[Bug 794744] Data is none in buffer from GstVideo.VideoFrame

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=794744

Mathieu Duponchelle <mduponchelle1 at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |mduponchelle1 at gmail.com

--- Comment #1 from Mathieu Duponchelle <mduponchelle1 at gmail.com> ---
Investigated, this is unfortunately a gobject-introspection bug:

pygobject obtains the value of the field with g_field_info_get_field(), which
in turn uses the offset returned by g_field_info_get_offset(), which is wrong
(differs from G_STRUCT_OFFSET (GstVideoFrame, buffer).

Digging a bit deeper, the VideoFrame is defined as:

struct _GstVideoFrame {
  GstVideoInfo info;                                                            
  GstVideoFrameFlags flags;                                                     

  GstBuffer *buffer;
}

and GstVideoInfo ends with a union:

struct _GstVideoInfo {
  ...

  union {
    struct {
      GstVideoMultiviewMode     multiview_mode;
      GstVideoMultiviewFlags    multiview_flags;
      GstVideoFieldOrder        field_order;                                    
    } abi;
    /*< private >*/
    gpointer _gst_reserved[GST_PADDING];
  } ABI;
}

The union isn't taken into account by the return of get_offset, this is easy to
determine as g_field_info_get_offset(buffer_field_info) == G_STRUCT_OFFSET
(GstVideoInfo, ABI).

The implementation of the get_offset function ends with:

  blob = (FieldBlob *)&rinfo->typelib->data[rinfo->offset];

  return blob->struct_offset;

I haven't investigated past this point, I would suggest reporting this issue to
the gobject-introspection devs, ideally with a smaller test case, though the
problem here is pretty clear cut.

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