Merge branch 'siri/stdlib/dialyzer-supervisor/OTP-9741'

* siri/stdlib/dialyzer-supervisor/OTP-9741:
  Add test for upgrade of supervisor
  Fix dialyzer warnings in supervisor

15 files changed, 355 insertions, 11 deletions

diff --git a/lib/sasl/src/release_handler_1.erl b/lib/sasl/src/release_handler_1.erl
index 37275eff45..93d12cf609 100644
--- a/lib/sasl/src/release_handler_1.erl
+++ b/lib/sasl/src/release_handler_1.erl
@@ -505,15 +505,20 @@ resume(Pids) ->
 
 change_code(Pids, Mod, Vsn, Extra, Timeout) ->
     Fun = fun(Pid) -> 
-		  case Timeout of
-		      default ->
-			  ok = sys:change_code(Pid, Mod, Vsn, Extra);
-		      _Else ->
-			  ok = sys:change_code(Pid, Mod, Vsn, Extra, Timeout)
+		  case sys_change_code(Pid, Mod, Vsn, Extra, Timeout) of
+		      ok ->
+			  ok;
+		      {error,Reason} ->
+			  throw({code_change_failed,Pid,Mod,Vsn,Reason})
 		  end
 	  end,
     lists:foreach(Fun, Pids).
 
+sys_change_code(Pid, Mod, Vsn, Extra, default) ->
+    sys:change_code(Pid, Mod, Vsn, Extra);
+sys_change_code(Pid, Mod, Vsn, Extra, Timeout) ->
+    sys:change_code(Pid, Mod, Vsn, Extra, Timeout).
+
 stop(Mod, Procs) ->
     lists:zf(fun({undefined, _Name, _Pid, _Mods}) ->
 		     false;
diff --git a/lib/sasl/test/release_handler_SUITE.erl b/lib/sasl/test/release_handler_SUITE.erl
index 4e5742a5d4..ac616dab72 100644
--- a/lib/sasl/test/release_handler_SUITE.erl
+++ b/lib/sasl/test/release_handler_SUITE.erl
@@ -62,7 +62,8 @@ cases() ->
      otp_9395_update_many_mods, otp_9395_rm_many_mods,
      instructions, eval_appup, eval_appup_with_restart,
      supervisor_which_children_timeout,
-     release_handler_which_releases, install_release_syntax_check].
+     release_handler_which_releases, install_release_syntax_check,
+     upgrade_supervisor, upgrade_supervisor_fail].
 
 groups() ->
     [{release,[],
@@ -1205,6 +1206,109 @@ otp_9395_rm_many_mods(cleanup,_Conf) ->
     stop_node(node_name(otp_9395_rm_many_mods)).
 
