webrtc: SessionDescription: access the inner SDPMessage as & or &mut

`WebRTCSessionDescription` owns its `SDPMessage`. The `sdp()` accessor used to
return a copy of the `SDPMessage` which prevented the user from getting a ref
and by extension from getting a mutable ref for in-place modification.

This commit makes the accessor return a reference to the inner `SDPMessage` and
adds a mutable accessor.

Previous behaviour (getting an owned copy of the `SDPMessage`) is available by
calling `to_owned()` on the reference returned by `sdp()`.

Users who wish to change the type of `WebRTCSessionDescription` can call
`set_type()`.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/1406>

gstreamer-webrtc/src/web_rtc_session_description.rs

@@ -1,7 +1,5 @@
 // Take a look at the license at the top of the repository in the LICENSE file.
 
-use std::mem;
-
 use glib::translate::*;
 
 use crate::{WebRTCSDPType, WebRTCSessionDescription};
@@ -11,10 +9,9 @@ impl WebRTCSessionDescription {
     pub fn new(type_: WebRTCSDPType, sdp: gst_sdp::SDPMessage) -> WebRTCSessionDescription {
         skip_assert_initialized!();
         unsafe {
-            let mut sdp = mem::ManuallyDrop::new(sdp);
             from_glib_full(ffi::gst_webrtc_session_description_new(
                 type_.into_glib(),
-                sdp.to_glib_none_mut().0,
+                sdp.into_glib_ptr(),
             ))
         }
     }
@@ -24,8 +21,58 @@ impl WebRTCSessionDescription {
         unsafe { from_glib((*self.as_ptr()).type_) }
     }
 
+    // rustdoc-stripper-ignore-next
+    /// Changes the type of this Session Description to the specified variant.
+    pub fn set_type(&mut self, type_: WebRTCSDPType) {
+        unsafe {
+            (*self.as_ptr()).type_ = type_.into_glib();
+        }
+    }
+
     #[doc(alias = "get_sdp")]
-    pub fn sdp(&self) -> gst_sdp::SDPMessage {
-        unsafe { from_glib_none((*self.as_ptr()).sdp) }
+    pub fn sdp(&self) -> &gst_sdp::SDPMessageRef {
+        unsafe { &*((*self.as_ptr()).sdp as *const gst_sdp::SDPMessageRef) }
+    }
+
+    pub fn sdp_mut(&mut self) -> &mut gst_sdp::SDPMessageRef {
+        unsafe { &mut *((*self.as_ptr()).sdp as *mut gst_sdp::SDPMessageRef) }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::WebRTCSDPType;
+    use gst_sdp::SDPMessage;
+
+    #[test]
+    fn change_type() {
+        gst::init().unwrap();
+
+        let mut desc =
+            crate::WebRTCSessionDescription::new(crate::WebRTCSDPType::Offer, SDPMessage::new());
+        assert_eq!(desc.type_(), WebRTCSDPType::Offer);
+
+        desc.set_type(WebRTCSDPType::Rollback);
+        assert_eq!(desc.type_(), WebRTCSDPType::Rollback);
+    }
+
+    #[test]
+    fn update_inner_msg() {
+        gst::init().unwrap();
+
+        let mut sdp = SDPMessage::new();
+        sdp.set_information("init");
+
+        let mut desc = crate::WebRTCSessionDescription::new(WebRTCSDPType::Offer, sdp);
+        assert_eq!(desc.sdp().information(), Some("init"));
+
+        let sdp_owned = desc.sdp().to_owned();
+
+        // update inner sdp message
+        desc.sdp_mut().set_information("update");
+        assert_eq!(desc.sdp().information(), Some("update"));
+
+        // previously acquired owned sdp message unchanged
+        assert_eq!(sdp_owned.information(), Some("init"));
     }
 }