gstreamer: add gst_macos_main()

Unlike the C version, this allows for any arbitrary type of return value.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1377>

gstreamer/src/lib.rs

@@ -58,6 +58,11 @@ pub use crate::log::{
     CAT_REGISTRY, CAT_RUST, CAT_SCHEDULING, CAT_SIGNAL, CAT_STATES,
 };
 
+#[cfg(target_os = "macos")]
+mod macos;
+#[cfg(target_os = "macos")]
+pub use crate::macos::*;
+
 #[macro_use]
 mod error;
 pub use crate::error::*;

gstreamer/src/macos.rs

@@ -0,0 +1,37 @@
+// Take a look at the license at the top of the repository in the LICENSE file.
+
+type GstMainFuncSimple = Option<unsafe extern "C" fn(glib::ffi::gpointer)>;
+
+#[link(name = "gstreamer-1.0")]
+extern "C" {
+    #[cfg(feature = "v1_22")]
+    fn gst_macos_main_simple(func: GstMainFuncSimple, user_data: glib::ffi::gpointer);
+}
+
+#[cfg(feature = "v1_22")]
+#[cfg_attr(docsrs, doc(cfg(feature = "v1_22")))]
+#[doc(alias = "gst_macos_main")]
+pub fn macos_main<T, F>(func: F) -> T
+where
+    F: FnOnce() -> T + Send,
+{
+    skip_assert_initialized!();
+    unsafe extern "C" fn trampoline<T, F: FnOnce() -> T + Send>(user_data: glib::ffi::gpointer) {
+        let data = &mut *(user_data as *mut (Option<F>, Option<T>));
+        let func = data.0.take().unwrap();
+        let res = func();
+
+        data.1 = Some(res);
+    }
+
+    let mut func: (Option<F>, Option<T>) = (Some(func), None);
+
+    unsafe {
+        gst_macos_main_simple(
+            Some(trampoline::<T, F>),
+            &mut func as *mut (Option<F>, Option<T>) as *mut _,
+        );
+    }
+
+    func.1.unwrap()
+}