 
+upgrade_supervisor(Conf) when is_list(Conf) ->
+    %% Set some paths
+    PrivDir = priv_dir(Conf),
+    Dir = filename:join(PrivDir,"upgrade_supervisor"),
+    LibDir = filename:join(?config(data_dir, Conf), "lib"),
+
+    %% Create the releases
+    Lib1 = [{a,"1.0",LibDir}],
+    Lib2 = [{a,"9.0",LibDir}],
+    Rel1 = create_and_install_fake_first_release(Dir,Lib1),
+    Rel2 = create_fake_upgrade_release(Dir,"2",Lib2,{[Rel1],[Rel1],[LibDir]}),
+    Rel1Dir = filename:dirname(Rel1),
+    Rel2Dir = filename:dirname(Rel2),
+
+    %% Start a slave node
+    {ok, Node} = t_start_node(upgrade_supervisor, Rel1,
+			      filename:join(Rel1Dir,"sys.config")),
+
+    %% Check path
+    Dir1 = filename:join([LibDir, "a-1.0"]),
+    Dir1 = rpc:call(Node, code, lib_dir, [a]),
+    ASupBeam1 = filename:join([Dir1,ebin,"a_sup.beam"]),
+    ASupBeam1 = rpc:call(Node, code, which, [a_sup]),
+
+    %% Install second release, with no changed modules
+    {ok, RelVsn2} = rpc:call(Node, release_handler, set_unpacked,
+			     [Rel2++".rel", Lib2]),
+    ok = rpc:call(Node, release_handler, install_file,
+		  [RelVsn2, filename:join(Rel2Dir, "relup")]),
+    ok = rpc:call(Node, release_handler, install_file,
+		  [RelVsn2, filename:join(Rel2Dir, "start.boot")]),
+    ok = rpc:call(Node, release_handler, install_file,
+		  [RelVsn2, filename:join(Rel2Dir, "sys.config")]),
+
+    {ok, _RelVsn1, []} =
+	rpc:call(Node, release_handler, install_release, [RelVsn2]),
+
+    %% Check that libdir is changed
+    Dir2 = filename:join([LibDir, "a-9.0"]),
+    Dir2 = rpc:call(Node, code, lib_dir, [a]),
+    ASupBeam2 = filename:join([Dir2,ebin,"a_sup.beam"]),
+    ASupBeam2 = rpc:call(Node, code, which, [a_sup]),
+
+    %% Check that the restart strategy and child spec is updated
+    {status, _, {module, _}, [_, _, _, _, [_,_,{data,[{"State",State}]}]]} =
+	rpc:call(Node,sys,get_status,[a_sup]),
+    {state,_,RestartStrategy,[Child],_,_,_,_,_,_} = State,
+    one_for_all = RestartStrategy, % changed from one_for_one
+    {child,_,_,_,_,brutal_kill,_,_} = Child, % changed from timeout 2000
+
+    ok.
+
+%% Check that if the supervisor fails, then the upgrade is rolled back
+%% and an ok error message is returned
+upgrade_supervisor_fail(Conf) when is_list(Conf) ->
+    %% Set some paths
+    PrivDir = priv_dir(Conf),
+    Dir = filename:join(PrivDir,"upgrade_supervisor_fail"),
+    LibDir = filename:join(?config(data_dir, Conf), "lib"),
+
+    %% Create the releases
+    Lib1 = [{a,"1.0",LibDir}],
+    Lib2 = [{a,"9.1",LibDir}],
+    Rel1 = create_and_install_fake_first_release(Dir,Lib1),
+    Rel2 = create_fake_upgrade_release(Dir,"2",Lib2,{[Rel1],[Rel1],[LibDir]}),
+    Rel1Dir = filename:dirname(Rel1),
+    Rel2Dir = filename:dirname(Rel2),
+
+    %% Start a slave node
+    {ok, Node} = t_start_node(upgrade_supervisor_fail, Rel1,
+			      filename:join(Rel1Dir,"sys.config")),
+
+    %% Check path
+    Dir1 = filename:join([LibDir, "a-1.0"]),
+    Dir1 = rpc:call(Node, code, lib_dir, [a]),
+    ASupBeam1 = filename:join([Dir1,ebin,"a_sup.beam"]),
+    ASupBeam1 = rpc:call(Node, code, which, [a_sup]),
+
+    %% Install second release, with no changed modules
+    {ok, RelVsn2} = rpc:call(Node, release_handler, set_unpacked,
+			     [Rel2++".rel", Lib2]),
+    ok = rpc:call(Node, release_handler, install_file,
+		  [RelVsn2, filename:join(Rel2Dir, "relup")]),
+    ok = rpc:call(Node, release_handler, install_file,
+		  [RelVsn2, filename:join(Rel2Dir, "start.boot")]),
+    ok = rpc:call(Node, release_handler, install_file,
+		  [RelVsn2, filename:join(Rel2Dir, "sys.config")]),
+    ok = net_kernel:monitor_nodes(true),
+
+    {error,{code_change_failed,_Pid,a_sup,_Vsn,
+	    {error,{invalid_shutdown,brutal_kil}}}} =
+	rpc:call(Node, release_handler, install_release, [RelVsn2]),
+
+    %% Check that the upgrade is terminated - normally this would mean
+    %% rollback, but since this testcase is very simplified the node
+    %% is not started with heart supervision and will therefore not be
+    %% restarted. So we just check that the node goes down.
+    receive {nodedown,Node} -> ok
+    after 10000 -> ct:fail(failed_upgrade_never_restarted_node)
+    end,
+
+    ok.
+
 
