Registering an element that's derived from another element in the same plugin

[Tony Houghton]

I want my plugin to have one "public" (registered) element, GstHtspSrc,
that acts as a URI handler which chooses between two different source
elements depending on the path of the URL. This is because tvheadend's HTS
protocol has major differences depending on whether it's streaming live TV
(pushes packet messages) or a recording (client reads chunks of a file in
mpeg transport stream format on demand).

The latter src element seemed easy enough to implement by basing it on
filesrc, but I thought the easiest way to test it would be to write a
preliminary version of GstHtspSrc which derives from GstHtspRecordingSrc
(which in turn derives from GstBaseSrc) and implements GstUriHandler for it
instead of diving straight into the extra complication of a manager element.

The trouble is my plugin isn't working. It causes gst-inspect-1.0 to hang
when I call gst_element_register on GstHtspRecordingSrc. It seems to be
stuck in a loop of:

0:00:09.116526506 324933 0x55bbe89c7e00 LOG       GST_PLUGIN_LOADING
gstpluginloader.c:1027:exchange_packets: Poll res = 0. 0 bytes pending for
write
0:00:09.116557976 324933 0x55bbe89c7e00 DEBUG               GST_POLL
gstpoll.c:1241:gst_poll_fd_has_error: 0x55bbe8bee800: fd (fd:8, idx:2) 0
0:00:09.116565199 324933 0x55bbe89c7e00 DEBUG               GST_POLL
gstpoll.c:1266:gst_poll_fd_can_read_unlocked: 0x55bbe8bee800: fd (fd:8,
idx:2) 0
0:00:09.116571431 324933 0x55bbe89c7e00 DEBUG               GST_POLL
gstpoll.c:1195:gst_poll_fd_has_closed: 0x55bbe8bee800: fd (fd:8, idx:2) 0
0:00:09.116578815 324933 0x55bbe89c7e00 DEBUG               GST_POLL
gstpoll.c:1414:gst_poll_wait: 0x55bbe8bee800: timeout :0:00:01.000000000

and never ultimately returns from gst_element_register.

I thought the problem could be caused by deriving from an unregistered
element class. So I tried calling gst_element_register on the base
class, GstHtspRecordingSrc, before the subclass GstHtspSrc. The
registration of GstHtspRecordingSrc succeeds, but GstHtspSrc still fails.
If I don't try to register GstHtspSrc, gst-inspect runs successfully and
shows my plugin to be present with that element.

I've also added debugging to gst_htsp_src_class_init and seen that it
never gets called. So the only explanation seems to be that gst doesn't
like that GstHtspSrc is derived from another element in the same plugin,
whether or not the base element is registered first. Is this a known
limitation?

-- 
TH
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