%% This file is a copy of gen_server.erl from the R13B-1 Erlang/OTP
%% distribution, with the following modifications:
%%
%% 1) the module name is gen_server2
%%
%% 2) more efficient handling of selective receives in callbacks
%% gen_server2 processes drain their message queue into an internal
%% buffer before invoking any callback module functions. Messages are
%% dequeued from the buffer for processing. Thus the effective message
%% queue of a gen_server2 process is the concatenation of the internal
%% buffer and the real message queue.
%% As a result of the draining, any selective receive invoked inside a
%% callback is less likely to have to scan a large message queue.
%%
%% 3) gen_server2:cast is guaranteed to be order-preserving
%% The original code could reorder messages when communicating with a
%% process on a remote node that was not currently connected.
%%
%% 4) The callback module can optionally implement prioritise_call/4,
%% prioritise_cast/3 and prioritise_info/3.  These functions take
%% Message, From, Length and State or just Message, Length and State
%% (where Length is the current number of messages waiting to be
%% processed) and return a single integer representing the priority
%% attached to the message, or 'drop' to ignore it (for
%% prioritise_cast/3 and prioritise_info/3 only).  Messages with
%% higher priorities are processed before requests with lower
%% priorities. The default priority is 0.
%%
%% 5) The callback module can optionally implement
%% handle_pre_hibernate/1 and handle_post_hibernate/1. These will be
%% called immediately prior to and post hibernation, respectively. If
%% handle_pre_hibernate returns {hibernate, NewState} then the process
%% will hibernate. If the module does not implement
%% handle_pre_hibernate/1 then the default action is to hibernate.
%%
%% 6) init can return a 4th arg, {backoff, InitialTimeout,
%% MinimumTimeout, DesiredHibernatePeriod} (all in milliseconds,
%% 'infinity' does not make sense here). Then, on all callbacks which
%% can return a timeout (including init), timeout can be
%% 'hibernate'. When this is the case, the current timeout value will
%% be used (initially, the InitialTimeout supplied from init). After
%% this timeout has occurred, hibernation will occur as normal. Upon
%% awaking, a new current timeout value will be calculated.
%%
%% The purpose is that the gen_server2 takes care of adjusting the
%% current timeout value such that the process will increase the
%% timeout value repeatedly if it is unable to sleep for the
%% DesiredHibernatePeriod. If it is able to sleep for the
%% DesiredHibernatePeriod it will decrease the current timeout down to
%% the MinimumTimeout, so that the process is put to sleep sooner (and
%% hopefully stays asleep for longer). In short, should a process
%% using this receive a burst of messages, it should not hibernate
%% between those messages, but as the messages become less frequent,
%% the process will not only hibernate, it will do so sooner after
%% each message.
%%
%% When using this backoff mechanism, normal timeout values (i.e. not
%% 'hibernate') can still be used, and if they are used then the
%% handle_info(timeout, State) will be called as normal. In this case,
%% returning 'hibernate' from handle_info(timeout, State) will not
%% hibernate the process immediately, as it would if backoff wasn't
%% being used. Instead it'll wait for the current timeout as described
%% above.
%%
%% 7) The callback module can return from any of the handle_*
%% functions, a {become, Module, State} triple, or a {become, Module,
%% State, Timeout} quadruple. This allows the gen_server to
%% dynamically change the callback module. The State is the new state
%% which will be passed into any of the callback functions in the new
%% module. Note there is no form also encompassing a reply, thus if
%% you wish to reply in handle_call/3 and change the callback module,
%% you need to use gen_server2:reply/2 to issue the reply
%% manually. The init function can similarly return a 5th argument,
%% Module, in order to dynamically decide the callback module on init.
%%
%% 8) The callback module can optionally implement
%% format_message_queue/2 which is the equivalent of format_status/2
%% but where the second argument is specifically the priority_queue
%% which contains the prioritised message_queue.
%%
%% 9) The function with_state/2 can be used to debug a process with
%% heavyweight state (without needing to copy the entire state out of
%% process as sys:get_status/1 would). Pass through a function which
%% can be invoked on the state, get back the result. The state is not
%% modified.
%%
%% 10) an mcall/1 function has been added for performing multiple
%% call/3 in parallel. Unlike multi_call, which sends the same request
%% to same-named processes residing on a supplied list of nodes, it
%% operates on name/request pairs, where name is anything accepted by
%% call/3, i.e. a pid, global name, local name, or local name on a
%% particular node.
%%

%% All modifications are (C) 2009-2013 GoPivotal, Inc.

%% ``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(gen_server2).

%%% ---------------------------------------------------
%%%
%%% The idea behind THIS server is that the user module
%%% provides (different) functions to handle different
%%% kind of inputs.
%%% If the Parent process terminates the Module:terminate/2
%%% function is called.
%%%
%%% The user module should export:
%%%
%%%   init(Args)
%%%     ==> {ok, State}
%%%         {ok, State, Timeout}
%%%         {ok, State, Timeout, Backoff}
%%%         {ok, State, Timeout, Backoff, Module}
%%%         ignore
%%%         {stop, Reason}
%%%
%%%   handle_call(Msg, {From, Tag}, State)
%%%
%%%    ==> {reply, Reply, State}
%%%        {reply, Reply, State, Timeout}
%%%        {noreply, State}
%%%        {noreply, State, Timeout}
%%%        {stop, Reason, Reply, State}
%%%              Reason = normal | shutdown | Term terminate(State) is called
%%%
%%%   handle_cast(Msg, State)
%%%
%%%    ==> {noreply, State}
%%%        {noreply, State, Timeout}
%%%        {stop, Reason, State}
%%%              Reason = normal | shutdown | Term terminate(State) is called
%%%
%%%   handle_info(Info, State) Info is e.g. {'EXIT', P, R}, {nodedown, N}, ...
%%%
%%%    ==> {noreply, State}
%%%        {noreply, State, Timeout}
%%%        {stop, Reason, State}
%%%              Reason = normal | shutdown | Term, terminate(State) is called
%%%
%%%   terminate(Reason, State) Let the user module clean up
%%%              Reason = normal | shutdown | {shutdown, Term} | Term
%%%        always called when server terminates
%%%
%%%    ==> ok | Term
%%%
%%%   handle_pre_hibernate(State)
%%%
%%%    ==> {hibernate, State}
%%%        {stop, Reason, State}
%%%              Reason = normal | shutdown | Term, terminate(State) is called
%%%
%%%   handle_post_hibernate(State)
%%%
%%%    ==> {noreply, State}
%%%        {stop, Reason, State}
%%%              Reason = normal | shutdown | Term, terminate(State) is called
%%%
%%% The work flow (of the server) can be described as follows:
%%%
%%%   User module                          Generic
%%%   -----------                          -------
%%%     start            ----->             start
%%%     init             <-----              .
%%%
%%%                                         loop
%%%     handle_call      <-----              .
%%%                      ----->             reply
%%%
%%%     handle_cast      <-----              .
%%%
%%%     handle_info      <-----              .
%%%
%%%     terminate        <-----              .
%%%
%%%                      ----->             reply
%%%
%%%
%%% ---------------------------------------------------

%% API
-export([start/3, start/4,
         start_link/3, start_link/4,
         call/2, call/3,
         cast/2, reply/2,
         abcast/2, abcast/3,
         multi_call/2, multi_call/3, multi_call/4,
         mcall/1,
         with_state/2,
         enter_loop/3, enter_loop/4, enter_loop/5, enter_loop/6, wake_hib/1]).

