kernel: add a resource reference counter

System resources/functionality may need to be reference counted
to be handled correctly when used or enabled/disabled from more
than one process or application.

It is easier to handle this in erlang code than in erts, so make a
process that deals with the housekeeping.

5 files changed, 218 insertions, 4 deletions

diff --git a/lib/kernel/src/Makefile b/lib/kernel/src/Makefile
index 5946620f0f..acfcca55e5 100644
--- a/lib/kernel/src/Makefile
+++ b/lib/kernel/src/Makefile
@@ -106,6 +106,7 @@ MODULES = \
 	inet_sctp \
 	kernel \
 	kernel_config \
+	kernel_refc \
 	local_udp \
 	local_tcp \
 	net \
diff --git a/lib/kernel/src/kernel.app.src b/lib/kernel/src/kernel.app.src
index 080b11fc4d..3136db8267 100644
--- a/lib/kernel/src/kernel.app.src
+++ b/lib/kernel/src/kernel.app.src
@@ -57,6 +57,7 @@
 	     inet_tcp_dist,
 	     kernel,
 	     kernel_config,
+	     kernel_refc,
 	     local_tcp,
 	     local_udp,
 	     net,
@@ -107,6 +108,7 @@
 		heart,
 		init,
 		kernel_config,
+		kernel_refc,
 		kernel_sup,
 		net_kernel,
 		net_sup,
diff --git a/lib/kernel/src/kernel.erl b/lib/kernel/src/kernel.erl
index cba57088ec..0382764b39 100644
--- a/lib/kernel/src/kernel.erl
+++ b/lib/kernel/src/kernel.erl
@@ -111,6 +111,13 @@ init([]) ->
                type => worker,
                modules => [kernel_config]},
 
+    RefC = #{id => kernel_refc,
+             start => {kernel_refc, start_link, []},
+             restart => permanent,
+             shutdown => 2000,
+             type => worker,
+             modules => [kernel_refc]},
+
     Code = #{id => code_server,
              start => {code, start_link, []},
              restart => permanent,
@@ -148,7 +155,7 @@ init([]) ->
 
     case init:get_argument(mode) of
         {ok, [["minimal"]]} ->
-            {ok, {SupFlags, [Code, File, StdError, User, Config, SafeSup]}};
+            {ok, {SupFlags, [Code, File, StdError, User, Config, RefC, SafeSup]}};
         _ ->
             Rpc = #{id => rex,
                     start => {rpc, start_link, []},
@@ -199,7 +206,7 @@ init([]) ->
             {ok, {SupFlags,
                   [Code, Rpc, Global, InetDb | DistAC] ++
                   [NetSup, GlGroup, File, SigSrv,
-                   StdError, User, Config, SafeSup] ++ Timer}}
+                   StdError, User, Config, RefC, SafeSup] ++ Timer}}
     end;
 init(safe) ->
     SupFlags = #{strategy => one_for_one,
diff --git a/lib/kernel/src/kernel_refc.erl b/lib/kernel/src/kernel_refc.erl
new file mode 100644
index 0000000000..3ad68e9ff9
--- /dev/null
+++ b/lib/kernel/src/kernel_refc.erl
@@ -0,0 +1,139 @@
+%%
+%% %CopyrightBegin%
+%%
+%% Copyright Ericsson AB 2017. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%
+%% %CopyrightEnd%
+%%
+-module(kernel_refc).
+
+-behaviour(gen_server).
+
+%% External exports
+-export([start_link/0, scheduler_wall_time/1]).
+%% Internal exports
+-export([init/1, handle_info/2, terminate/2]).
+-export([handle_call/3, handle_cast/2, code_change/3]).
+
+%%%-----------------------------------------------------------------
+%%% This module implements a process that handles reference counters for
+%%% various erts or other kernel resources which needs reference counting.
+%%%
+%%% Should not be documented nor used directly by user applications.
+%%%-----------------------------------------------------------------
+start_link() ->
+    gen_server:start_link({local,kernel_refc}, kernel_refc, [], []).
+
+-spec scheduler_wall_time(boolean()) -> boolean().
+scheduler_wall_time(Bool) ->
+    gen_server:call(kernel_refc, {scheduler_wall_time, self(), Bool}).
+
+%%-----------------------------------------------------------------
+%% Callback functions from gen_server
+%%-----------------------------------------------------------------
+
+-spec init([]) -> {'ok', map()} | {'stop', term()}.
+
+init([]) ->
+    resource(scheduler_wall_time, false),
+    {ok, #{scheduler_wall_time=>#{}}}.
+
+-spec handle_call(term(), term(), State) -> {'reply', term(), State}.
+handle_call({What, Who, false}, _From, State) ->
+    {Reply, Cnt} = do_stop(What, maps:get(What, State), Who),
+    {reply, Reply, State#{What:=Cnt}};
+handle_call({What, Who, true}, _From, State) ->
+    {Reply, Cnt} = do_start(What, maps:get(What, State), Who),
+    {reply, Reply, State#{What:=Cnt}};
+handle_call(_, _From, State) ->
+    {reply, badarg, State}.
+
+-spec handle_cast(term(), State) -> {'noreply', State}.
+handle_cast(_, State) ->
+    {noreply, State}.
+
+-spec handle_info(term(), State) -> {'noreply', State}.
+handle_info({'DOWN', _Ref, process, Pid, _}, State) ->
+    Cleanup = fun(Resource, Cnts) ->
+                      cleanup(Resource, Cnts, Pid)
+              end,
+    {noreply, maps:map(Cleanup, State)};
+handle_info(_, State) ->
+    {noreply, State}.
+
+-spec terminate(term(), term()) -> 'ok'.
+terminate(_Reason, _State) ->
+    ok.
+
+-spec code_change(term(), State, term()) -> {'ok', State}.
+code_change(_OldVsn, State, _Extra) ->
+    {ok, State}.
+
+%%-----------------------------------------------------------------
+%% Internal functions
+%%-----------------------------------------------------------------
+
+do_start(Resource, Cnt, Pid) ->
+    case maps:get(Pid, Cnt, undefined) of
+        undefined ->
+            Ref = erlang:monitor(process, Pid),
+            case any(Cnt) of
+                true ->
+                    {true, Cnt#{Pid=>{1, Ref}}};
+                false ->
+                    resource(Resource, true),
+                    {false, Cnt#{Pid=>{1, Ref}}}
+            end;
+        {N, Ref} ->
+            {true, Cnt#{Pid=>{N+1, Ref}}}
+    end.
+
+do_stop(Resource, Cnt0, Pid) ->
+    case maps:get(Pid, Cnt0, undefined) of
+        undefined ->
+            {any(Cnt0), Cnt0};
+        {1, Ref} ->
+            erlang:demonitor(Ref, [flush]),
+            Cnt = maps:remove(Pid, Cnt0),
+            case any(Cnt) of
+                true ->
+                    {true, Cnt};
+                false ->
+                    resource(Resource, false),
+                    {true, Cnt}
+            end;
+        {N, Ref} ->
+            {true, Cnt0#{Pid=>{N-1, Ref}}}
+    end.
+
+cleanup(Resource, Cnt0, Pid) ->
+    case maps:is_key(Pid, Cnt0) of
+        true ->
+            Cnt = maps:remove(Pid, Cnt0),
+            case any(Cnt) of
+                true ->
+                    Cnt;
+                false ->
+                    resource(Resource, false),
+                    Cnt
+            end;
+        false ->
+            Cnt0
+    end.
+
+any(Cnt) -> maps:size(Cnt) > 0.
+
+resource(scheduler_wall_time, Enable) ->
+    _ = erlang:system_flag(scheduler_wall_time, Enable).
diff --git a/lib/kernel/test/kernel_SUITE.erl b/lib/kernel/test/kernel_SUITE.erl
index da56359294..7898988dbe 100644
--- a/lib/kernel/test/kernel_SUITE.erl
+++ b/lib/kernel/test/kernel_SUITE.erl
@@ -30,14 +30,14 @@
 -export([init_per_testcase/2, end_per_testcase/2]).
 
