Enable a GMainLoop when capturing using GStreamer

A running GMainLoop processes many events on the GLib/GStreamer
world. While some things may work without it, many others wont.
Examples of these are signals, timers and many other source
events. The problem becomes more concerning by the fact that
some GStreamer elements rely on signals to work.

This commit allows the user to specify an OpenCV option to
start a main loop, if needed. Since the loop blocks, this is
done in a separate thread.

modules/videoio/src/cap_gstreamer.cpp

@@ -54,6 +54,7 @@
 
 #include <iostream>
 #include <string.h>
+#include <thread>
 
 #include <gst/gst.h>
 #include <gst/gstbuffer.h>
@@ -107,6 +108,7 @@ template<> inline void GSafePtr_release<GstBuffer>(GstBuffer** pPtr) { if (pPtr)
 template<> inline void GSafePtr_release<GstSample>(GstSample** pPtr) { if (pPtr) { gst_sample_unref(*pPtr); *pPtr = NULL; } }
 template<> inline void GSafePtr_release<GstBus>(GstBus** pPtr) { if (pPtr) { gst_object_unref(G_OBJECT(*pPtr)); *pPtr = NULL; } }
 template<> inline void GSafePtr_release<GstMessage>(GstMessage** pPtr) { if (pPtr) { gst_message_unref(*pPtr); *pPtr = NULL; } }
+template<> inline void GSafePtr_release<GMainLoop>(GMainLoop** pPtr) { if (pPtr) { g_main_loop_unref(*pPtr); *pPtr = NULL; } }
 
 template<> inline void GSafePtr_release<GstEncodingVideoProfile>(GstEncodingVideoProfile** pPtr) { if (pPtr) { gst_encoding_profile_unref(*pPtr); *pPtr = NULL; } }
 template<> inline void GSafePtr_release<GstEncodingContainerProfile>(GstEncodingContainerProfile** pPtr) { if (pPtr) { gst_object_unref(G_OBJECT(*pPtr)); *pPtr = NULL; } }
@@ -194,10 +196,15 @@ public:
 private:
     bool isFailed;
     bool call_deinit;
+    bool start_loop;
+    GSafePtr<GMainLoop> loop;
+    std::thread thread;
+
     gst_initializer() :
         isFailed(false)
     {
         call_deinit = utils::getConfigurationParameterBool("OPENCV_VIDEOIO_GSTREAMER_CALL_DEINIT", false);
+        start_loop = utils::getConfigurationParameterBool("OPENCV_VIDEOIO_GSTREAMER_START_MAINLOOP", false);
 
         GSafePtr<GError> err;
         gst_init_check(NULL, NULL, err.getRef());
@@ -215,6 +222,14 @@ private:
             isFailed = true;
             return;
         }
+
+        if (start_loop)
+        {
+            loop.attach(g_main_loop_new (NULL, FALSE));
+            thread = std::thread([this](){
+                g_main_loop_run (loop);
+            });
+        }
     }
     ~gst_initializer()
     {
@@ -223,6 +238,12 @@ private:
             // Debug leaks: GST_LEAKS_TRACER_STACK_TRACE=1 GST_DEBUG="GST_TRACER:7" GST_TRACERS="leaks"
             gst_deinit();
         }
+
+        if (start_loop)
+        {
+            g_main_loop_quit(loop);
+            thread.join();
+        }
     }
 };