[libcamera-devel] [RFC PATCH v4] gstreamer: Added virtual functions needed to support request pads

[Vedant Paranjape]

Using request pads needs virtual functions to create new request pads and
release the allocated pads. Added way to generate unique stream_id in
gst_libcamera_src_task_enter(). It failed to execute when more than one
pad was added.

This patch defines the functions gst_libcamera_src_request_new_pad() and
gst_libcamera_src_release_pad() which handle, creating and releasing
request pads. Also assigns these functions to their callback
handlers in GstLibcameraSrcClass.

gst_pad_create_stream_id() failed as this gstreamer element has more than 2
source pads, and a nullptr was passed to it's stream_id argument.
This function fails to auto generate a stream id for scenario of more
than one source pad present.

Used a gint which increments everytime new stream_id is requested and
group_id to generate an unique stream_id by appending them togther as
%i%i.

This doesn't enable request pad support in gstreamer element. This is
one of the series of changes needed to make it work.

Signed-off-by: Vedant Paranjape <vedantparanjape160201 at gmail.com>
---
In _request_new_pad and _release_pad, a GLibLock is taken on
self->state. But since self->state is a C++ object,
GST_OBJECT(self->state) fails and returns null, so GLibLocker throws a
warning "invalid cast from '(null)' to 'GstObject'. This means that it
actually fails to get lock the resource that needs to be protected,
i.e., the srcpads_ vector.

The following changes were tested using the following test code: https://pastebin.com/gNDF1cyG

 src/gstreamer/gstlibcamerasrc.cpp | 57 ++++++++++++++++++++++++++++++-
 1 file changed, 56 insertions(+), 1 deletion(-)

diff --git a/src/gstreamer/gstlibcamerasrc.cpp b/src/gstreamer/gstlibcamerasrc.cpp
index ccc61590..5bb8c46c 100644
--- a/src/gstreamer/gstlibcamerasrc.cpp
+++ b/src/gstreamer/gstlibcamerasrc.cpp
@@ -361,10 +361,11 @@ gst_libcamera_src_task_enter(GstTask *task, [[maybe_unused]] GThread *thread,
 	GST_DEBUG_OBJECT(self, "Streaming thread has started");
 
 	guint group_id = gst_util_group_id_next();
+	gint stream_id_num = 0;
 	StreamRoles roles;
 	for (GstPad *srcpad : state->srcpads_) {
 		/* Create stream-id and push stream-start. */
-		g_autofree gchar *stream_id = gst_pad_create_stream_id(srcpad, GST_ELEMENT(self), nullptr);
+		g_autofree gchar *stream_id = gst_pad_create_stream_id(srcpad, GST_ELEMENT(self), g_strdup_printf("%i%i", group_id, stream_id_num++));
 		GstEvent *event = gst_event_new_stream_start(stream_id);
 		gst_event_set_group_id(event, group_id);
 		gst_pad_push_event(srcpad, event);
@@ -640,6 +641,58 @@ gst_libcamera_src_init(GstLibcameraSrc *self)
 	self->state = state;
 }
 
+static GstPad*
+gst_libcamera_src_request_new_pad(GstElement *element, GstPadTemplate *templ,
+					const gchar *name, [[maybe_unused]] const GstCaps *caps)
+{
+	GstLibcameraSrc *self = GST_LIBCAMERA_SRC(element);
+	g_autoptr(GstPad) pad = NULL;
+
+	GST_DEBUG_OBJECT(self, "new request pad created");
+
+	pad = gst_pad_new_from_template(templ, name);
+	g_object_ref_sink(pad);
+	gboolean result = gst_element_add_pad(element, pad);
+
+	if (result)
+	{
+		GLibLocker lock(GST_OBJECT(self));
+		self->state->srcpads_.push_back(reinterpret_cast<GstPad*>(g_object_ref(pad)));
+	}
+	else
+	{
+		GST_ELEMENT_ERROR (element, STREAM, FAILED,
+					("Internal data stream error."),
+					("Could not add pad to element"));
+		return NULL;		
+	}
+	
+	return reinterpret_cast<GstPad*>(g_steal_pointer(&pad));
+}
+
+static void
+gst_libcamera_src_release_pad(GstElement *element, GstPad *pad)
+{
+	GstLibcameraSrc *self = GST_LIBCAMERA_SRC(element);
+	
+	GST_DEBUG_OBJECT (self, "Pad %" GST_PTR_FORMAT " being released", pad);
+
+	{
+		GLibLocker lock(GST_OBJECT(self));
+		std::vector<GstPad*> &pads = self->state->srcpads_;
+		auto begin_iterator = pads.begin();
+		auto end_iterator = pads.end();
+		auto pad_iterator = std::find(begin_iterator, end_iterator, pad);
+
+		if (pad_iterator != end_iterator)
+		{
+			g_object_unref(*pad_iterator);
+			pads.erase(pad_iterator);
+		}
+	}
+	gst_element_remove_pad(element, pad);
+}	
+
 static void
 gst_libcamera_src_class_init(GstLibcameraSrcClass *klass)
 {
@@ -650,6 +703,8 @@ gst_libcamera_src_class_init(GstLibcameraSrcClass *klass)
 	object_class->get_property = gst_libcamera_src_get_property;
 	object_class->finalize = gst_libcamera_src_finalize;
 
+	element_class->request_new_pad = gst_libcamera_src_request_new_pad;
+	element_class->release_pad = gst_libcamera_src_release_pad;
 	element_class->change_state = gst_libcamera_src_change_state;
 
 	gst_element_class_set_metadata(element_class,
-- 
2.25.1