It's possible somehow we could be left with an empty mnesia directory (spotted with "make cleandb" but maybe other things could do this). So instead check for the existence of the guid serial, which has not changed since at least 1.7.0.

Author: Simon MacMullen

src/rabbit_guid.erl

@@ -19,6 +19,7 @@
 -behaviour(gen_server).
 
 -export([start_link/0]).
+-export([filename/0]).
 -export([gen/0, gen_secure/0, string/2, binary/2]).
 
 -export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2,
@@ -38,6 +39,7 @@
 -type(guid() :: binary()).
 
 -spec(start_link/0 :: () -> rabbit_types:ok_pid_or_error()).
+-spec(filename/0 :: () -> string()).
 -spec(gen/0 :: () -> guid()).
 -spec(gen_secure/0 :: () -> guid()).
 -spec(string/2 :: (guid(), any()) -> string()).
@@ -51,8 +53,14 @@ start_link() ->
     gen_server:start_link({local, ?SERVER}, ?MODULE,
                           [update_disk_serial()], []).
 
+%% We use this to detect a (possibly rather old) Mnesia directory,
+%% since it has existed since at least 1.7.0 (as far back as I cared
+%% to go).
+filename() ->
+    filename:join(rabbit_mnesia:dir(), ?SERIAL_FILENAME).
+
 update_disk_serial() ->
-    Filename = filename:join(rabbit_mnesia:dir(), ?SERIAL_FILENAME),
+    Filename = filename(),
     Serial = case rabbit_file:read_term_file(Filename) of
                  {ok, [Num]}     -> Num;
                  {error, enoent} -> 0;

src/rabbit_version.erl

@@ -96,7 +96,7 @@ record_desired_for_scope(Scope) ->
 upgrades_required(Scope) ->
     case recorded_for_scope(Scope) of
         {error, enoent} ->
-            case filelib:is_dir(rabbit_mnesia:dir() ++ "/") of
+            case filelib:is_file(rabbit_guid:filename()) of
                 false -> {ok, []}; %% We're blank, no upgrade needed
                 true  -> {error, version_not_available}
             end;