%% System exports
-export([system_continue/3,
         system_terminate/4,
         system_code_change/4,
         format_status/2]).

%% Internal exports
-export([init_it/6]).

-import(error_logger, [format/2]).

%% State record
-record(gs2_state, {parent, name, state, mod, time,
                    timeout_state, queue, debug, prioritisers}).

%%%=========================================================================
%%%  Specs. These exist only to shut up dialyzer's warnings
%%%=========================================================================

-type gs2_state() :: #gs2_state{}.

-spec handle_common_termination(any(), atom(), gs2_state()) -> no_return().
-spec hibernate(gs2_state()) -> no_return().
-spec pre_hibernate(gs2_state()) -> no_return().
-spec system_terminate(_, _, _, gs2_state()) -> no_return().

-type millis() :: non_neg_integer().

%%%=========================================================================
%%%  API
%%%=========================================================================

-callback init(Args :: term()) ->
    {ok, State :: term()} |
    {ok, State :: term(), timeout() | hibernate} |
    {ok, State :: term(), timeout() | hibernate,
     {backoff, millis(), millis(), millis()}} |
    {ok, State :: term(), timeout() | hibernate,
     {backoff, millis(), millis(), millis()}, atom()} |
    ignore |
    {stop, Reason :: term()}.
-callback handle_call(Request :: term(), From :: {pid(), Tag :: term()},
                      State :: term()) ->
    {reply, Reply :: term(), NewState :: term()} |
    {reply, Reply :: term(), NewState :: term(), timeout() | hibernate} |
    {noreply, NewState :: term()} |
    {noreply, NewState :: term(), timeout() | hibernate} |
    {stop, Reason :: term(),
     Reply :: term(), NewState :: term()}.
-callback handle_cast(Request :: term(), State :: term()) ->
    {noreply, NewState :: term()} |
    {noreply, NewState :: term(), timeout() | hibernate} |
    {stop, Reason :: term(), NewState :: term()}.
-callback handle_info(Info :: term(), State :: term()) ->
    {noreply, NewState :: term()} |
    {noreply, NewState :: term(), timeout() | hibernate} |
    {stop, Reason :: term(), NewState :: term()}.
-callback terminate(Reason :: (normal | shutdown | {shutdown, term()} | term()),
                    State :: term()) ->
    ok | term().
-callback code_change(OldVsn :: (term() | {down, term()}), State :: term(),
                      Extra :: term()) ->
    {ok, NewState :: term()} | {error, Reason :: term()}.

%% It's not possible to define "optional" -callbacks, so putting specs
%% for handle_pre_hibernate/1 and handle_post_hibernate/1 will result
%% in warnings (the same applied for the behaviour_info before).

%%%  -----------------------------------------------------------------
%%% Starts a generic server.
%%% start(Mod, Args, Options)
%%% start(Name, Mod, Args, Options)
%%% start_link(Mod, Args, Options)
%%% start_link(Name, Mod, Args, Options) where:
%%%    Name ::= {local, atom()} | {global, atom()}
%%%    Mod  ::= atom(), callback module implementing the 'real' server
%%%    Args ::= term(), init arguments (to Mod:init/1)
%%%    Options ::= [{timeout, Timeout} | {debug, [Flag]}]
%%%      Flag ::= trace | log | {logfile, File} | statistics | debug
%%%          (debug == log && statistics)
%%% Returns: {ok, Pid} |
%%%          {error, {already_started, Pid}} |
%%%          {error, Reason}
%%% -----------------------------------------------------------------
start(Mod, Args, Options) ->
    gen:start(?MODULE, nolink, Mod, Args, Options).

start(Name, Mod, Args, Options) ->
    gen:start(?MODULE, nolink, Name, Mod, Args, Options).

start_link(Mod, Args, Options) ->
    gen:start(?MODULE, link, Mod, Args, Options).

start_link(Name, Mod, Args, Options) ->
    gen:start(?MODULE, link, Name, Mod, Args, Options).


%% -----------------------------------------------------------------
%% Make a call to a generic server.
%% If the server is located at another node, that node will
%% be monitored.
%% If the client is trapping exits and is linked server termination
%% is handled here (? Shall we do that here (or rely on timeouts) ?).
%% -----------------------------------------------------------------
call(Name, Request) ->
    case catch gen:call(Name, '$gen_call', Request) of
        {ok,Res} ->
            Res;
        {'EXIT',Reason} ->
            exit({Reason, {?MODULE, call, [Name, Request]}})
    end.

call(Name, Request, Timeout) ->
    case catch gen:call(Name, '$gen_call', Request, Timeout) of
        {ok,Res} ->
            Res;
        {'EXIT',Reason} ->
            exit({Reason, {?MODULE, call, [Name, Request, Timeout]}})
    end.

%% -----------------------------------------------------------------
%% Make a cast to a generic server.
%% -----------------------------------------------------------------
cast({global,Name}, Request) ->
    catch global:send(Name, cast_msg(Request)),
    ok;
cast({Name,Node}=Dest, Request) when is_atom(Name), is_atom(Node) ->
    do_cast(Dest, Request);
cast(Dest, Request) when is_atom(Dest) ->
    do_cast(Dest, Request);
cast(Dest, Request) when is_pid(Dest) ->
    do_cast(Dest, Request).

do_cast(Dest, Request) ->
    do_send(Dest, cast_msg(Request)),
    ok.

cast_msg(Request) -> {'$gen_cast',Request}.

%% -----------------------------------------------------------------
%% Send a reply to the client.
%% -----------------------------------------------------------------
reply({To, Tag}, Reply) ->
    catch To ! {Tag, Reply}.

