%% The contents of this file are subject to the Mozilla Public License

%% Version 1.1 (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.mozilla.org/MPL/

%%

%% 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 Original Code is RabbitMQ.

%%

%% The Initial Developer of the Original Code is VMware, Inc.

%% Copyright (c) 2007-2012 VMware, Inc.  All rights reserved.

%%

-module(worker_pool).

%% Generic worker pool manager.

%%

%% Supports nested submission of jobs (nested jobs always run

%% immediately in current worker process).

%%

%% Possible future enhancements:

%%

%% 1. Allow priorities (basically, change the pending queue to a

%% priority_queue).

-behaviour(gen_server2).

-export([start_link/0, submit/1, submit_async/1, idle/1]).

-export([init/1, handle_call/3, handle_cast/2, handle_info/2,

         terminate/2, code_change/3]).

%%----------------------------------------------------------------------------

-ifdef(use_specs).

-type(mfargs() :: {atom(), atom(), [any()]}).

-spec(start_link/0 :: () -> {'ok', pid()} | {'error', any()}).

-spec(submit/1 :: (fun (() -> A) | mfargs()) -> A).

-spec(submit_async/1 :: (fun (() -> any()) | mfargs()) -> 'ok').

-spec(idle/1 :: (any()) -> 'ok').

-endif.

%%----------------------------------------------------------------------------

-define(SERVER, ?MODULE).

-define(HIBERNATE_AFTER_MIN, 1000).

-define(DESIRED_HIBERNATE, 10000).

-record(state, { available, pending }).

%%----------------------------------------------------------------------------

start_link() ->

    gen_server2:start_link({local, ?SERVER}, ?MODULE, [],

                           [{timeout, infinity}]).

submit(Fun) ->

    case get(worker_pool_worker) of

        true -> worker_pool_worker:run(Fun);

        _    -> Pid = gen_server2:call(?SERVER, next_free, infinity),

                worker_pool_worker:submit(Pid, Fun)

    end.

submit_async(Fun) ->

    gen_server2:cast(?SERVER, {run_async, Fun}).

idle(WId) ->

    gen_server2:cast(?SERVER, {idle, WId}).

%%----------------------------------------------------------------------------

init([]) ->

    {ok, #state { pending = queue:new(), available = queue:new() }, hibernate,

     {backoff, ?HIBERNATE_AFTER_MIN, ?HIBERNATE_AFTER_MIN, ?DESIRED_HIBERNATE}}.

handle_call(next_free, From, State = #state { available = Avail,

                                              pending = Pending }) ->

    case queue:out(Avail) of

        {empty, _Avail} ->

            {noreply,

             State #state { pending = queue:in({next_free, From}, Pending) },

             hibernate};

        {{value, WId}, Avail1} ->

            {reply, get_worker_pid(WId), State #state { available = Avail1 },

             hibernate}

    end;

handle_call(Msg, _From, State) ->

    {stop, {unexpected_call, Msg}, State}.

handle_cast({idle, WId}, State = #state { available = Avail,

                                          pending = Pending }) ->

    {noreply, case queue:out(Pending) of

                  {empty, _Pending} ->

                      State #state { available = queue:in(WId, Avail) };

                  {{value, {next_free, From}}, Pending1} ->

                      gen_server2:reply(From, get_worker_pid(WId)),

                      State #state { pending = Pending1 };

                  {{value, {run_async, Fun}}, Pending1} ->

                      worker_pool_worker:submit_async(get_worker_pid(WId), Fun),

                      State #state { pending = Pending1 }

              end, hibernate};

handle_cast({run_async, Fun}, State = #state { available = Avail,

                                               pending = Pending }) ->

    {noreply,

     case queue:out(Avail) of

         {empty, _Avail} ->

             State #state { pending = queue:in({run_async, Fun}, Pending)};

         {{value, WId}, Avail1} ->

             worker_pool_worker:submit_async(get_worker_pid(WId), Fun),

             State #state { available = Avail1 }

     end, hibernate};

handle_cast(Msg, State) ->

    {stop, {unexpected_cast, Msg}, State}.

handle_info(Msg, State) ->

    {stop, {unexpected_info, Msg}, State}.

code_change(_OldVsn, State, _Extra) ->

    {ok, State}.

terminate(_Reason, State) ->

    State.

%%----------------------------------------------------------------------------

get_worker_pid(WId) ->

    [{WId, Pid, _Type, _Modules} | _] =

        lists:dropwhile(fun ({Id, _Pid, _Type, _Modules})

                              when Id =:= WId -> false;

                            (_)               -> true

                        end,

                        supervisor:which_children(worker_pool_sup)),

    Pid.