fix(wayland): subscribe to portal Response before making the request (#15726)

`request_remote_desktop` and its response handlers call the portal method
first and only then subscribe to the resulting Request's `Response` signal,
using the object path returned by the call. The comment above
`create_session` already describes why that is wrong:

> To avoid a race condition between the caller subscribing to the signal
> after receiving the reply for the method call and the signal getting
> emitted, a convention for Request object paths has been established that
> allows the caller to subscribe to the signal before making the method
> call.

The code then does the opposite of what the comment says. When the portal
emits `Response` before our match rule is installed, the signal is dropped
and the flow stalls: `request_remote_desktop` spins its 3-minute wait loop
and gives up, so the user sees the screen picker again (or a failure) even
when a valid restore token would have restored the session silently.

Build the request path from our unique bus name plus the `handle_token` we
pass in the call arguments, per the Request documentation, and subscribe
before calling. Applied to all five portal calls: CreateSession,
SelectSources (both the ScreenCast and post-SelectDevices paths),
SelectDevices, and Start. The `handle_token` values are unchanged; they are
now named locals so the path and the argument cannot drift apart.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

libs/scrap/src/wayland/pipewire.rs

@@ -507,6 +507,22 @@ where
     })
 }
 
+// The request object path a portal method call will use, derived from our unique
+// bus name and the `handle_token` we pass in the call arguments. Knowing it up
+// front lets us subscribe to the `Response` signal *before* making the call.
+// https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.Request.html
+fn get_request_path(
+    conn: &SyncConnection,
+    handle_token: &str,
+) -> Result<dbus::Path<'static>, dbus::Error> {
+    let sender = conn.unique_name().trim_start_matches(':').replace('.', "_");
+    dbus::Path::new(format!(
+        "/org/freedesktop/portal/desktop/request/{}/{}",
+        sender, handle_token
+    ))
+    .map_err(|_| dbus::Error::new_failed("Failed to construct portal request path"))
+}
+
 pub fn get_portal(conn: &SyncConnection) -> Proxy<&SyncConnection> {
     conn.with_proxy(
         "org.freedesktop.portal.Desktop",
@@ -632,13 +648,14 @@ pub fn request_remote_desktop(
     let failure_res = failure.clone();
     let session: Arc<Mutex<Option<dbus::Path>>> = Arc::new(Mutex::new(None));
     let session_res = session.clone();
+    let create_session_handle_token = "u1";
     args.insert(
         "session_handle_token".to_string(),
-        Variant(Box::new("u1".to_string())),
+        Variant(Box::new(create_session_handle_token.to_string())),
     );
     args.insert(
         "handle_token".to_string(),
-        Variant(Box::new("u1".to_string())),
+        Variant(Box::new(create_session_handle_token.to_string())),
     );
 
     let mut is_support_restore_token = false;
@@ -654,15 +671,9 @@ pub fn request_remote_desktop(
     // between the caller subscribing to the signal after receiving the reply for the method call and the signal getting emitted,
     // a convention for Request object paths has been established that allows
     // the caller to subscribe to the signal before making the method call.
-    let path;
-    if is_server_running() {
-        path = screencast_portal::create_session(&portal, args)?;
-    } else {
-        path = remote_desktop_portal::create_session(&portal, args)?;
-    }
     handle_response(
         &conn,
-        path,
+        get_request_path(&conn, create_session_handle_token)?,
         on_create_session_response(
             fd.clone(),
             streams.clone(),
@@ -673,6 +684,11 @@ pub fn request_remote_desktop(
         ),
         failure_res.clone(),
     )?;
+    if is_server_running() {
+        let _ = screencast_portal::create_session(&portal, args)?;
+    } else {
+        let _ = remote_desktop_portal::create_session(&portal, args)?;
+    }
 
     // wait 3 minutes for user interaction
     for _ in 0..1800 {
@@ -751,9 +767,10 @@ fn on_create_session_response(
                 // persist_mode may be configured by the user.
                 args.insert("persist_mode".to_string(), Variant(Box::new(2u32)));
             }
+            let select_sources_handle_token = "u3";
             args.insert(
                 "handle_token".to_string(),
-                Variant(Box::new("u3".to_string())),
+                Variant(Box::new(select_sources_handle_token.to_string())),
             );
             // https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.ScreenCast.html
             if is_server_running() {
@@ -769,42 +786,43 @@ fn on_create_session_response(
                 });
             }
 
