webrtcsrc: expose MSID property on source pad

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1789>

docs/plugins/gst_plugins_cache.json

@@ -11261,7 +11261,21 @@
                     "GInitiallyUnowned",
                     "GObject"
                 ],
-                "kind": "object"
+                "kind": "object",
+                "properties": {
+                    "msid": {
+                        "blurb": "Remote MediaStream ID in use for this pad",
+                        "conditionally-available": false,
+                        "construct": false,
+                        "construct-only": false,
+                        "controllable": false,
+                        "default": "NULL",
+                        "mutable": "null",
+                        "readable": true,
+                        "type": "gchararray",
+                        "writable": false
+                    }
+                }
             }
         },
         "package": "gst-plugin-webrtc",

net/webrtc/src/webrtcsrc/imp.rs

@@ -491,6 +491,8 @@ impl Session {
                 self.id
             );
             webrtcbin_pad.store_sticky_event(&builder.build()).ok();
+
+            srcpad.imp().set_webrtc_pad(webrtcbin_pad.downgrade());
         }
 
         let ghostpad = gst::GhostPad::builder(gst::PadDirection::Src)

net/webrtc/src/webrtcsrc/pad.rs

@@ -1,7 +1,9 @@
 // SPDX-License-Identifier: MPL-2.0
 
 use gst::glib;
+use gst::prelude::*;
 use gst::subclass::prelude::*;
+use once_cell::sync::Lazy;
 use std::sync::atomic::AtomicBool;
 use std::sync::atomic::Ordering;
 use std::sync::Mutex;
@@ -10,6 +12,7 @@ use std::sync::Mutex;
 pub struct WebRTCSrcPad {
     needs_raw: AtomicBool,
     stream_id: Mutex<Option<String>>,
+    webrtcbin_pad: Mutex<Option<gst::glib::WeakRef<gst::Pad>>>,
 }
 
 impl WebRTCSrcPad {
@@ -29,6 +32,10 @@ impl WebRTCSrcPad {
         let stream_id = self.stream_id.lock().unwrap();
         stream_id.as_ref().unwrap().clone()
     }
+
+    pub fn set_webrtc_pad(&self, pad: glib::object::WeakRef<gst::Pad>) {
+        *self.webrtcbin_pad.lock().unwrap() = Some(pad);
+    }
 }
 
 #[glib::object_subclass]
@@ -38,7 +45,33 @@ impl ObjectSubclass for WebRTCSrcPad {
     type ParentType = gst::GhostPad;
 }
 
-impl ObjectImpl for WebRTCSrcPad {}
+impl ObjectImpl for WebRTCSrcPad {
+    fn properties() -> &'static [glib::ParamSpec] {
+        static PROPS: Lazy<Vec<glib::ParamSpec>> = Lazy::new(|| {
+            vec![glib::ParamSpecString::builder("msid")
+                .flags(glib::ParamFlags::READABLE)
+                .blurb("Remote MediaStream ID in use for this pad")
+                .build()]
+        });
+        PROPS.as_ref()
+    }
+    fn property(&self, _id: usize, pspec: &glib::ParamSpec) -> glib::Value {
+        match pspec.name() {
+            "msid" => {
+                let msid = self
+                    .webrtcbin_pad
+                    .lock()
+                    .unwrap()
+                    .as_ref()
+                    .and_then(|p| p.upgrade())
+                    .map(|p| p.property::<String>("msid"));
+
+                msid.to_value()
+            }
+            name => panic!("no readable property {name:?}"),
+        }
+    }
+}
 impl GstObjectImpl for WebRTCSrcPad {}
 impl PadImpl for WebRTCSrcPad {}
 impl ProxyPadImpl for WebRTCSrcPad {}