%% -----------------------------------------------------------------
%% Asyncronous broadcast, returns nothing, it's just send'n pray
%% -----------------------------------------------------------------
abcast(Name, Request) when is_atom(Name) ->
    do_abcast([node() | nodes()], Name, cast_msg(Request)).

abcast(Nodes, Name, Request) when is_list(Nodes), is_atom(Name) ->
    do_abcast(Nodes, Name, cast_msg(Request)).

do_abcast([Node|Nodes], Name, Msg) when is_atom(Node) ->
    do_send({Name,Node},Msg),
    do_abcast(Nodes, Name, Msg);
do_abcast([], _,_) -> abcast.

%%% -----------------------------------------------------------------
%%% Make a call to servers at several nodes.
%%% Returns: {[Replies],[BadNodes]}
%%% A Timeout can be given
%%%
%%% A middleman process is used in case late answers arrives after
%%% the timeout. If they would be allowed to glog the callers message
%%% queue, it would probably become confused. Late answers will
%%% now arrive to the terminated middleman and so be discarded.
%%% -----------------------------------------------------------------
multi_call(Name, Req)
  when is_atom(Name) ->
    do_multi_call([node() | nodes()], Name, Req, infinity).

multi_call(Nodes, Name, Req)
  when is_list(Nodes), is_atom(Name) ->
    do_multi_call(Nodes, Name, Req, infinity).

multi_call(Nodes, Name, Req, infinity) ->
    do_multi_call(Nodes, Name, Req, infinity);
multi_call(Nodes, Name, Req, Timeout)
  when is_list(Nodes), is_atom(Name), is_integer(Timeout), Timeout >= 0 ->
    do_multi_call(Nodes, Name, Req, Timeout).

%%% -----------------------------------------------------------------
%%% Make multiple calls to multiple servers, given pairs of servers
%%% and messages.
%%% Returns: {[{Dest, Reply}], [{Dest, Error}]}
%%%
%%% Dest can be pid() | RegName :: atom() |
%%%             {Name :: atom(), Node :: atom()} | {global, Name :: atom()}
%%%
%%% A middleman process is used to avoid clogging up the callers
%%% message queue.
%%% -----------------------------------------------------------------
mcall(CallSpecs) ->
    Tag = make_ref(),
    {_, MRef} = spawn_monitor(
                  fun() ->
                          Refs = lists:foldl(
                                   fun ({Dest, _Request}=S, Dict) ->
                                           dict:store(do_mcall(S), Dest, Dict)
                                   end, dict:new(), CallSpecs),
                          collect_replies(Tag, Refs, [], [])
                  end),
    receive
        {'DOWN', MRef, _, _, {Tag, Result}} -> Result;
        {'DOWN', MRef, _, _, Reason}        -> exit(Reason)
    end.

do_mcall({{global,Name}=Dest, Request}) ->
    %% whereis_name is simply an ets lookup, and is precisely what
    %% global:send/2 does, yet we need a Ref to put in the call to the
    %% server, so invoking whereis_name makes a lot more sense here.
    case global:whereis_name(Name) of
        Pid when is_pid(Pid) ->
            MRef = erlang:monitor(process, Pid),
            catch msend(Pid, MRef, Request),
            MRef;
        undefined ->
            Ref = make_ref(),
            self() ! {'DOWN', Ref, process, Dest, noproc},
            Ref
    end;
do_mcall({{Name,Node}=Dest, Request}) when is_atom(Name), is_atom(Node) ->
    {_Node, MRef} = start_monitor(Node, Name), %% NB: we don't handle R6
    catch msend(Dest, MRef, Request),
    MRef;
do_mcall({Dest, Request}) when is_atom(Dest); is_pid(Dest) ->
    MRef = erlang:monitor(process, Dest),
    catch msend(Dest, MRef, Request),
    MRef.

msend(Dest, MRef, Request) ->
    erlang:send(Dest, {'$gen_call', {self(), MRef}, Request}, [noconnect]).

collect_replies(Tag, Refs, Replies, Errors) ->
    case dict:size(Refs) of
        0 -> exit({Tag, {Replies, Errors}});
        _ -> receive
                 {MRef, Reply} ->
                     {Refs1, Replies1} = handle_call_result(MRef, Reply,
                                                            Refs, Replies),
                     collect_replies(Tag, Refs1, Replies1, Errors);
                 {'DOWN', MRef, _, _, Reason} ->
                     Reason1 = case Reason of
                                   noconnection -> nodedown;
                                   _            -> Reason
                               end,
                     {Refs1, Errors1} = handle_call_result(MRef, Reason1,
                                                           Refs, Errors),
                     collect_replies(Tag, Refs1, Replies, Errors1)
             end
    end.

handle_call_result(MRef, Result, Refs, AccList) ->
    %% we avoid the mailbox scanning cost of a call to erlang:demonitor/{1,2}
    %% here, so we must cope with MRefs that we've already seen and erased
    case dict:find(MRef, Refs) of
        {ok, Pid} -> {dict:erase(MRef, Refs), [{Pid, Result}|AccList]};
        _         -> {Refs, AccList}
    end.

%% -----------------------------------------------------------------
%% Apply a function to a generic server's state.
%% -----------------------------------------------------------------
with_state(Name, Fun) ->
    case catch gen:call(Name, '$with_state', Fun, infinity) of
        {ok,Res} ->
            Res;
        {'EXIT',Reason} ->
            exit({Reason, {?MODULE, with_state, [Name, Fun]}})
    end.

%%-----------------------------------------------------------------
%% enter_loop(Mod, Options, State, <ServerName>, <TimeOut>, <Backoff>) ->_
%%
%% Description: Makes an existing process into a gen_server.
%%              The calling process will enter the gen_server receive
%%              loop and become a gen_server process.
%%              The process *must* have been started using one of the
%%              start functions in proc_lib, see proc_lib(3).
%%              The user is responsible for any initialization of the
%%              process, including registering a name for it.
%%-----------------------------------------------------------------
enter_loop(Mod, Options, State) ->
    enter_loop(Mod, Options, State, self(), infinity, undefined).

enter_loop(Mod, Options, State, Backoff = {backoff, _, _ , _}) ->
    enter_loop(Mod, Options, State, self(), infinity, Backoff);

enter_loop(Mod, Options, State, ServerName = {_, _}) ->
    enter_loop(Mod, Options, State, ServerName, infinity, undefined);

enter_loop(Mod, Options, State, Timeout) ->
    enter_loop(Mod, Options, State, self(), Timeout, undefined).

enter_loop(Mod, Options, State, ServerName, Backoff = {backoff, _, _, _}) ->
    enter_loop(Mod, Options, State, ServerName, infinity, Backoff);

enter_loop(Mod, Options, State, ServerName, Timeout) ->
    enter_loop(Mod, Options, State, ServerName, Timeout, undefined).

enter_loop(Mod, Options, State, ServerName, Timeout, Backoff) ->
    Name = get_proc_name(ServerName),
    Parent = get_parent(),
    Debug = debug_options(Name, Options),
    Queue = priority_queue:new(),
    Backoff1 = extend_backoff(Backoff),
    loop(find_prioritisers(
           #gs2_state { parent = Parent, name = Name, state = State,
                        mod = Mod, time = Timeout, timeout_state = Backoff1,
                        queue = Queue, debug = Debug })).

