factor server_name change check into a reusable fn

src/service/globals.rs

@@ -305,6 +305,37 @@ impl Service {
         Ok(s)
     }
 
+    /// Check if `server_name` in the DB and config differ, return error if so
+    ///
+    /// Matrix resource ownership is based on the server name; changing it
+    /// requires recreating the database from scratch. This check needs to be
+    /// done before background tasks are started to avoid data races.
+    // Allowed because this function calls `services()`
+    #[allow(clippy::unused_self)]
+    pub(crate) fn err_if_server_name_changed(
+        &self,
+    ) -> Result<(), crate::error::ServerNameChanged> {
+        use crate::error::ServerNameChanged as Error;
+
+        if services()
+            .users
+            .count()
+            .map(|x| x > 0)
+            .map_err(Error::NonZeroUsers)?
+        {
+            let admin_bot = self.admin_bot_user_id.as_ref();
+            if !services()
+                .users
+                .exists(admin_bot)
+                .map_err(Error::AdminBotExists)?
+            {
+                return Err(Error::Renamed);
+            }
+        }
+
+        Ok(())
+    }
+
     /// Returns this server's keypair.
     pub(crate) fn keypair(&self) -> &ruma::signatures::Ed25519KeyPair {
         &self.keypair