[Bug 778298] [REGRESSION]: decodebin2: Wrong stream-topology reported for some streams

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=778298

--- Comment #2 from Thibault Saunier <tsaunier at gnome.org> ---
(In reply to Sebastian Dröge (slomo) from comment #1)
> This would seem like a bug in discoverer then. The decodebin change only
> makes it so that the topology actually contains the pads it's supposed to
> contain instead of NULL.

Well this is the theory to me :)

# Old topology:

stream-topology, next=(structure){
  "stream-topology, next=(structure)"stream-topology,
caps=(GstCaps)subpicture/x-pgs, pad=(GstPad)"(GstDecodePad) src_2",
    element-srcpad=(GstPad)"(GstPad) src_2";", caps=(GstCaps)subpicture/x-pgs,
element-srcpad=(GstPad)"(GstPad) subpicture_0_1200";",
  "stream-topology, next=(structure)"stream-topology,
caps=(GstCaps)"audio/x-raw, format=(string)F32LE, layout=(string)interleaved,
rate=(int)[ 4000, 96000 ], channels=(int)[ 1, 6 ]", pad=(GstPad)"(GstDecodePad#
    element-srcpad=(GstPad)"(GstPad) src";", caps=(GstCaps)audio/x-ac3,
    element-srcpad=(GstPad)"(GstPad) audio_0_1100";",
  "stream-topology, next=(structure)"stream-topology,
caps=(GstCaps)"video/x-raw, format=(string){ I420, YUY2, RGB, BGR, Y42B, Y444,
YUV9, Y41B, GRAY8, RGB8P, I420, Y42B, Y444, UYVY, NV12, NV21, ARGB, RGBA, ABG#
    element-srcpad=(GstPad)"(GstPad) src";", caps=(GstCaps)"video/x-h264,
stream-format=(string)byte-stream, alignment=(string)nal",                      
    element-srcpad=(GstPad)"(GstPad) video_0_1011";"
  }, caps=(GstCaps)"video/mpegts, systemstream=(boolean)true,
packetsize=(int)192", element-srcpad=(GstPad)"(GstPad) src";


# Topology since 46835f550d0a49338a3477973df842b1443c7a81

0:00:02.553599408 31385 0x55edcfa4db00 ERROR             discoverer
gstdiscoverer.c:1320:discoverer_collect: Topology is stream-topology,
    next=(structure){
      "stream-topology, next=(structure)"stream-topology,
caps=(GstCaps)subpicture/x-pgs, pad=(GstPad)"(GstDecodePad) src_2",
        element-srcpad=(GstPad)"(GstPad) src_2";",
caps=(GstCaps)subpicture/x-pgs,
        element-srcpad=(GstPad)"(GstPad) subpicture_0_1200";",
      "stream-topology, next=(structure)"stream-topology,
next=(structure)"stream-topology, caps=(GstCaps)"audio/x-raw,
format=(string)F32LE, layout=(string)interleaved, rate=(int)[ 4000, 96000 ],
channels=(int#
        element-srcpad=(GstPad)"(GstPad) src";", caps=(GstCaps)"audio/x-ac3,
framed=(boolean)true, rate=(int)48000, channels=(int)6,
alignment=(string)frame";",
        element-srcpad=(GstPad)"(GstPad) audio_0_1100",
caps=(GstCaps)audio/x-ac3;",
      "stream-topology, next=(structure)"stream-topology,
next=(structure)"stream-topology, caps=(GstCaps)"video/x-raw, format=(string){
I420, YUY2, RGB, BGR, Y42B, Y444, YUV9, Y41B, GRAY8, RGB8P, I420, Y42B, Y#
        element-srcpad=(GstPad)"(GstPad) src";", caps=(GstCaps)"video/x-h264,
stream-format=(string)avc, alignment=(string)au,
pixel-aspect-ratio=(fraction)1/1, width=(int)1920, height=(int)1080,
framerate=(fra#
        element-srcpad=(GstPad)"(GstPad) video_0_1011",
caps=(GstCaps)"video/x-h264, stream-format=(string)byte-stream,
alignment=(string)nal";"
    },
    caps=(GstCaps)"video/mpegts, systemstream=(boolean)true,
packetsize=(int)192", element-srcpad=(GstPad)"(GstPad) src";

So here we can see that we used not to have caps for the repetition of the
srcpad stream and we now have. The discoverer is not able to handle that change
because h264parse is doing some conversion, this 'hack' would solve our issue
here though:

> diff --git a/gst-libs/gst/pbutils/gstdiscoverer.c b/gst-libs/gst/pbutils/gstdiscoverer.c
> index 96fa39712..bf55a0fa8 100644
> --- a/gst-libs/gst/pbutils/gstdiscoverer.c
> +++ b/gst-libs/gst/pbutils/gstdiscoverer.c
> @@ -1070,6 +1070,8 @@ child_is_same_stream (const GstCaps * _parent, const GstCaps * child)
>    for (i = 0; i < size; i++) {
>      gst_structure_remove_field (gst_caps_get_structure (parent, i), "parsed");
>      gst_structure_remove_field (gst_caps_get_structure (parent, i), "framed");
> +    gst_structure_remove_field (gst_caps_get_structure (parent, i), "stream-format");
> +    gst_structure_remove_field (gst_caps_get_structure (parent, i), "alignment");
>    }
>    res = gst_caps_can_intersect (parent, child);
>   gst_caps_unref (parent);

It might make sense but I am not sure, what do you think?

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