
>>> to use jack_connect() from inside a jack_set_port_registration_callback
>>> function, using Jack2. My goal is to have the application remember and
>>> re-connect to a disconnected port if/when it "comes back." Am I going to
>>> need to create a mostly sleeping thread just for port re-connections
>>> attempts, or is there something obvious I am missing?
>>>
>>> Thanks,
>>> Ethan...
>>
>> If I understand your goal, I did this in MusE.
>> I called it 'persistent connections', or 'persistent ports'.
>>
>> For example you can unplug a USB midi device currently
>>    in use by MusE as a 'Jack device', then re-plug it,
>>    and MusE will automatically reconnect to its ports
>>    as if nothing happened and you may continue playing.
>> (I made our 'ALSA devices' do the same thing!)
>> It should also work with other Jack clients.
>>
>> Look in our driver/jack.cpp, at:
>>     static void registration_callback(...),
>>     static void port_connect_callback(...),
>>    and
>>     static int graph_callback(...),
>>     // This is called in the gui context,
>>     //  triggered by graph_callback()
>>     void JackAudioDevice::graphChanged().
>>
>> It was complicated. Very. I needed to support Jack 1 and 2.
>> So there are a few sequences of calls and resulting
>>    sequences of callbacks for example that are different
>>    in Jack 1 and 2. Several tricks and traps. See comments.
>>
>> My system relies HEAVILY on the jack port 'alias names',
>>    because it was pretty much the ONLY thing you can rely on,
>>    because the regular port names and port numbers change
>>    upon re-plugging. It automatically chooses which alias is
>>    best or falls back to the not so good canonical port name.
>>
>> So, that's what it 'remembers' between device plug ins -
>>    simply the port alias or name text.
>>
>> HTH.
>> Tim.
>> _______________________________________________
>> Jack-Devel mailing list
>> Jack-Devel at lists.jackaudio.org
>>  <mailto:Jack-Devel at lists.jackaudio.org>
>>
>> http://lists.jackaudio.org/listinfo.cgi/jack-devel-jackaudio.org
>>