 %% Test cases must be exported.
--export([app_test/1, appup_test/1]).
+-export([app_test/1, appup_test/1, refc/1]).
 
 suite() ->
     [{ct_hooks,[ts_install_cth]},
      {timetrap,{minutes,2}}].
 
 all() -> 
-    [app_test, appup_test].
+    [app_test, appup_test, refc].
 
 groups() -> 
     [].
@@ -163,3 +163,68 @@ check_appup([Vsn|Vsns],Instrs,Expected) ->
     end;
 check_appup([],_,_) ->
     ok.
+
+%%% Check that refc module handles the counters as expected
+refc(_Config) ->
+    Enable = fun(Enable) -> erlang:system_flag(scheduler_wall_time, Enable) end,
+    IsOn = fun() -> undefined /= erlang:statistics(scheduler_wall_time) end,
+    Tester = self(),
+    Loop = fun Loop() ->
+                   receive
+                       die -> normal;
+                       {apply, Bool} ->
+                           Res = Enable(Bool),
+                           Tester ! {self(), Res},
+                           Loop()
+                   end
+           end,
+
+    %% Counter should be 0
+    false = Enable(false),
+
+    false = Enable(true),
+    true  = Enable(true),
+    true  = Enable(false),
+    true  = Enable(false),
+
+    %% Counter should be 0
+    false = IsOn(),
+
+    P1 = spawn_link(Loop),
+    P1 ! {apply, true},
+    receive {P1, R1} -> false = R1 end,
+
+    %% P1 has turned it on counter should be one
+    true = IsOn(),
+    true = Enable(true),
+    true = Enable(false),
+    true = IsOn(),
+
+    P1 ! {apply, false},
+    receive {P1, R2} -> true = R2 end,
+    false = IsOn(),
+
+    P1 ! {apply, true},
+    receive {P1, R3} -> false = R3 end,
+    true = IsOn(),
+    true = Enable(false),
+
+
+    P1 ! die,
+    timer:sleep(100),
+    false = IsOn(),
+    false = Enable(false),
+
+    P2 = spawn_link(Loop),
+    P2 ! {apply, true},
+    receive {P2, R4} -> false = R4 end,
+    true = IsOn(),
+    P2 ! {apply, true},
+    receive {P2, R5} -> true = R5 end,
+    true = IsOn(),
+
+    P2 ! die,
+    timer:sleep(100),
+    false = IsOn(),
+
+    ok.