%%%========================================================================
%%% Gen-callback functions
%%%========================================================================

%%% ---------------------------------------------------
%%% Initiate the new process.
%%% Register the name using the Rfunc function
%%% Calls the Mod:init/Args function.
%%% Finally an acknowledge is sent to Parent and the main
%%% loop is entered.
%%% ---------------------------------------------------
init_it(Starter, self, Name, Mod, Args, Options) ->
    init_it(Starter, self(), Name, Mod, Args, Options);
init_it(Starter, Parent, Name0, Mod, Args, Options) ->
    Name = name(Name0),
    Debug = debug_options(Name, Options),
    Queue = priority_queue:new(),
    GS2State = find_prioritisers(
                 #gs2_state { parent  = Parent,
                              name    = Name,
                              mod     = Mod,
                              queue   = Queue,
                              debug   = Debug }),
    case catch Mod:init(Args) of
        {ok, State} ->
            proc_lib:init_ack(Starter, {ok, self()}),
            loop(GS2State #gs2_state { state         = State,
                                       time          = infinity,
                                       timeout_state = undefined });
        {ok, State, Timeout} ->
            proc_lib:init_ack(Starter, {ok, self()}),
            loop(GS2State #gs2_state { state         = State,
                                       time          = Timeout,
                                       timeout_state = undefined });
        {ok, State, Timeout, Backoff = {backoff, _, _, _}} ->
            Backoff1 = extend_backoff(Backoff),
            proc_lib:init_ack(Starter, {ok, self()}),
            loop(GS2State #gs2_state { state         = State,
                                       time          = Timeout,
                                       timeout_state = Backoff1 });
        {ok, State, Timeout, Backoff = {backoff, _, _, _}, Mod1} ->
            Backoff1 = extend_backoff(Backoff),
            proc_lib:init_ack(Starter, {ok, self()}),
            loop(find_prioritisers(
                  GS2State #gs2_state { mod           = Mod1,
                                        state         = State,
                                        time          = Timeout,
                                        timeout_state = Backoff1 }));
        {stop, Reason} ->
            %% For consistency, we must make sure that the
            %% registered name (if any) is unregistered before
            %% the parent process is notified about the failure.
            %% (Otherwise, the parent process could get
            %% an 'already_started' error if it immediately
            %% tried starting the process again.)
            unregister_name(Name0),
            proc_lib:init_ack(Starter, {error, Reason}),
            exit(Reason);
        ignore ->
            unregister_name(Name0),
            proc_lib:init_ack(Starter, ignore),
            exit(normal);
        {'EXIT', Reason} ->
            unregister_name(Name0),
            proc_lib:init_ack(Starter, {error, Reason}),
            exit(Reason);
        Else ->
            Error = {bad_return_value, Else},
            proc_lib:init_ack(Starter, {error, Error}),
            exit(Error)
    end.

name({local,Name}) -> Name;
name({global,Name}) -> Name;
%% name(Pid) when is_pid(Pid) -> Pid;
%% when R12 goes away, drop the line beneath and uncomment the line above
name(Name) -> Name.

unregister_name({local,Name}) ->
    _ = (catch unregister(Name));
unregister_name({global,Name}) ->
    _ = global:unregister_name(Name);
unregister_name(Pid) when is_pid(Pid) ->
    Pid;
%% Under R12 let's just ignore it, as we have a single term as Name.
%% On R13 it will never get here, as we get tuple with 'local/global' atom.
unregister_name(_Name) -> ok.

extend_backoff(undefined) ->
    undefined;
extend_backoff({backoff, InitialTimeout, MinimumTimeout, DesiredHibPeriod}) ->
    {backoff, InitialTimeout, MinimumTimeout, DesiredHibPeriod,
      rand_compat:seed(exsplus)}.

%%%========================================================================
%%% Internal functions
%%%========================================================================
%%% ---------------------------------------------------
%%% The MAIN loop.
%%% ---------------------------------------------------
loop(GS2State = #gs2_state { time          = hibernate,
                             timeout_state = undefined,
                             queue         = Queue }) ->
    case priority_queue:is_empty(Queue) of
        true  ->
            pre_hibernate(GS2State);
        false ->
            process_next_msg(GS2State)
    end;

loop(GS2State) ->
    process_next_msg(drain(GS2State)).

drain(GS2State) ->
    receive
        Input -> drain(in(Input, GS2State))
    after 0 -> GS2State
    end.

process_next_msg(GS2State = #gs2_state { time          = Time,
                                         timeout_state = TimeoutState,
                                         queue         = Queue }) ->
    case priority_queue:out(Queue) of
        {{value, Msg}, Queue1} ->
            process_msg(Msg, GS2State #gs2_state { queue = Queue1 });
        {empty, Queue1} ->
            {Time1, HibOnTimeout}
                = case {Time, TimeoutState} of
                      {hibernate, {backoff, Current, _Min, _Desired, _RSt}} ->
                          {Current, true};
                      {hibernate, _} ->
                          %% wake_hib/7 will set Time to hibernate. If
                          %% we were woken and didn't receive a msg
                          %% then we will get here and need a sensible
                          %% value for Time1, otherwise we crash.
                          %% R13B1 always waits infinitely when waking
                          %% from hibernation, so that's what we do
                          %% here too.
                          {infinity, false};
                      _ -> {Time, false}
                  end,
            receive
                Input ->
                    %% Time could be 'hibernate' here, so *don't* call loop
                    process_next_msg(
                      drain(in(Input, GS2State #gs2_state { queue = Queue1 })))
            after Time1 ->
                    case HibOnTimeout of
                        true ->
                            pre_hibernate(
                              GS2State #gs2_state { queue = Queue1 });
                        false ->
                            process_msg(timeout,
                                        GS2State #gs2_state { queue = Queue1 })
                    end
            end
    end.

