gstreamer-app: Assert that GStreamer is initialized before creating instances

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1193>

gstreamer-app/src/app_sink.rs

@@ -273,6 +273,7 @@ impl AppSink {
     ///
     /// This method returns an instance of [`AppSinkBuilder`](crate::builders::AppSinkBuilder) which can be used to create [`AppSink`] objects.
     pub fn builder() -> AppSinkBuilder {
+        assert_initialized_main_thread!();
         AppSinkBuilder::default()
     }
 

gstreamer-app/src/app_src.rs

@@ -210,6 +210,7 @@ impl AppSrc {
     ///
     /// This method returns an instance of [`AppSrcBuilder`](crate::builders::AppSrcBuilder) which can be used to create [`AppSrc`] objects.
     pub fn builder() -> AppSrcBuilder {
+        assert_initialized_main_thread!();
         AppSrcBuilder::default()
     }
 

gstreamer-app/src/lib.rs

@@ -10,6 +10,19 @@ pub use glib;
 pub use gst;
 pub use gst_base;
 
+macro_rules! assert_initialized_main_thread {
+    () => {
+        if !gst::INITIALIZED.load(std::sync::atomic::Ordering::SeqCst) {
+            #[allow(unused_unsafe)]
+            if unsafe { gst::ffi::gst_is_initialized() } != glib::ffi::GTRUE {
+                panic!("GStreamer has not been initialized. Call `gst::init` first.");
+            } else {
+                gst::INITIALIZED.store(true, std::sync::atomic::Ordering::SeqCst);
+            }
+        }
+    };
+}
+
 macro_rules! skip_assert_initialized {
     () => {};
 }