From c291c20a3b8350a6960311afc8f08923c4b40d47 Mon Sep 17 00:00:00 2001
From: Badlop <badlop@process-one.net>
Date: Fri, 22 Nov 2024 19:13:46 +0100
Subject: [PATCH] Fix problem starting ejabberd when first host uses SQL, other
 one mnesia

The problem appeared when there are several vhosts,
the first vhost uses SQL for persistent data (and RAM for volatile),
and another vhost wants to use Mnesia

Example config to trigger the problem:

  hosts:
    - mysql.localhost
    - localhost
  host_config:
    mysql.localhost:
      default_db: sql

In that case, ejabberd crashed at start with an error like:
[critical] Internal error of module mod_muc has occurred during start:
...
** exception exit: {aborted,
                     {no_exists,
                         [muc_room,
                          [{{muc_room,{'_',<<"conference.localhost">>},'_'},
                            [],
                            ['$_']}]]}}
---
 src/mod_muc_mnesia.erl | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/mod_muc_mnesia.erl b/src/mod_muc_mnesia.erl
index 5be5b7928..d5a89b36d 100644
--- a/src/mod_muc_mnesia.erl
+++ b/src/mod_muc_mnesia.erl
@@ -57,6 +57,10 @@ init(Host, Opts) ->
 	    transient, 5000, worker, [?MODULE]},
     case supervisor:start_child(ejabberd_backend_sup, Spec) of
 	{ok, _Pid} -> ok;
+        %% Maybe started for a vhost which only wanted mnesia for ram
+        %% and this vhost wants mnesia for persitent storage too
+        {error, {already_started, _Pid}} ->
+            init([Host, Opts]);
 	Err -> Err
     end.