kernel: Fix loading of prim_tty nif in embedded mode

When running in embedded mode the on_load function is called
by the init process after the user processes are started. So
we add a way for user to call the on_load function earlier
in the boot sequence so that the shell can be started when
it should.

We cannot move the on_load calls for all modules to an earlier
place in the kernel boot sequence as the code in on_load may
depend on kernel services being started.

erts/preloaded/src/init.erl

@@ -53,7 +53,7 @@
 	 get_argument/1,script_id/0,script_name/0]).
 
 %% for the on_load functionality; not for general use
--export([run_on_load_handlers/0]).
+-export([run_on_load_handlers/0, run_on_load_handlers/1]).
 
 %% internal exports
 -export([fetch_loaded/0,ensure_loaded/1,make_permanent/2,
@@ -1463,8 +1463,11 @@ archive_extension() ->
 %%%
 
 run_on_load_handlers() ->
+    run_on_load_handlers(all).
+
+run_on_load_handlers(Mods) when is_list(Mods); Mods =:= all ->
     Ref = monitor(process, ?ON_LOAD_HANDLER),
-    catch ?ON_LOAD_HANDLER ! run_on_load,
+    catch ?ON_LOAD_HANDLER ! {run_on_load, self(), Ref, Mods},
     receive
 	{'DOWN',Ref,process,_,noproc} ->
 	    %% There is no on_load handler process,
@@ -1477,7 +1480,9 @@ run_on_load_handlers() ->
 	{'DOWN',Ref,process,_,Res} ->
 	    %% Failure to run an on_load handler.
 	    %% This is fatal during start-up.
-	    exit(Res)
+	    exit(Res);
+        {reply, Ref, on_load_done} ->
+            ok
     end.
 
 start_on_load_handler_process() ->
@@ -1493,9 +1498,14 @@ on_load_loop(Mods, Debug0) ->
 	    on_load_loop(Mods, Debug);
 	{loaded,Mod} ->
 	    on_load_loop([Mod|Mods], Debug0);
-	run_on_load ->
+	{run_on_load, _, _, all} ->
 	    run_on_load_handlers(Mods, Debug0),
-	    exit(on_load_done)
+	    exit(on_load_done);
+        {run_on_load, From, Ref, ModsToRun} ->
+            [run_on_load_handlers([Mod], Debug0)
+             || Mod <- ModsToRun, lists:member(Mod, Mods)],
+            From ! {reply, Ref, on_load_done},
+            on_load_loop(Mods -- ModsToRun, Debug0)
     end.
 
 run_on_load_handlers([M|Ms], Debug) ->

lib/kernel/src/user_drv.erl

@@ -119,6 +119,9 @@ callback_mode() -> state_functions.
 -spec init(arguments()) -> gen_statem:init_result(init).
 init(Args) ->
     process_flag(trap_exit, true),
+    %% When running in embedded mode we need to call prim_tty:on_load manually here
+    %% as the automatic call happens after user is started.
+    ok = init:run_on_load_handlers([prim_tty]),
     IsTTY = prim_tty:isatty(stdin) =:= true andalso prim_tty:isatty(stdout) =:= true,
     StartShell = maps:get(initial_shell, Args, undefined) =/= noshell,
     try