wake_hib(GS2State = #gs2_state { timeout_state = TS }) ->
    TimeoutState1 = case TS of
                        undefined ->
                            undefined;
                        {SleptAt, TimeoutState} ->
                            adjust_timeout_state(SleptAt,
                                                 time_compat:monotonic_time(),
                                                 TimeoutState)
                    end,
    post_hibernate(
      drain(GS2State #gs2_state { timeout_state = TimeoutState1 })).

hibernate(GS2State = #gs2_state { timeout_state = TimeoutState }) ->
    TS = case TimeoutState of
             undefined             -> undefined;
             {backoff, _, _, _, _} -> {time_compat:monotonic_time(),
                                       TimeoutState}
         end,
    proc_lib:hibernate(?MODULE, wake_hib,
                       [GS2State #gs2_state { timeout_state = TS }]).

pre_hibernate(GS2State = #gs2_state { state   = State,
                                      mod     = Mod }) ->
    case erlang:function_exported(Mod, handle_pre_hibernate, 1) of
        true ->
            case catch Mod:handle_pre_hibernate(State) of
                {hibernate, NState} ->
                    hibernate(GS2State #gs2_state { state = NState } );
                Reply ->
                    handle_common_termination(Reply, pre_hibernate, GS2State)
            end;
        false ->
            hibernate(GS2State)
    end.

post_hibernate(GS2State = #gs2_state { state = State,
                                       mod   = Mod }) ->
    case erlang:function_exported(Mod, handle_post_hibernate, 1) of
        true ->
            case catch Mod:handle_post_hibernate(State) of
                {noreply, NState} ->
                    process_next_msg(GS2State #gs2_state { state = NState,
                                                           time  = infinity });
                {noreply, NState, Time} ->
                    process_next_msg(GS2State #gs2_state { state = NState,
                                                           time  = Time });
                Reply ->
                    handle_common_termination(Reply, post_hibernate, GS2State)
            end;
        false ->
            %% use hibernate here, not infinity. This matches
            %% R13B. The key is that we should be able to get through
            %% to process_msg calling sys:handle_system_msg with Time
            %% still set to hibernate, iff that msg is the very msg
            %% that woke us up (or the first msg we receive after
            %% waking up).
            process_next_msg(GS2State #gs2_state { time = hibernate })
    end.

adjust_timeout_state(SleptAt, AwokeAt, {backoff, CurrentTO, MinimumTO,
                                        DesiredHibPeriod, RandomState}) ->
    NapLengthMicros = time_compat:convert_time_unit(AwokeAt - SleptAt,
                                                    native, micro_seconds),
    CurrentMicros = CurrentTO * 1000,
    MinimumMicros = MinimumTO * 1000,
    DesiredHibMicros = DesiredHibPeriod * 1000,
    GapBetweenMessagesMicros = NapLengthMicros + CurrentMicros,
    Base =
        %% If enough time has passed between the last two messages then we
        %% should consider sleeping sooner. Otherwise stay awake longer.
        case GapBetweenMessagesMicros > (MinimumMicros + DesiredHibMicros) of
            true -> lists:max([MinimumTO, CurrentTO div 2]);
            false -> CurrentTO
        end,
    {Extra, RandomState1} = rand_compat:uniform_s(Base, RandomState),
    CurrentTO1 = Base + Extra,
    {backoff, CurrentTO1, MinimumTO, DesiredHibPeriod, RandomState1}.

in({'$gen_cast', Msg} = Input,
   GS2State = #gs2_state { prioritisers = {_, F, _} }) ->
    in(Input, F(Msg, GS2State), GS2State);
in({'$gen_call', From, Msg} = Input,
   GS2State = #gs2_state { prioritisers = {F, _, _} }) ->
    in(Input, F(Msg, From, GS2State), GS2State);
in({'$with_state', _From, _Fun} = Input, GS2State) ->
    in(Input, 0, GS2State);
in({'EXIT', Parent, _R} = Input, GS2State = #gs2_state { parent = Parent }) ->
    in(Input, infinity, GS2State);
in({system, _From, _Req} = Input, GS2State) ->
    in(Input, infinity, GS2State);
in(Input, GS2State = #gs2_state { prioritisers = {_, _, F} }) ->
    in(Input, F(Input, GS2State), GS2State).

in(_Input, drop, GS2State) ->
    GS2State;

in(Input, Priority, GS2State = #gs2_state { queue = Queue }) ->
    GS2State # gs2_state { queue = priority_queue:in(Input, Priority, Queue) }.

process_msg({system, From, Req},
            GS2State = #gs2_state { parent = Parent, debug  = Debug }) ->
    %% gen_server puts Hib on the end as the 7th arg, but that version
    %% of the fun seems not to be documented so leaving out for now.
    sys:handle_system_msg(Req, From, Parent, ?MODULE, Debug, GS2State);
process_msg({'$with_state', From, Fun},
           GS2State = #gs2_state{state = State}) ->
    reply(From, catch Fun(State)),
    loop(GS2State);
process_msg({'EXIT', Parent, Reason} = Msg,
            GS2State = #gs2_state { parent = Parent }) ->
    terminate(Reason, Msg, GS2State);
process_msg(Msg, GS2State = #gs2_state { debug  = [] }) ->
    handle_msg(Msg, GS2State);
process_msg(Msg, GS2State = #gs2_state { name = Name, debug  = Debug }) ->
    Debug1 = sys:handle_debug(Debug, fun print_event/3, Name, {in, Msg}),
    handle_msg(Msg, GS2State #gs2_state { debug = Debug1 }).

%%% ---------------------------------------------------
%%% Send/recive functions
%%% ---------------------------------------------------
do_send(Dest, Msg) ->
    catch erlang:send(Dest, Msg).

do_multi_call(Nodes, Name, Req, infinity) ->
    Tag = make_ref(),
    Monitors = send_nodes(Nodes, Name, Tag, Req),
    rec_nodes(Tag, Monitors, Name, undefined);
do_multi_call(Nodes, Name, Req, Timeout) ->
    Tag = make_ref(),
    Caller = self(),
    Receiver =
        spawn(
          fun () ->
                  %% Middleman process. Should be unsensitive to regular
                  %% exit signals. The sychronization is needed in case
                  %% the receiver would exit before the caller started
                  %% the monitor.
                  process_flag(trap_exit, true),
                  Mref = erlang:monitor(process, Caller),
                  receive
                      {Caller,Tag} ->
                          Monitors = send_nodes(Nodes, Name, Tag, Req),
                          TimerId = erlang:start_timer(Timeout, self(), ok),
                          Result = rec_nodes(Tag, Monitors, Name, TimerId),
                          exit({self(),Tag,Result});
                      {'DOWN',Mref,_,_,_} ->
                          %% Caller died before sending us the go-ahead.
                          %% Give up silently.
                          exit(normal)
                  end
          end),
    Mref = erlang:monitor(process, Receiver),
    Receiver ! {self(),Tag},
    receive
        {'DOWN',Mref,_,_,{Receiver,Tag,Result}} ->
            Result;
        {'DOWN',Mref,_,_,Reason} ->
            %% The middleman code failed. Or someone did
            %% exit(_, kill) on the middleman process => Reason==killed
            exit(Reason)
    end.

