make webrtc-instance-creation testable

src/chat.rs

@@ -1629,13 +1629,7 @@ pub async fn send_videochat_invitation(context: &Context, chat_id: ChatId) -> Re
         bail!("webrtc_instance not set");
     };
 
-    let room = dc_create_id();
-
-    let instance = if instance.contains("$ROOM") {
-        instance.replace("$ROOM", &room)
-    } else {
-        format!("{}{}", instance, room)
-    };
+    let instance = Message::create_webrtc_instance(&instance, &dc_create_id());
 
     let mut msg = Message::new(Viewtype::VideochatInvitation);
     msg.param.set(Param::WebrtcRoom, &instance);

src/message.rs

@@ -643,6 +643,16 @@ impl Message {
         None
     }
 
+    // add room to a webrtc_instance as defined by the corresponding config-value;
+    // the result may still be prefixed by the type
+    pub fn create_webrtc_instance(instance: &str, room: &str) -> String {
+        if instance.contains("$ROOM") {
+            instance.replace("$ROOM", &room)
+        } else {
+            format!("{}{}", instance, room)
+        }
+    }
+
     /// split a webrtc_instance as defined by the corresponding config-value into a type and a url
     pub fn parse_webrtc_instance(instance: &str) -> (VideochatType, String) {
         let mut split = instance.splitn(2, ':');