[PATCH v6 5/5] winegstreamer: Replace source pad interface with GstAppSrc.

[Derek Lesho]

On 9/24/21 12:38, Zebediah Figura wrote:
> On 9/24/21 3:05 AM, Derek Lesho wrote:
>> On 9/21/21 13:02, Zebediah Figura wrote:
>>
>>>
>>>
>>> 1. Rely on the fact that seek-data is sent during a seek, and use it
>>> to effectively invalidate any buffers sent before the seek. Unlike the
>>> aforementioned problem with validation, this will actually work: at
>>> the time that seek-data is sent appsrc should be flushing, so any
>>> buffers we send before it will be discarded by appsrc, and any buffers
>>> we send afterward can be discarded by us if they aren't valid. This is
>>> very fragile, though. There's really no reason for appsrc to send
>>> seek-data for random-access streams in the first place, and this kind
>>> of synchronization is easy to get wrong if I haven't already.
>>
>> This sounds like the best way forward in my opinion, how is it fragile?
>
> It's fragile in general because we're making a lot of assumptions, 
> that aren't documented, about when and from which thread gstappsrc 
> will send these signals, and what synchronization guarantees it 
> applies when doing so.

My perspective is that if we can get it working, documenting the two 
required quirks to the GStreamer project, eventually the problem will be 
be cleared up and/or fixed, and we can remove the quirks then.

>
>> It seems that if we just ensure that a pushed buffer takes into account
>> the latest seek/need data pair, not much can go wrong.
>>
>
> That's not enough by itself. We can, generally speaking, send a buffer 
> after a seek-data/need-data is triggered but before it is actually sent. 
Before what is sent?
> This happens in practice with flushes.
>
> In order to get around that, you need the hack I mentioned: assume 
> that a flush will be accompanied by a seek-data signal. This is 
> especially fragile because there's no reason for appsrc to even be 
> sending this signal in the first place (it only really makes sense in 
> "seekable" mode), and they could easily decide to not do that.

I'm not sure I understand, whenever the next buffer app source wants is 
not consecutively after the last, seek-data is required (and sent).  
What does "assume the flush will be accompanied by a seek-data signal" 
mean?  The app source client doesn't have any conception of a flush, 
just seek-data and need-data, and to fix this problem all we need to do 
is add a quite rational check that push_data is responding to the latest 
seek/need data pair.