send_nodes(Nodes, Name, Tag, Req) ->
    send_nodes(Nodes, Name, Tag, Req, []).

send_nodes([Node|Tail], Name, Tag, Req, Monitors)
  when is_atom(Node) ->
    Monitor = start_monitor(Node, Name),
    %% Handle non-existing names in rec_nodes.
    catch {Name, Node} ! {'$gen_call', {self(), {Tag, Node}}, Req},
    send_nodes(Tail, Name, Tag, Req, [Monitor | Monitors]);
send_nodes([_Node|Tail], Name, Tag, Req, Monitors) ->
    %% Skip non-atom Node
    send_nodes(Tail, Name, Tag, Req, Monitors);
send_nodes([], _Name, _Tag, _Req, Monitors) ->
    Monitors.

%% Against old nodes:
%% If no reply has been delivered within 2 secs. (per node) check that
%% the server really exists and wait for ever for the answer.
%%
%% Against contemporary nodes:
%% Wait for reply, server 'DOWN', or timeout from TimerId.

rec_nodes(Tag, Nodes, Name, TimerId) ->
    rec_nodes(Tag, Nodes, Name, [], [], 2000, TimerId).

rec_nodes(Tag, [{N,R}|Tail], Name, Badnodes, Replies, Time, TimerId ) ->
    receive
        {'DOWN', R, _, _, _} ->
            rec_nodes(Tag, Tail, Name, [N|Badnodes], Replies, Time, TimerId);
        {{Tag, N}, Reply} ->  %% Tag is bound !!!
            unmonitor(R),
            rec_nodes(Tag, Tail, Name, Badnodes,
                      [{N,Reply}|Replies], Time, TimerId);
        {timeout, TimerId, _} ->
            unmonitor(R),
            %% Collect all replies that already have arrived
            rec_nodes_rest(Tag, Tail, Name, [N|Badnodes], Replies)
    end;
rec_nodes(Tag, [N|Tail], Name, Badnodes, Replies, Time, TimerId) ->
    %% R6 node
    receive
        {nodedown, N} ->
            monitor_node(N, false),
            rec_nodes(Tag, Tail, Name, [N|Badnodes], Replies, 2000, TimerId);
        {{Tag, N}, Reply} ->  %% Tag is bound !!!
            receive {nodedown, N} -> ok after 0 -> ok end,
            monitor_node(N, false),
            rec_nodes(Tag, Tail, Name, Badnodes,
                      [{N,Reply}|Replies], 2000, TimerId);
        {timeout, TimerId, _} ->
            receive {nodedown, N} -> ok after 0 -> ok end,
            monitor_node(N, false),
            %% Collect all replies that already have arrived
            rec_nodes_rest(Tag, Tail, Name, [N | Badnodes], Replies)
    after Time ->
            case rpc:call(N, erlang, whereis, [Name]) of
                Pid when is_pid(Pid) -> % It exists try again.
                    rec_nodes(Tag, [N|Tail], Name, Badnodes,
                              Replies, infinity, TimerId);
                _ -> % badnode
                    receive {nodedown, N} -> ok after 0 -> ok end,
                    monitor_node(N, false),
                    rec_nodes(Tag, Tail, Name, [N|Badnodes],
                              Replies, 2000, TimerId)
            end
    end;
rec_nodes(_, [], _, Badnodes, Replies, _, TimerId) ->
    case catch erlang:cancel_timer(TimerId) of
        false ->  % It has already sent it's message
            receive
                {timeout, TimerId, _} -> ok
            after 0 ->
                    ok
            end;
        _ -> % Timer was cancelled, or TimerId was 'undefined'
            ok
    end,
    {Replies, Badnodes}.

%% Collect all replies that already have arrived
rec_nodes_rest(Tag, [{N,R}|Tail], Name, Badnodes, Replies) ->
    receive
        {'DOWN', R, _, _, _} ->
            rec_nodes_rest(Tag, Tail, Name, [N|Badnodes], Replies);
        {{Tag, N}, Reply} -> %% Tag is bound !!!
            unmonitor(R),
            rec_nodes_rest(Tag, Tail, Name, Badnodes, [{N,Reply}|Replies])
    after 0 ->
            unmonitor(R),
            rec_nodes_rest(Tag, Tail, Name, [N|Badnodes], Replies)
    end;
rec_nodes_rest(Tag, [N|Tail], Name, Badnodes, Replies) ->
    %% R6 node
    receive
        {nodedown, N} ->
            monitor_node(N, false),
            rec_nodes_rest(Tag, Tail, Name, [N|Badnodes], Replies);
        {{Tag, N}, Reply} ->  %% Tag is bound !!!
            receive {nodedown, N} -> ok after 0 -> ok end,
            monitor_node(N, false),
            rec_nodes_rest(Tag, Tail, Name, Badnodes, [{N,Reply}|Replies])
    after 0 ->
            receive {nodedown, N} -> ok after 0 -> ok end,
            monitor_node(N, false),
            rec_nodes_rest(Tag, Tail, Name, [N|Badnodes], Replies)
    end;
rec_nodes_rest(_Tag, [], _Name, Badnodes, Replies) ->
    {Replies, Badnodes}.


%%% ---------------------------------------------------
%%% Monitor functions
%%% ---------------------------------------------------

start_monitor(Node, Name) when is_atom(Node), is_atom(Name) ->
    if node() =:= nonode@nohost, Node =/= nonode@nohost ->
            Ref = make_ref(),
            self() ! {'DOWN', Ref, process, {Name, Node}, noconnection},
            {Node, Ref};
       true ->
            case catch erlang:monitor(process, {Name, Node}) of
                {'EXIT', _} ->
                    %% Remote node is R6
                    monitor_node(Node, true),
                    Node;
                Ref when is_reference(Ref) ->
                    {Node, Ref}
            end
    end.

%% Cancels a monitor started with Ref=erlang:monitor(_, _).
unmonitor(Ref) when is_reference(Ref) ->
    erlang:demonitor(Ref),
    receive
        {'DOWN', Ref, _, _, _} ->
            true
    after 0 ->
            true
    end.

