From d4341cd249d728174fce580bd018e1e8b402d161 Mon Sep 17 00:00:00 2001
From: Dan Gudmundsson <dgud@erlang.org>
Date: Mon, 20 Nov 2017 15:23:24 +0100
Subject: 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.
---
 lib/kernel/src/kernel_refc.erl | 139 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 139 insertions(+)
 create mode 100644 lib/kernel/src/kernel_refc.erl

(limited to 'lib/kernel/src/kernel_refc.erl')

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).
-- 
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