Why is GstBuffer not writable in the _fill method of GstPushSrc?

[Ben Rush]

When implementing a GstPushSrc-derived class, I set the fill method to my
own method. This fill method gets passed to it a GstBuffer that,
presumably, is to be filled by the method. However, if I call

GstMapInfo info;
gst_buffer_map(buffer, &info, GST_MAP_WRITE);

The data property of the info object is null. I can't copy data to a null
pointer. I've been able to successfully use GstVideoFrame
and gst_video_frame_map and write data to the video frame. So why can't I
directly write to the buffer that's passed?

As it turns out I'm reading data from a camera in YUV420, and don't want to
incur any penalty for copying buffers around. I want to be able to just
copy the frame data directly onto my buffer object.
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