 %% Test upgrade and downgrade of applications
 eval_appup(Conf) when is_list(Conf) ->
diff --git a/lib/sasl/test/release_handler_SUITE_data/Makefile.src b/lib/sasl/test/release_handler_SUITE_data/Makefile.src
index 6f40088161..55d20aa8b6 100644
--- a/lib/sasl/test/release_handler_SUITE_data/Makefile.src
+++ b/lib/sasl/test/release_handler_SUITE_data/Makefile.src
@@ -5,6 +5,10 @@ P2B= \
      P2B/a-2.0/ebin/a_sup.@EMULATOR@
 
 LIB= \
+     lib/a-9.1/ebin/a.@EMULATOR@ \
+     lib/a-9.1/ebin/a_sup.@EMULATOR@ \
+     lib/a-9.0/ebin/a.@EMULATOR@ \
+     lib/a-9.0/ebin/a_sup.@EMULATOR@ \
      lib/a-1.2/ebin/a.@EMULATOR@ \
      lib/a-1.2/ebin/a_sup.@EMULATOR@ \
      lib/a-1.1/ebin/a.@EMULATOR@ \
@@ -101,6 +105,17 @@ lib/a-1.2/ebin/a.@EMULATOR@: lib/a-1.2/src/a.erl
 lib/a-1.2/ebin/a_sup.@EMULATOR@: lib/a-1.2/src/a_sup.erl
 	erlc $(EFLAGS) -olib/a-1.2/ebin lib/a-1.2/src/a_sup.erl
 
+lib/a-9.0/ebin/a.@EMULATOR@: lib/a-9.0/src/a.erl
+	erlc $(EFLAGS) -olib/a-9.0/ebin lib/a-9.0/src/a.erl
+lib/a-9.0/ebin/a_sup.@EMULATOR@: lib/a-9.0/src/a_sup.erl
+	erlc $(EFLAGS) -olib/a-9.0/ebin lib/a-9.0/src/a_sup.erl
+
+lib/a-9.1/ebin/a.@EMULATOR@: lib/a-9.1/src/a.erl
+	erlc $(EFLAGS) -olib/a-9.1/ebin lib/a-9.1/src/a.erl
+lib/a-9.1/ebin/a_sup.@EMULATOR@: lib/a-9.1/src/a_sup.erl
+	erlc $(EFLAGS) -olib/a-9.1/ebin lib/a-9.1/src/a_sup.erl
+
+
 lib/b-1.0/ebin/b_server.@EMULATOR@: lib/b-1.0/src/b_server.erl
 	erlc $(EFLAGS) -olib/b-1.0/ebin lib/b-1.0/src/b_server.erl
 lib/b-1.0/ebin/b_lib.@EMULATOR@: lib/b-1.0/src/b_lib.erl
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/README b/lib/sasl/test/release_handler_SUITE_data/lib/README
index 639a4ca0fb..ffb8c5120b 100644
--- a/lib/sasl/test/release_handler_SUITE_data/lib/README
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/README
@@ -8,6 +8,14 @@ a-1.2:
 can be upgraded to from a-1.1.
 No module have changed, but priv dir is added including one 'file'
 
+a-9.0:
+can be upgrade to from a-1.0
+Changes a_sup correctly - to test successful upgrade of supervisor
+
+a-9.1:
+can be upgrade to from a-1.0
+Changes a_sup faulty - to test failing upgrade of supervisor
+
 b-1.0:
 start version, includes b_lib and b_server
 
