Pipeline not going into Playing state when uridecodebin with custom handler is used

[Krzysztof Konopko]

Interestingly /decodebin2/ does not propagate instantiated element's
GstStateChangeReturn return value as it's a type-found signal callback.
gst-plugin-base/gst/playback/gstdecodebin2.c:connect_pad():

/* Bring the element to the state of the parent */
    if ((gst_element_set_state (element,
                GST_STATE_PAUSED)) == GST_STATE_CHANGE_FAILURE) {

so no way to say there will be no pre-roll.

Does it make any sense to try to instantiate a live element through
typfinding?

Cheers,
Kris

On 11/01/13 17:05, Krzysztof Konopko wrote:
> Hi,
>
> I had a closer look into it. The problem can be reduced to something
> like this (hope you don't mind if I rehash some bits of previous
> description):
>
> gst-launch-1.0 \
>   uridecodebin uri=file:///tmp/test.sdp caps=video/mpegts \
>   ! fakesink
>
> On my laptop it doesn't matter whether it's a genuine HTTP or filesystem
> request. It deadlocks.
>
>
> There's a custom vqesdpdemux element which has application/sdp
> capability on its sink. Here's how the gist of the problem goes:
>
> 1. uridecodebin instantiates filesrc, queue2 and vqesdpdemux (typefinder
> magic)
>
> 2. vqesdpdemux receives data (buffers) from filesrc until it gets EOS
> event on its sink pad (receives SDP content, simple text file)
>
> 3. In the same thread were the EOS event was received vqesdpdemux
> (GstBin) creates a genuine custom vqesrc element which is supposed to
> receive the RTP traffic
>
> 4. The new vqesrc element's state is synchronised with its parent
> (vqesdpdemux) by calling gst_element_sync_state_with_parent() but this
> can "trap" it in PAUSED or READY state (depends how far in the
> transition the pipeline got after sending EOS).
>
> Now because vqesrc is a *live* element, it's activated pad thread waits
> in gst_base_src_wait_playing() for vqesrc to get into PLAYING. But it
> won't transition to that state if it's "trapped" in pipeline's
> transition towards READY (depending on timing conditions).
>
>
>
> I'll be digging into it (*) but if it bells with anyone, please shout.
>
> Basically we're looking for (or trying to find/understand) a good
> pattern to create a new source element somewhere in the *middle* of the
> pipeline based on some information (buffers) already received and
> potentially ditch/replace (lop off) the old bit of upstream (if
> possible) after it's done its job.
>
> It works pretty well if the source is explicit (e. g. filesrc). Things
> get complicated if one wants to use typefind magic and
> auto-instantiating/connecting (some decodebin element).
>
> Cheers,
> Kris
>
> (*) It's a quite involving piece of work to looking into. I'm not doing
> it because I'm magnanimous but because Mariusz is sitting almost next to
> me :)
>
> On 07/01/13 17:57, Mariusz Buras wrote:
>> Hi all,
>>
>> I have a quite complex problem to describe so first I'll show you my
>> pipeline setup:
>>
>>    uridecodebin uri=http://some_address_somewhere.co.uk/test.sdp
>> caps=video/mpegts
>>        ! queue ! tsdemux ! video/x-h264 ! decodebin ! queue ! xvimagesink
>>
>> Where "test.sdp" is an SDP file which refers to a multicast RTP stream.
>>   I have created an element "vqesrc"[1] which is a live source for this
>> multicast RTP data.  It takes the contents of an SDP file as a property.
>>   To get it to be autoplugged by decodebin I have also created
>> vqesdpdemux which handles "application/sdp" caps much like hlsdemux or
>> sdpdemux works.
>>
>> vqesdpdemux is a bin.  It creates a vqesrc, adds it to itself, sets the SDP
>> file data as a one of the properties of vqesrc and adds a ghost pad on
>> vqesdpdemux exposing the source pad of the gstvqesrc to outside of the
>> bin. So when everything works I should be getting something like that on
>> the diagram[2].  As you can see everything gets autoplugged as expected
>> and I indeed can see live video on my screen. I have also attached a log
>> file that shows what's going on when it works [3].
>>
>> In vqesdpdemux when I create the vqesrc child element (which is a live
>> source) I am careful to call gst_element_sync_state_with_parent for the
>> child.  Also I am careful to return GST_STATE_CHANGE_NO_PREROLL from
>> vqesdpdemux whenever a request to enter state PAUSED is received as I
>> know the child I will be adding will be live.
>>
>>
>> The problem starts when uridecodebin is pointed to a locally stored SDP
>> file, something like this:
>>
>>    uridecodebin uri=file:///home/mariusz.buras/tmp/test.sdp
>> caps=video/mpegts
>>      ! queue ! tsdemux ! video/x-h264 ! decodebin  ! queue ! xvimagesink
>>
>> Basically, what seems to happen is that my pipeline never fully enters
>> PAUSED state (only vqesdpdemux does) and gets stuck in READY forever.
>> As shown in [4] pipeline start to preroll but never finishes.  My
>> experiments showed that in "http case" SDP file is delivered while the
>> pipline is changing it's state to PLAYING. While in the "broken" case
>> it's delivered when the pipeline is transitioning to the PAUSED state.
>> This leads me to believe that something wrong happens (or something good
>> doesn't happen) when vqesdpdemux receives a sink event callback with a
>> buffer containing the SDP file.
>>
>> What I have tried to fix/mitigate the problem*:
>>
>> 1. Forcing vqesrc element state to PLAYING just after it's added to
>> vqesdpdemux bin with gst_set_state rather than
>> gst_element_sync_state_with_parent. This makes it work, but I don't
>> think this is a right thing to do.  It makes me a bit uneasy using  a
>> half baked "fix" to a problem I don't understand.
>> 2. Sending a reconfigure event. Doesn't help.
>> 3. Delaying the creation of vqesrc element - this doesn't work.
>> 4. Plenty of weird/desperate hacks on vqesdpdemux - same result.
>>
>> * when I write that "it doesn't work" I mean that it behaves in the same
>> or very similar fashion to when the "fix" wasn't applied.
>>
>> I've also attached [5] a diagram of the last state the pipeline gets
>> into when it doesn't work. This is not very helpful though, because
>> the last state my pipeline fully enters is READY when no auto-plugging
>> happened yet.
>>
>>
>> I think possibly what is happening is that the pipeline is prerolling in
>> the PAUSED state so vqesrc doesn't produce any data so the pipeline
>> never exits the prerolling state so makes it to state PLAYING but then
>> I'm unsure why it would work with the http case but not with local file.
>>
>>
>> Can someone help me with that?
>>
>> Regards,
>> Mariusz.
>>
>>
>> [1] The code for both of these can be found on github:
>> https://github.com/sqward/gst-vqe
>> [2] An example working pipeline attached as "example_working_pipeline.jpg"
>> [3] An example working log attached as "example_working_log.txt"
>> [4] Log of a broken case attached as "broken_case.txt"
>> [5] A diagram of "broken" pipeline when it gets stuck in READY state
>> attached as "broken_case.png"
>>
>>
>>
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>>
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