Change the expected return value for on_load functions

An on_load function is supposed to return 'true' to indicate
that the module should be loaded, and 'false' if it should be
unloaded. But returning any other term, as well as causing an
exception, will also unload the module.

Since we don't like boolean values mixed with other values,
change the expected return value as follows:

* If 'ok' is returned, the module will remain loaded and become
  callable.

* If any other value is returned (or an exception is generated),
  the module will be unloaded. Also, if the returned value is
  not an atom, send a warning message to the error_logger
  (using error_logger:warning_msg/2).

The new interpretation of the return value means that an on_load
function can now directly return the return value from
erlang:load_nif/2.

1 files changed, 22 insertions, 5 deletions

diff --git a/lib/kernel/src/code_server.erl b/lib/kernel/src/code_server.erl
index 018f7f41d2..d4e3f0bcf8 100644
--- a/lib/kernel/src/code_server.erl
+++ b/lib/kernel/src/code_server.erl
@@ -1479,13 +1479,12 @@ finish_on_load(Ref, OnLoadRes, #state{on_load=OnLoad0,moddb=Db}=State) ->
     end.
 
 finish_on_load_1(Mod, File, OnLoadRes, WaitingPids, Db) ->
-    Keep = if
-	       is_boolean(OnLoadRes) -> OnLoadRes;
-	       true -> false
-	   end,
+    Keep = OnLoadRes =:= ok,
     erlang:finish_after_on_load(Mod, Keep),
     Res = case Keep of
-	      false -> {error,on_load_failure};
+	      false ->
+		  finish_on_load_report(Mod, OnLoadRes),
+		  {error,on_load_failure};
 	      true ->
 		  ets:insert(Db, {Mod,File}),
 		  {module,Mod}
@@ -1493,6 +1492,24 @@ finish_on_load_1(Mod, File, OnLoadRes, WaitingPids, Db) ->
     [reply(Pid, Res) || Pid <- WaitingPids],
     ok.
 
+finish_on_load_report(_Mod, Atom) when is_atom(Atom) ->
+    %% No error reports for atoms.
+    ok;
+finish_on_load_report(Mod, Term) ->
+    %% Play it very safe here. The error_logger module and
+    %% modules it depend on may not be loaded yet and there
+    %% would be a dead-lock if we called it directly
+    %% from the code_server process.
+    spawn(fun() ->
+		  F = "The on_load function for module "
+		      "~s returned ~P\n",
+
+		  %% Express the call as an apply to simplify
+		  %% the ext_mod_dep/1 test case.
+		  E = error_logger,
+		  E:warning_msg(F, [Mod,Term,10])
+	  end).
+
 %% -------------------------------------------------------
 %% Internal functions.
 %% -------------------------------------------------------