Fix hang in on_load handlers in embedded mode

In embedded mode, all on_load handlers will be called after
loading all modules but before starting any servers. Therefore,
if an on_load handler calls any function in the code module that
calls the code server (such as code:priv_dir/1), there will be a
deadlock because the code server has not yet been started.

Fix this problem by invoking the on_load handlers after
having started most servers in the kernel application.

2 files changed, 22 insertions, 19 deletions

diff --git a/erts/preloaded/ebin/init.beam b/erts/preloaded/ebin/init.beam
index f1b54b7fcb..f8af88b899 100644
--- a/erts/preloaded/ebin/init.beam
+++ b/erts/preloaded/ebin/init.beam
diff --git a/erts/preloaded/src/init.erl b/erts/preloaded/src/init.erl
index 3b98b9cddc..255f33c299 100644
--- a/erts/preloaded/src/init.erl
+++ b/erts/preloaded/src/init.erl
@@ -51,6 +51,9 @@
 	 get_status/0,boot/1,get_arguments/0,get_plain_arguments/0,
 	 get_argument/1,script_id/0]).
 
+%% for the on_load functionality; not for general use
+-export([run_on_load_handlers/0]).
+
 %% internal exports
 -export([fetch_loaded/0,ensure_loaded/1,make_permanent/2,
 	 notify_when_started/1,wait_until_started/0, 
@@ -308,24 +311,6 @@ boot_loop(BootPid, State) ->
 	{stop,Reason} ->
 	    stop(Reason,State);
 	{From,fetch_loaded} ->   %% Fetch and reset initially loaded modules.
-	    case whereis(?ON_LOAD_HANDLER) of
-		undefined ->
-		    %% There is no on_load handler process,
-		    %% probably because init:restart/0 has been
-		    %% called and it is not the first time we
-		    %% pass through here.
-		    ok;
-		Pid when is_pid(Pid) ->
-		    Pid ! run_on_load,
-		    receive
-			{'EXIT',Pid,on_load_done} ->
-			    ok;
-			{'EXIT',Pid,Res} ->
-			    %% Failure to run an on_load handler.
-			    %% This is fatal during start-up.
-			    exit(Res)
-		    end
-	    end,
 	    From ! {init,State#state.loaded},
 	    garb_boot_loop(BootPid,State#state{loaded = []});
 	{From,{ensure_loaded,Module}} ->
@@ -1335,9 +1320,27 @@ archive_extension() ->
 %%% Support for handling of on_load functions.
 %%%
 
+run_on_load_handlers() ->
+    Ref = monitor(process, ?ON_LOAD_HANDLER),
+    catch ?ON_LOAD_HANDLER ! run_on_load,
+    receive
+	{'DOWN',Ref,process,_,noproc} ->
+	    %% There is no on_load handler process,
+	    %% probably because init:restart/0 has been
+	    %% called and it is not the first time we
+	    %% pass through here.
+	    ok;
+	{'DOWN',Ref,process,_,on_load_done} ->
+	    ok;
+	{'DOWN',Ref,process,_,Res} ->
+	    %% Failure to run an on_load handler.
+	    %% This is fatal during start-up.
+	    exit(Res)
+    end.
+
 start_on_load_handler_process() ->
     register(?ON_LOAD_HANDLER,
-	     spawn_link(fun on_load_handler_init/0)).
+	     spawn(fun on_load_handler_init/0)).
 
 on_load_handler_init() ->
     on_load_loop([]).