fix(dashboard): handle add default user race condition

This can happen at least in tests, when nodes boot concurrently.

apps/emqx_dashboard/src/emqx_dashboard.app.src

@@ -2,7 +2,7 @@
 {application, emqx_dashboard, [
     {description, "EMQX Web Dashboard"},
     % strict semver, bump manually!
-    {vsn, "5.1.2"},
+    {vsn, "5.1.3"},
     {modules, []},
     {registered, [emqx_dashboard_sup]},
     {applications, [

apps/emqx_dashboard/src/emqx_dashboard_admin.erl

@@ -63,6 +63,8 @@
 
 -type emqx_admin() :: #?ADMIN{}.
 
+-define(USERNAME_ALREADY_EXISTS_ERROR, <<"username_already_exists">>).
+
 %%--------------------------------------------------------------------
 %% Mnesia bootstrap
 %%--------------------------------------------------------------------
@@ -214,7 +216,7 @@ add_user_(Username, Password, Role, Desc) ->
                 username => Username,
                 role => Role
             }),
-            mnesia:abort(<<"username_already_exist">>)
+            mnesia:abort(?USERNAME_ALREADY_EXISTS_ERROR)
     end.
 
 -spec remove_user(dashboard_username()) -> {ok, any()} | {error, any()}.
@@ -411,8 +413,16 @@ add_default_user(Username, Password) when ?EMPTY_KEY(Username) orelse ?EMPTY_KEY
     {ok, empty};
 add_default_user(Username, Password) ->
     case lookup_user(Username) of
-        [] -> do_add_user(Username, Password, ?ROLE_SUPERUSER, <<"administrator">>);
-        _ -> {ok, default_user_exists}
+        [] ->
+            case do_add_user(Username, Password, ?ROLE_SUPERUSER, <<"administrator">>) of
+                {error, ?USERNAME_ALREADY_EXISTS_ERROR} ->
+                    %% race condition: multiple nodes booting at the same time?
+                    {ok, default_user_exists};
+                Res ->
+                    Res
+            end;
+        _ ->
+            {ok, default_user_exists}
     end.
 
 %% ensure the `role` is correct when it is directly read from the table

apps/emqx_dashboard/test/emqx_dashboard_admin_SUITE.erl

@@ -69,7 +69,7 @@ t_add_user(_) ->
     false = maps:is_key(password, NewUser),
 
     %% add again
-    {error, <<"username_already_exist">>} =
+    {error, <<"username_already_exists">>} =
         emqx_dashboard_admin:add_user(AddUser, AddPassword, ?ROLE_SUPERUSER, AddDescription),
 
     %% add bad username