[Bug 797142] New: caps: Using gst_caps_fixate() on src pad leads to lies in the caps

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=797142

            Bug ID: 797142
           Summary: caps: Using gst_caps_fixate() on src pad leads to lies
                    in the caps
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gstreamer (core)
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: nicolas at ndufresne.ca
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

The default implementation of fixate in basesrc is based on gst_caps_fixate().
This though can lead to terribly lies in the resulting caps if downstream
element uses ranges or set to explicitly state what is supported.

As in:

  udpsrc caps="video/x-h264" ! h264parse ! avdec_h264 ! fakesink

avdec_h264 sets the alignment to { au, nal }, in order to ensure that it's one
of these two alignment that are provided. udpsrc, which uses BaseSrc default,
will happily pretend to produce:

  video/x-h264, alignment=(string)au, stream-format=(string)avc

This is highly problematic, since we now lie to h264parse, triggering
optimization that should not.

The downstream solution to that is to remove the alignment and to write code in
When we receive that caps that check that alignment is present. That's not a
great solution because it requires coding the requirements and also it reduce
the quality of the documentation.

While I'm under the impression this may break some valid use cases, I'd like to
start experimenting a solution that would make our caps negotiation more
strict. My first proposal would be to create a new version of
gst_caps_fixate(), let's say gst_caps_fixate_known (caps, known_field), or
gst_caps_fixate_from_template (caps, template). The idea is that the opration
would first remove all the unkmown field before fixating. As a side effect, in
the previous example, udpsrc would negotiate just "

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