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/ebin/a.app b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/ebin/a.app
new file mode 100644
index 0000000000..aa436d3e8c
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/ebin/a.app
@@ -0,0 +1,8 @@
+{application, a,
+	     [{description, "A  CXC 138 11"},
+              {vsn, "9.0"},
+	      {modules, [a, a_sup]},
+	      {registered, [a_sup]},
+	      {applications, [kernel, stdlib]},
+	      {env, [{key1, val1}]},
+	      {mod, {a_sup, []}}]}.
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/ebin/a.appup b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/ebin/a.appup
new file mode 100644
index 0000000000..c4071d57a3
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/ebin/a.appup
@@ -0,0 +1,3 @@
+{"9.0",
+ [{"1.0",[{update,a_sup,{advanced,update_supervisor}}]}],
+ [{"1.0",[{update,a_sup,{advanced,update_supervisor}}]}]}.
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/src/a.erl b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/src/a.erl
new file mode 100644
index 0000000000..1050e53f35
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/src/a.erl
@@ -0,0 +1,56 @@
+%% ``The contents of this file are subject to the Erlang Public License,
+%% Version 1.1, (the "License"); you may not use this file except in
+%% compliance with the License. You should have received a copy of the
+%% Erlang Public License along with this software. If not, it can be
+%% retrieved via the world wide web at http://www.erlang.org/.
+%%
+%% Software distributed under the License is distributed on an "AS IS"
+%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+%% the License for the specific language governing rights and limitations
+%% under the License.
+%%
+%% The Initial Developer of the Original Code is Ericsson Utvecklings AB.
+%% Portions created by Ericsson are Copyright 1999, Ericsson Utvecklings
+%% AB. All Rights Reserved.''
+%%
+%%     $Id$
+%%
+-module(a).
+
+
+-behaviour(gen_server).
+
+%% External exports
+-export([start_link/0, a/0, b/0]).
+%% Internal exports
+-export([init/1, handle_call/3, handle_info/2, terminate/2, code_change/3]).
+
+start_link() -> gen_server:start_link({local, aa}, a, [], []).
+
+a() -> gen_server:call(aa, a).
+b() -> gen_server:call(aa, b).
+
+%%-----------------------------------------------------------------
+%% Callback functions from gen_server
+%%-----------------------------------------------------------------
+init([]) ->
+    process_flag(trap_exit, true),
+    {ok, {state, bval}}.
+
+handle_call(a, _From, State) ->
+    X = application:get_all_env(a),
+    {reply, X, State};
+
+handle_call(b, _From, State) ->
+    {reply, {ok, element(2, State)}, State}.
+
+handle_info(_, State) ->
+    {noreply, State}.
+
+terminate(_Reason, _State) ->
+    ok.
+
+code_change(1, Extra, State) ->
+    {ok, {state, bval}};
+code_change({down,1},Extra,State) ->
+    {ok, state}.
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/src/a_sup.erl b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/src/a_sup.erl
new file mode 100644
index 0000000000..ae1d080f58
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.0/src/a_sup.erl
@@ -0,0 +1,37 @@
+%% ``The contents of this file are subject to the Erlang Public License,
+%% Version 1.1, (the "License"); you may not use this file except in
+%% compliance with the License. You should have received a copy of the
+%% Erlang Public License along with this software. If not, it can be
+%% retrieved via the world wide web at http://www.erlang.org/.
+%%
+%% Software distributed under the License is distributed on an "AS IS"
+%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+%% the License for the specific language governing rights and limitations
+%% under the License.
+%%
+%% The Initial Developer of the Original Code is Ericsson Utvecklings AB.
