rabbit_db_cluster: Figure out members list even if Mnesia is stopped

[Why]
If Mnesia is stopped and the local node is a disk-less member, the list
returned by `mnesia:system_info(db_nodes)` will be empty. This is
problematic for any users of this list of members obviously.

[How]
The `members_using_mnesia/0` function copies the logic of
`rabbit_mnesia:cluster_status/1` but simplifies it to only work with all
members (not running members or disk members). Therefore, if Mnesia is
not running, we look at the cluster status files stored on disk. If they
are missing, the code falls back to a list made of the local node name
only.

Author: dumbbell

deps/rabbit/src/rabbit_db_cluster.erl

@@ -8,7 +8,6 @@
 -module(rabbit_db_cluster).
 
 -include_lib("kernel/include/logger.hrl").
--include_lib("stdlib/include/assert.hrl").
 
 -include_lib("rabbit_common/include/logging.hrl").
 
@@ -103,7 +102,25 @@ members() ->
     members_using_mnesia().
 
 members_using_mnesia() ->
-    mnesia:system_info(db_nodes).
+    case rabbit_mnesia:is_running() andalso rabbit_table:is_present() of
+        true ->
+            %% If Mnesia is running locally and some tables exist, we can know
+            %% the database was initialized and we can query the list of
+            %% members.
+            mnesia:system_info(db_nodes);
+        false ->
+            try
+                %% When Mnesia is not running, we fall back to reading the
+                %% cluster status files stored on disk, if they exist.
+                {Members, _, _} = rabbit_node_monitor:read_cluster_status(),
+                Members
+            catch
+                throw:{error, _Reason}:_Stacktrace ->
+                    %% If we couldn't read those files, we consider that only
+                    %% this node is part of the "cluster".
+                    [node()]
+            end
+    end.
 
 -spec disc_members() -> Members when
       Members :: [node()].

deps/rabbit/src/rabbit_mnesia.erl

@@ -19,6 +19,7 @@
 
          %% Various queries to get the status of the db
          status/0,
+         is_running/0,
          is_clustered/0,
          on_running_node/1,
          is_process_alive/1,