Implement handler for NewAccessMethodEvent

This commit implements the daemon logic for handling a
`NewAccessMethodEvent`. Such an event occur when `AccessModeSelector`
announces that a new access method is active, and it will cause the
daemon to except some API endpoint in the firewall. It may conditionally
broadcast the new access method to all clients.

mullvad-daemon/src/lib.rs

@@ -1326,7 +1326,29 @@ where
         }
     }
 
-    fn handle_access_method_event(&mut self, event: NewAccessMethodEvent) {}
+    fn handle_access_method_event(&mut self, event: NewAccessMethodEvent) {
+        // Update the firewall to exempt a new API endpoint.
+        let (completion_tx, completion_rx) = oneshot::channel();
+        self.send_tunnel_command(TunnelCommand::AllowEndpoint(event.endpoint, completion_tx));
+        // If the `NewAccessMethodEvent` should be announced to any client
+        // listening for updates of the currently active access method, we need
+        // to clone the handle to the broadcaster of such events. The
+        // announcement should be made after the firewall policy has been
+        // updated, since the new access method will be useless before then.
+        let event_listener = self.event_listener.clone();
+        tokio::spawn(async move {
+            // Wait for the firewall policy to be updated.
+            let _ = completion_rx.await;
+            // Let the emitter of this event know that the firewall has been updated.
+            if let Some(event_source) = event.update_finished_tx {
+                let _ = event_source.send(());
+            }
+            // Notify clients about the change if necessary.
+            if event.announce {
+                event_listener.notify_new_access_method_event(event.setting);
+            }
+        });
+    }
 
     fn handle_device_migration_event(
         &mut self,