fix(ipc): never adopt an empty id from the main IPC (#15626)

src/ipc.rs

@@ -1689,7 +1689,12 @@ pub fn clear_trusted_devices() {
 }
 
 pub fn get_id() -> String {
+    // An empty id may come from a process that took over the main IPC with a
+    // config scope that has no id yet (e.g. a user GUI that became the server
+    // while the installed service was restarting). Treat it as no answer,
+    // otherwise the empty id is adopted below and wipes the local one.
     if let Ok(Some(v)) = get_config("id") {
+        if !v.is_empty() {
             // update salt also, so that next time reinstallation not causing first-time auto-login failure
             if let Ok(Some(v2)) = get_config("salt") {
                 Config::set_salt(&v2);
@@ -1698,10 +1703,10 @@ pub fn get_id() -> String {
                 Config::set_key_confirmed(false);
                 Config::set_id(&v);
             }
-        v
+            return v;
-    } else {
-        Config::get_id()
         }
+    }
+    Config::get_id()
 }
 
 pub async fn get_rendezvous_server(ms_timeout: u64) -> (String, Vec<String>) {