+%% Portions created by Ericsson are Copyright 1999, Ericsson Utvecklings
+%% AB. All Rights Reserved.''
+%%
+%%     $Id$
+%%
+-module(a_sup).
+
+
+-behaviour(supervisor).
+
+%% External exports
+-export([start/2]).
+
+%% Internal exports
+-export([init/1]).
+
+start(_, _) ->
+    supervisor:start_link({local, a_sup}, a_sup, []).
+
+init([]) ->
+    SupFlags = {one_for_all, 4, 3600},
+    Config = {a,
+	      {a, start_link, []},
+	      permanent, brutal_kill, worker, [a]},
+    {ok, {SupFlags, [Config]}}.
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/ebin/a.app b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/ebin/a.app
new file mode 100644
index 0000000000..5b467ec4e8
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/ebin/a.app
@@ -0,0 +1,8 @@
+{application, a,
+	     [{description, "A  CXC 138 11"},
+              {vsn, "9.1"},
+	      {modules, [a, a_sup]},
+	      {registered, [a_sup]},
+	      {applications, [kernel, stdlib]},
+	      {env, [{key1, val1}]},
+	      {mod, {a_sup, []}}]}.
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/ebin/a.appup b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/ebin/a.appup
new file mode 100644
index 0000000000..efeb7f1fe3
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/ebin/a.appup
@@ -0,0 +1,3 @@
+{"9.1",
+ [{"1.0",[{update,a_sup,{advanced,update_supervisor}}]}],
+ [{"1.0",[{update,a_sup,{advanced,update_supervisor}}]}]}.
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/src/a.erl b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/src/a.erl
new file mode 100644
index 0000000000..1050e53f35
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/src/a.erl
@@ -0,0 +1,56 @@
+%% ``The contents of this file are subject to the Erlang Public License,
+%% Version 1.1, (the "License"); you may not use this file except in
+%% compliance with the License. You should have received a copy of the
+%% Erlang Public License along with this software. If not, it can be
+%% retrieved via the world wide web at http://www.erlang.org/.
+%%
+%% Software distributed under the License is distributed on an "AS IS"
+%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+%% the License for the specific language governing rights and limitations
+%% under the License.
+%%
+%% The Initial Developer of the Original Code is Ericsson Utvecklings AB.
+%% Portions created by Ericsson are Copyright 1999, Ericsson Utvecklings
+%% AB. All Rights Reserved.''
+%%
+%%     $Id$
+%%
+-module(a).
+
+
+-behaviour(gen_server).
+
+%% External exports
+-export([start_link/0, a/0, b/0]).
+%% Internal exports
+-export([init/1, handle_call/3, handle_info/2, terminate/2, code_change/3]).
+
+start_link() -> gen_server:start_link({local, aa}, a, [], []).
+
+a() -> gen_server:call(aa, a).
+b() -> gen_server:call(aa, b).
+
+%%-----------------------------------------------------------------
+%% Callback functions from gen_server
+%%-----------------------------------------------------------------
+init([]) ->
+    process_flag(trap_exit, true),
+    {ok, {state, bval}}.
+
+handle_call(a, _From, State) ->
+    X = application:get_all_env(a),
+    {reply, X, State};
+
+handle_call(b, _From, State) ->
+    {reply, {ok, element(2, State)}, State}.
+
+handle_info(_, State) ->
+    {noreply, State}.
+
+terminate(_Reason, _State) ->
+    ok.
+
+code_change(1, Extra, State) ->
+    {ok, {state, bval}};
+code_change({down,1},Extra,State) ->
+    {ok, state}.
diff --git a/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/src/a_sup.erl b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/src/a_sup.erl
new file mode 100644
index 0000000000..b0597dc5c3
--- /dev/null
+++ b/lib/sasl/test/release_handler_SUITE_data/lib/a-9.1/src/a_sup.erl
@@ -0,0 +1,37 @@
+%% ``The contents of this file are subject to the Erlang Public License,