%%% ---------------------------------------------------
%%% Message handling functions
%%% ---------------------------------------------------

dispatch({'$gen_cast', Msg}, Mod, State) ->
    Mod:handle_cast(Msg, State);
dispatch(Info, Mod, State) ->
    Mod:handle_info(Info, State).

common_reply(_Name, From, Reply, _NState, [] = _Debug) ->
    reply(From, Reply),
    [];
common_reply(Name, {To, _Tag} = From, Reply, NState, Debug) ->
    reply(From, Reply),
    sys:handle_debug(Debug, fun print_event/3, Name, {out, Reply, To, NState}).

common_noreply(_Name, _NState, [] = _Debug) ->
    [];
common_noreply(Name, NState, Debug) ->
    sys:handle_debug(Debug, fun print_event/3, Name, {noreply, NState}).

common_become(_Name, _Mod, _NState, [] = _Debug) ->
    [];
common_become(Name, Mod, NState, Debug) ->
    sys:handle_debug(Debug, fun print_event/3, Name, {become, Mod, NState}).

handle_msg({'$gen_call', From, Msg}, GS2State = #gs2_state { mod = Mod,
                                                             state = State,
                                                             name = Name,
                                                             debug = Debug }) ->
    case catch Mod:handle_call(Msg, From, State) of
        {reply, Reply, NState} ->
            Debug1 = common_reply(Name, From, Reply, NState, Debug),
            loop(GS2State #gs2_state { state = NState,
                                       time  = infinity,
                                       debug = Debug1 });
        {reply, Reply, NState, Time1} ->
            Debug1 = common_reply(Name, From, Reply, NState, Debug),
            loop(GS2State #gs2_state { state = NState,
                                       time  = Time1,
                                       debug = Debug1});
        {stop, Reason, Reply, NState} ->
            {'EXIT', R} =
                (catch terminate(Reason, Msg,
                                 GS2State #gs2_state { state = NState })),
            common_reply(Name, From, Reply, NState, Debug),
            exit(R);
        Other ->
            handle_common_reply(Other, Msg, GS2State)
    end;
handle_msg(Msg, GS2State = #gs2_state { mod = Mod, state = State }) ->
    Reply = (catch dispatch(Msg, Mod, State)),
    handle_common_reply(Reply, Msg, GS2State).

handle_common_reply(Reply, Msg, GS2State = #gs2_state { name  = Name,
                                                        debug = Debug}) ->
    case Reply of
        {noreply, NState} ->
            Debug1 = common_noreply(Name, NState, Debug),
            loop(GS2State #gs2_state {state = NState,
                                      time  = infinity,
                                      debug = Debug1});
        {noreply, NState, Time1} ->
            Debug1 = common_noreply(Name, NState, Debug),
            loop(GS2State #gs2_state {state = NState,
                                      time  = Time1,
                                      debug = Debug1});
        {become, Mod, NState} ->
            Debug1 = common_become(Name, Mod, NState, Debug),
            loop(find_prioritisers(
                   GS2State #gs2_state { mod   = Mod,
                                         state = NState,
                                         time  = infinity,
                                         debug = Debug1 }));
        {become, Mod, NState, Time1} ->
            Debug1 = common_become(Name, Mod, NState, Debug),
            loop(find_prioritisers(
                   GS2State #gs2_state { mod   = Mod,
                                         state = NState,
                                         time  = Time1,
                                         debug = Debug1 }));
        _ ->
            handle_common_termination(Reply, Msg, GS2State)
    end.

handle_common_termination(Reply, Msg, GS2State) ->
    case Reply of
        {stop, Reason, NState} ->
            terminate(Reason, Msg, GS2State #gs2_state { state = NState });
        {'EXIT', What} ->
            terminate(What, Msg, GS2State);
        _ ->
            terminate({bad_return_value, Reply}, Msg, GS2State)
    end.

%%-----------------------------------------------------------------
%% Callback functions for system messages handling.
%%-----------------------------------------------------------------
system_continue(Parent, Debug, GS2State) ->
    loop(GS2State #gs2_state { parent = Parent, debug = Debug }).

system_terminate(Reason, _Parent, Debug, GS2State) ->
    terminate(Reason, [], GS2State #gs2_state { debug = Debug }).

system_code_change(GS2State = #gs2_state { mod   = Mod,
                                           state = State },
                   _Module, OldVsn, Extra) ->
    case catch Mod:code_change(OldVsn, State, Extra) of
        {ok, NewState} ->
            NewGS2State = find_prioritisers(
                            GS2State #gs2_state { state = NewState }),
            {ok, [NewGS2State]};
        Else ->
            Else
    end.

%%-----------------------------------------------------------------
%% Format debug messages.  Print them as the call-back module sees
%% them, not as the real erlang messages.  Use trace for that.
%%-----------------------------------------------------------------
print_event(Dev, {in, Msg}, Name) ->
    case Msg of
        {'$gen_call', {From, _Tag}, Call} ->
            io:format(Dev, "*DBG* ~p got call ~p from ~w~n",
                      [Name, Call, From]);
        {'$gen_cast', Cast} ->
            io:format(Dev, "*DBG* ~p got cast ~p~n",
                      [Name, Cast]);
        _ ->
            io:format(Dev, "*DBG* ~p got ~p~n", [Name, Msg])
    end;
print_event(Dev, {out, Msg, To, State}, Name) ->
    io:format(Dev, "*DBG* ~p sent ~p to ~w, new state ~w~n",
              [Name, Msg, To, State]);
print_event(Dev, {noreply, State}, Name) ->
    io:format(Dev, "*DBG* ~p new state ~w~n", [Name, State]);
print_event(Dev, Event, Name) ->
    io:format(Dev, "*DBG* ~p dbg  ~p~n", [Name, Event]).