-            let path = portal.select_sources(ses.clone(), args)?;
             handle_response(
                 c,
-                path,
+                get_request_path(c, select_sources_handle_token)?,
                 on_select_sources_response(
                     fd.clone(),
                     streams.clone(),
                     failure.clone(),
-                    ses,
+                    ses.clone(),
                     is_support_restore_token,
                 ),
                 failure.clone(),
             )?;
+            let _ = portal.select_sources(ses.clone(), args)?;
         } else {
             // TODO: support persist_mode for remote_desktop_portal
             // https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.RemoteDesktop.html
 
+            let select_devices_handle_token = "u2";
             args.insert(
                 "handle_token".to_string(),
-                Variant(Box::new("u2".to_string())),
+                Variant(Box::new(select_devices_handle_token.to_string())),
             );
             args.insert("types".to_string(), Variant(Box::new(7u32)));
 
-            let path = portal.select_devices(ses.clone(), args)?;
             handle_response(
                 c,
-                path,
+                get_request_path(c, select_devices_handle_token)?,
                 on_select_devices_response(
                     fd.clone(),
                     streams.clone(),
                     failure.clone(),
-                    ses,
+                    ses.clone(),
                     is_support_restore_token,
                 ),
                 failure.clone(),
             )?;
+            let _ = portal.select_devices(ses.clone(), args)?;
         }
 
         Ok(())
@@ -825,9 +843,10 @@ fn on_select_devices_response(
     move |_: OrgFreedesktopPortalRequestResponse, c, _| {
         let portal = get_portal(c);
         let mut args: PropMap = HashMap::new();
+        let select_sources_handle_token = "u3";
         args.insert(
             "handle_token".to_string(),
-            Variant(Box::new("u3".to_string())),
+            Variant(Box::new(select_sources_handle_token.to_string())),
         );
         // https://flatpak.github.io/xdg-desktop-portal/docs/doc-org.freedesktop.portal.ScreenCast.html
         if is_server_running() {
@@ -836,19 +855,19 @@ fn on_select_devices_response(
         args.insert("types".into(), Variant(Box::new(1u32))); //| 2u32)));
 
         let session = session.clone();
-        let path = portal.select_sources(session.clone(), args)?;
         handle_response(
             c,
-            path,
+            get_request_path(c, select_sources_handle_token)?,
             on_select_sources_response(
                 fd.clone(),
                 streams.clone(),
                 failure.clone(),
-                session,
+                session.clone(),
                 is_support_restore_token,
             ),
             failure.clone(),
         )?;
+        let _ = portal.select_sources(session.clone(), args)?;
 
         Ok(())
     }
@@ -868,19 +887,14 @@ fn on_select_sources_response(
     move |_: OrgFreedesktopPortalRequestResponse, c, _| {
         let portal = get_portal(c);
         let mut args: PropMap = HashMap::new();
+        let start_handle_token = "u4";
         args.insert(
             "handle_token".to_string(),
-            Variant(Box::new("u4".to_string())),
+            Variant(Box::new(start_handle_token.to_string())),
         );
-        let path;
-        if is_server_running() {
-            path = screencast_portal::start(&portal, session.clone(), "", args)?;
-        } else {
-            path = remote_desktop_portal::start(&portal, session.clone(), "", args)?;
-        }
         handle_response(
             c,
-            path,
+            get_request_path(c, start_handle_token)?,
             on_start_response(
                 fd.clone(),
                 streams.clone(),
@@ -889,6 +903,11 @@ fn on_select_sources_response(
             ),
             failure.clone(),
         )?;
+        if is_server_running() {
+            let _ = screencast_portal::start(&portal, session.clone(), "", args)?;
+        } else {
+            let _ = remote_desktop_portal::start(&portal, session.clone(), "", args)?;
+        }
 
         Ok(())
     }