[Bug 797332] New: basesrc: Allow for suppressing the initial negotiation

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=797332

            Bug ID: 797332
           Summary: basesrc: Allow for suppressing the initial negotiation
    Classification: Platform
           Product: GStreamer
           Version: unspecified
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: Normal
         Component: gstreamer (core)
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: crg7475 at mailbox.org
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

Run this example:

gst-launch-1.0 audiotestsrc ! "audio/x-raw,rate=44100" ! interaudiosink   
interaudiosrc ! fakesink sync=true silent=false -v

We'd expect this to negotiate the downstream caps to 44100 Hz. This is also
what happens - but initially, the caps are set to 48000 Hz instead. Here's the
output:

/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = event   *******
(fakesink0:sink) E (type: stream-start (10254), GstEventStreamStart,
stream-id=(string)ea61450f33201961286324e03482a1c5,
flags=(GstStreamFlags)GST_STREAM_FLAG_NONE, group-id=(uint)2;) 0x7f0e5c004c20
/GstPipeline:pipeline0/GstInterAudioSrc:interaudiosrc0.GstPad:src: caps =
audio/x-raw, format=(string)S16LE, rate=(int)48000, channels=(int)2,
layout=(string)interleaved
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = event   *******
(fakesink0:sink) E (type: caps (12814), GstEventCaps,
caps=(GstCaps)"audio/x-raw\,\ format\=\(string\)S16LE\,\ rate\=\(int\)48000\,\
channels\=\(int\)2\,\ layout\=\(string\)interleaved";) 0x7f0e5c004c90
/GstPipeline:pipeline0/GstAudioTestSrc:audiotestsrc0.GstPad:src: caps =
audio/x-raw, format=(string)S16LE, layout=(string)interleaved, rate=(int)44100,
channels=(int)1
/GstPipeline:pipeline0/GstCapsFilter:capsfilter0.GstPad:src: caps =
audio/x-raw, format=(string)S16LE, layout=(string)interleaved, rate=(int)44100,
channels=(int)1
/GstPipeline:pipeline0/GstInterAudioSink:interaudiosink0.GstPad:sink: caps =
audio/x-raw, format=(string)S16LE, layout=(string)interleaved, rate=(int)44100,
channels=(int)1
/GstPipeline:pipeline0/GstCapsFilter:capsfilter0.GstPad:sink: caps =
audio/x-raw, format=(string)S16LE, layout=(string)interleaved, rate=(int)44100,
channels=(int)1
Setting pipeline to PLAYING ...
New clock: GstSystemClock
/GstPipeline:pipeline0/GstInterAudioSrc:interaudiosrc0.GstPad:src: caps =
audio/x-raw, format=(string)S16LE, layout=(string)interleaved, rate=(int)44100,
channels=(int)1
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = event   *******
(fakesink0:sink) E (type: caps (12814), GstEventCaps,
caps=(GstCaps)"audio/x-raw\,\ format\=\(string\)S16LE\,\
layout\=\(string\)interleaved\,\ rate\=\(int\)44100\,\ channels\=\(int\)1";)
0x7f0e5c004d00
/GstPipeline:pipeline0/GstFakeSink:fakesink0.GstPad:sink: caps = audio/x-raw,
format=(string)S16LE, layout=(string)interleaved, rate=(int)44100,
channels=(int)1
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = event   *******
(fakesink0:sink) E (type: segment (17934), GstEventSegment,
segment=(GstSegment)"GstSegment, flags=(GstSegmentFlags)GST_SEGMENT_FLAG_NONE,
rate=(double)1, applied-rate=(double)1, format=(GstFormat)GST_FORMAT_TIME,
base=(guint64)0, offset=(guint64)0, start=(guint64)0,
stop=(guint64)18446744073709551615, time=(guint64)0, position=(guint64)0,
duration=(guint64)18446744073709551615;";) 0x7f0e5c004de0
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = preroll   ******* 
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = chain   *******
(fakesink0:sink) (2400 bytes, dts: none, pts: 0:00:00.000470130, duration:
0:00:00.027210884, offset: 0, offset_end: 1200, flags: 00004840 discont gap
tag-memory , meta: none) 0x7f0e540086b0
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = chain   *******
(fakesink0:sink) (2204 bytes, dts: none, pts: 0:00:00.027681014, duration:
0:00:00.024988662, offset: 1200, offset_end: 2302, flags: 00000000 , meta:
none) 0x7f0e540087c0
/GstPipeline:pipeline0/GstFakeSink:fakesink0: last-message = chain   *******
(fakesink0:sink) (2204 bytes, dts: none, pts: 0:00:00.052669676, duration:
0:00:00.024988662, offset: 2302, offset_end: 3404, flags: 00000000 , meta:
none) 0x7f0e540088d0

The reason for these initial 48000 Hz is that right at the beginning, basesrc
tries to come up with fixated src caps. However, at that point, interaudiosrc
does not have the actual caps yet (that is, the caps for the data that comes
from interaudiosink). So, instead, interaudiosrc's fixate() function is called
to fixate the template caps. The interaudiosrc.c fixate function contains this
line:

  gst_structure_fixate_field_nearest_int (structure, "rate", 48000);

This is not a bug in interaudiosrc. Rather, the basesrc class tries to output a
caps event too early. It would be better if it were possible to inform basesrc
that the srccaps will be known later, and that until then, it should not try to
push a caps event downstream. It would be good to add functions to the API for
this purpose.

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