%%% ---------------------------------------------------
%%% Terminate the server.
%%% ---------------------------------------------------

terminate(Reason, Msg, #gs2_state { name  = Name,
                                    mod   = Mod,
                                    state = State,
                                    debug = Debug }) ->
    case catch Mod:terminate(Reason, State) of
        {'EXIT', R} ->
            error_info(R, Reason, Name, Msg, State, Debug),
            exit(R);
        _ ->
            case Reason of
                normal ->
                    exit(normal);
                shutdown ->
                    exit(shutdown);
                {shutdown,_}=Shutdown ->
                    exit(Shutdown);
                _ ->
                    error_info(Reason, undefined, Name, Msg, State, Debug),
                    exit(Reason)
            end
    end.

error_info(_Reason, _RootCause, application_controller, _Msg, _State, _Debug) ->
    %% OTP-5811 Don't send an error report if it's the system process
    %% application_controller which is terminating - let init take care
    %% of it instead
    ok;
error_info(Reason, RootCause, Name, Msg, State, Debug) ->
    Reason1 = error_reason(Reason),
    Fmt =
        "** Generic server ~p terminating~n"
        "** Last message in was ~p~n"
        "** When Server state == ~p~n"
        "** Reason for termination == ~n** ~p~n",
    case RootCause of
        undefined -> format(Fmt, [Name, Msg, State, Reason1]);
        _         -> format(Fmt ++ "** In 'terminate' callback "
                            "with reason ==~n** ~p~n",
                            [Name, Msg, State, Reason1,
                             error_reason(RootCause)])
    end,
    sys:print_log(Debug),
    ok.

error_reason({undef,[{M,F,A}|MFAs]} = Reason) ->
    case code:is_loaded(M) of
        false -> {'module could not be loaded',[{M,F,A}|MFAs]};
        _     -> case erlang:function_exported(M, F, length(A)) of
                     true  -> Reason;
                     false -> {'function not exported',[{M,F,A}|MFAs]}
                 end
    end;
error_reason(Reason) ->
    Reason.

%%% ---------------------------------------------------
%%% Misc. functions.
%%% ---------------------------------------------------

opt(Op, [{Op, Value}|_]) ->
    {ok, Value};
opt(Op, [_|Options]) ->
    opt(Op, Options);
opt(_, []) ->
    false.

debug_options(Name, Opts) ->
    case opt(debug, Opts) of
        {ok, Options} -> dbg_options(Name, Options);
        _ -> dbg_options(Name, [])
    end.

dbg_options(Name, []) ->
    Opts =
        case init:get_argument(generic_debug) of
            error ->
                [];
            _ ->
                [log, statistics]
        end,
    dbg_opts(Name, Opts);
dbg_options(Name, Opts) ->
    dbg_opts(Name, Opts).

dbg_opts(Name, Opts) ->
    case catch sys:debug_options(Opts) of
        {'EXIT',_} ->
            format("~p: ignoring erroneous debug options - ~p~n",
                   [Name, Opts]),
            [];
        Dbg ->
            Dbg
    end.

get_proc_name(Pid) when is_pid(Pid) ->
    Pid;
get_proc_name({local, Name}) ->
    case process_info(self(), registered_name) of
        {registered_name, Name} ->
            Name;
        {registered_name, _Name} ->
            exit(process_not_registered);
        [] ->
            exit(process_not_registered)
    end;
get_proc_name({global, Name}) ->
    case whereis_name(Name) of
        undefined ->
            exit(process_not_registered_globally);
        Pid when Pid =:= self() ->
            Name;
        _Pid ->
            exit(process_not_registered_globally)
    end.

get_parent() ->
    case get('$ancestors') of
        [Parent | _] when is_pid(Parent)->
            Parent;
        [Parent | _] when is_atom(Parent)->
            name_to_pid(Parent);
        _ ->
            exit(process_was_not_started_by_proc_lib)
    end.

name_to_pid(Name) ->
    case whereis(Name) of
        undefined ->
            case whereis_name(Name) of
                undefined ->
                    exit(could_not_find_registerd_name);
                Pid ->
                    Pid
            end;
        Pid ->
            Pid
    end.

whereis_name(Name) ->
    case ets:lookup(global_names, Name) of
    [{_Name, Pid, _Method, _RPid, _Ref}] ->
        if node(Pid) == node() ->
            case is_process_alive(Pid) of
            true  -> Pid;
            false -> undefined
            end;
           true ->
            Pid
        end;
    [] -> undefined
    end.

find_prioritisers(GS2State = #gs2_state { mod = Mod }) ->
    PCall = function_exported_or_default(Mod, 'prioritise_call', 4,
                                         fun (_Msg, _From, _State) -> 0 end),
    PCast = function_exported_or_default(Mod, 'prioritise_cast', 3,
                                         fun (_Msg, _State) -> 0 end),
    PInfo = function_exported_or_default(Mod, 'prioritise_info', 3,
                                         fun (_Msg, _State) -> 0 end),
    GS2State #gs2_state { prioritisers = {PCall, PCast, PInfo} }.

function_exported_or_default(Mod, Fun, Arity, Default) ->
    case erlang:function_exported(Mod, Fun, Arity) of
        true -> case Arity of
                    3 -> fun (Msg, GS2State = #gs2_state { queue = Queue,
                                                           state = State }) ->
                                 Length = priority_queue:len(Queue),
                                 case catch Mod:Fun(Msg, Length, State) of
                                     drop ->
                                         drop;
                                     Res when is_integer(Res) ->
                                         Res;
                                     Err ->
                                         handle_common_termination(Err, Msg, GS2State)
                                 end
                         end;
                    4 -> fun (Msg, From, GS2State = #gs2_state { queue = Queue,
                                                                 state = State }) ->
                                 Length = priority_queue:len(Queue),
                                 case catch Mod:Fun(Msg, From, Length, State) of
                                     Res when is_integer(Res) ->
                                         Res;
                                     Err ->
                                         handle_common_termination(Err, Msg, GS2State)
                                 end
                         end
                end;
        false -> Default
    end.

%%-----------------------------------------------------------------
%% Status information
%%-----------------------------------------------------------------
format_status(Opt, StatusData) ->
    [PDict, SysState, Parent, Debug,
     #gs2_state{name = Name, state = State, mod = Mod, queue = Queue}] =
        StatusData,
    NameTag = if is_pid(Name) ->
                      pid_to_list(Name);
                 is_atom(Name) ->
                      Name
              end,
    Header = lists:concat(["Status for generic server ", NameTag]),
    Log = sys:get_debug(log, Debug, []),
    Specfic = callback(Mod, format_status, [Opt, [PDict, State]],
                       fun () -> [{data, [{"State", State}]}] end),
    Messages = callback(Mod, format_message_queue, [Opt, Queue],
                        fun () -> priority_queue:to_list(Queue) end),
    [{header, Header},
     {data, [{"Status", SysState},
             {"Parent", Parent},
             {"Logged events", Log},
             {"Queued messages", Messages}]} |
     Specfic].

callback(Mod, FunName, Args, DefaultThunk) ->
    case erlang:function_exported(Mod, FunName, length(Args)) of
        true  -> case catch apply(Mod, FunName, Args) of
                     {'EXIT', _} -> DefaultThunk();
                     Success     -> Success
                 end;
        false -> DefaultThunk()
    end.
