%% 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.