factor server_name change check into a reusable fn

src/cli/serve.rs

@@ -84,15 +84,7 @@ pub(crate) async fn run(args: ServeArgs) -> Result<(), error::ServeCommand> {
             .map_err(Error::InitializeServices)?,
     )));
 
-    // Matrix resource ownership is based on the server name; changing it
+    services().globals.err_if_server_name_changed()?;
-    // requires recreating the database from scratch. This check needs to be
-    // done before background tasks are started to avoid data races.
-    if services().users.count().map(|x| x > 0).map_err(Error::NonZeroUsers)? {
-        let admin_bot = services().globals.admin_bot_user_id.as_ref();
-        if !services().users.exists(admin_bot).map_err(Error::AdminBotExists)? {
-            return Err(Error::Renamed);
-        }
-    }
 
     db.apply_migrations().await.map_err(Error::DatabaseError)?;
 

src/error.rs

@@ -65,6 +65,16 @@ pub(crate) enum ServeCommand {
     #[error("failed to initialize services")]
     InitializeServices(#[source] crate::utils::error::Error),
 
+    #[error("`server_name` change check failed")]
+    ServerNameChanged(#[from] ServerNameChanged),
+}
+
+/// Error generated if `server_name` has changed or if checking this failed
+// Missing docs are allowed here since that kind of information should be
+// encoded in the error messages themselves anyway.
+#[allow(missing_docs)]
+#[derive(Error, Debug)]
+pub(crate) enum ServerNameChanged {
     #[error("failed to check if there are any users")]
     NonZeroUsers(#[source] crate::utils::error::Error),
 

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