+%% Version 1.1, (the "License"); you may not use this file except in
+%% compliance with the License. You should have received a copy of the
+%% Erlang Public License along with this software. If not, it can be
+%% retrieved via the world wide web at http://www.erlang.org/.
+%%
+%% Software distributed under the License is distributed on an "AS IS"
+%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
+%% the License for the specific language governing rights and limitations
+%% under the License.
+%%
+%% The Initial Developer of the Original Code is Ericsson Utvecklings AB.
+%% Portions created by Ericsson are Copyright 1999, Ericsson Utvecklings
+%% AB. All Rights Reserved.''
+%%
+%%     $Id$
+%%
+-module(a_sup).
+
+
+-behaviour(supervisor).
+
+%% External exports
+-export([start/2]).
+
+%% Internal exports
+-export([init/1]).
+
+start(_, _) ->
+    supervisor:start_link({local, a_sup}, a_sup, []).
+
+init([]) ->
+    SupFlags = {one_for_all, 4, 3600},
+    Config = {a,
+	      {a, start_link, []},
+	      permanent, brutal_kil, worker, [a]},
+    {ok, {SupFlags, [Config]}}.
diff --git a/lib/stdlib/doc/src/gen_server.xml b/lib/stdlib/doc/src/gen_server.xml
index 1045766e01..edeb7dff91 100644
--- a/lib/stdlib/doc/src/gen_server.xml
+++ b/lib/stdlib/doc/src/gen_server.xml
@@ -4,7 +4,7 @@
 <erlref>
   <header>
     <copyright>
-      <year>1996</year><year>2010</year>
+      <year>1996</year><year>2011</year>
       <holder>Ericsson AB. All Rights Reserved.</holder>
     </copyright>
     <legalnotice>
@@ -570,13 +570,14 @@ gen_server:abcast     -----> Module:handle_cast/2
       </desc>
     </func>
     <func>
-      <name>Module:code_change(OldVsn, State, Extra) -> {ok, NewState}</name>
+      <name>Module:code_change(OldVsn, State, Extra) -> {ok, NewState} | {error, Reason}</name>
       <fsummary>Update the internal state during upgrade/downgrade.</fsummary>
       <type>
         <v>OldVsn = Vsn | {down, Vsn}</v>
         <v>&nbsp;&nbsp;Vsn = term()</v>
         <v>State = NewState = term()</v>
         <v>Extra = term()</v>
+	<v>Reason = term()</v>
       </type>
       <desc>
         <p>This function is called by a gen_server when it should
@@ -595,7 +596,10 @@ gen_server:abcast     -----> Module:handle_cast/2
         <p><c>State</c> is the internal state of the gen_server.</p>
         <p><c>Extra</c> is passed as-is from the <c>{advanced,Extra}</c>
           part of the update instruction.</p>
-        <p>The function should return the updated internal state.</p>
+        <p>If successful, the function shall return the updated
+          internal state.</p>
+	<p>If the function returns <c>{error,Reason}</c>, the ongoing
+	  upgrade will fail and roll back to the old release.</p>
       </desc>
     </func>
     <func>
diff --git a/lib/stdlib/src/gen_server.erl b/lib/stdlib/src/gen_server.erl
index 6f075bbe5a..af07bc988a 100644
--- a/lib/stdlib/src/gen_server.erl
+++ b/lib/stdlib/src/gen_server.erl
@@ -134,7 +134,7 @@
     term().
 -callback code_change(OldVsn :: (term() | {down, term()}), State :: term(),
                       Extra :: term()) ->
-    {ok, NewState :: term()}.
+    {ok, NewState :: term()} | {error, Reason :: term()}.
 
 %%%  -----------------------------------------------------------------
 %%% Starts a generic server.
diff --git a/lib/stdlib/src/supervisor.erl b/lib/stdlib/src/supervisor.erl
index 2dd5ccce7a..42ea42f42e 100644
--- a/lib/stdlib/src/supervisor.erl
+++ b/lib/stdlib/src/supervisor.erl
@@ -37,7 +37,7 @@
 
 %%--------------------------------------------------------------------------
 
--type child()    :: pid() | 'undefined'.
+-type child()    :: 'undefined' | pid() | [pid()].
 -type child_id() :: term().
 -type mfargs()   :: {M :: module(), F :: atom(), A :: [term()] | undefined}.
 -type modules()  :: [module()] | 'dynamic'.