horus: Work around race in cover:is_compiled/1

[Why]
The `cover:call/1` internal checks if the cover server is running and
starts it if it's not:

    Ref = erlang:monitor(process,?SERVER),
    receive {'DOWN', Ref, _Type, _Object, noproc} ->
            erlang:demonitor(Ref),
            {ok,_} = start(),
	    ...

However, there is no lock between the check and
the start. Therefore, any concurrent uses of this code may cause a
`badmatch` crash on `{ok,_} = start()` because it might have been
started in parallel and the function returns the `already_started`
error.

[How]
To work around the problem, we catch the `badmatch` and retry once after
a sleep of 100 ms. We don't try to catch any exception from that second
attempt.

Author: dumbbell

src/horus.erl

@@ -1998,7 +1998,7 @@ get_object_code(Module) ->
 %% @private
 
 do_get_object_code(Module) ->
-    case cover:is_compiled(Module) of
+    case is_cover_compiled(Module) of
         false ->
             get_object_code_from_code_server(Module);
         {file, Filename} ->
@@ -2008,6 +2008,18 @@ do_get_object_code(Module) ->
             erpc:call(CoverMainNode, ?MODULE, do_get_object_code, [Module])
     end.
 
+is_cover_compiled(Module) ->
+    try
+        cover:is_compiled(Module)
+    catch
+        error:{badmatch, {error, {already_started, _}}} ->
+            %% The code in `cover' that checks if the cover server is started
+            %% seem racy. We get an exception if it thinks it has to start it
+            %% but it is already started. In this case, we retry the call.
+            timer:sleep(100),
+            cover:is_compiled(Module)
+    end.
+
 get_object_code_from_code_server(Module) ->
     case code:get_object_code(Module) of
         {Module, Beam, Filename} ->