gen_event.erl

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server_add_handler({Mod,Id}, Args, MSL) ->
    Handler = #handler{module = Mod,
		       id = Id},
    server_add_handler(Mod, Handler, Args, MSL);
server_add_handler(Mod, Args, MSL) -> 
    Handler = #handler{module = Mod},
    server_add_handler(Mod, Handler, Args, MSL).

server_add_handler(Mod, Handler, Args, MSL) ->
    case catch Mod:init(Args) of
        {ok, State} ->
	    {ok, [Handler#handler{state = State}|MSL]};
        Other ->
            {Other, MSL}
    end.

%% Set up a link to the supervising process.
%% (Ought to be unidirected links here, Erl5.0 !!)
%% NOTE: This link will not be removed then the
%% handler is removed in case another handler has
%% own link to this process.
server_add_sup_handler({Mod,Id}, Args, MSL, Parent) ->
    link(Parent),
    Handler = #handler{module = Mod,
		       id = Id,
		       supervised = Parent},
    server_add_handler(Mod, Handler, Args, MSL);
server_add_sup_handler(Mod, Args, MSL, Parent) -> 
    link(Parent),
    Handler = #handler{module = Mod,
		       supervised = Parent},
    server_add_handler(Mod, Handler, Args, MSL).

%% server_delete_handler(HandlerId, Args, MSL) -> {Ret, MSL'}

server_delete_handler(HandlerId, Args, MSL, SName) -> 
    case split(HandlerId, MSL) of
	{Mod, Handler, MSL1} ->
	    {do_terminate(Mod, Handler, Args,
			  Handler#handler.state, delete, SName, normal),
	     MSL1};
	error ->
	    {{error, module_not_found}, MSL}
    end.

%% server_swap_handler(Handler1, Args1, Handler2, Args2, MSL, SN)= -> MSL'
%% server_swap_handler(Handler1, Args1, Handler2, Args2, MSL, Sup, SN)= -> MSL'

server_swap_handler(Handler1, Args1, Handler2, Args2, MSL, SName) ->
    {State2, Sup, MSL1} = split_and_terminate(Handler1, Args1, MSL,
					      SName, Handler2, false),
    case s_s_h(Sup, Handler2, {Args2, State2}, MSL1) of
	{ok, MSL2} ->
	    {ok, MSL2};
	{What, MSL2} ->
	    {{error, What}, MSL2}
    end.

server_swap_handler(Handler1, Args1, Handler2, Args2, MSL, Sup, SName) ->
    {State2, _, MSL1} = split_and_terminate(Handler1, Args1, MSL,
					    SName, Handler2, Sup),
    case s_s_h(Sup, Handler2, {Args2, State2}, MSL1) of
	{ok, MSL2} ->
	    {ok, MSL2};
	{What, MSL2} ->
	    {{error, What}, MSL2}
    end.

s_s_h(false, Handler, Args, MSL) ->
    server_add_handler(Handler, Args, MSL);
s_s_h(Pid, Handler, Args, MSL) ->
    server_add_sup_handler(Handler, Args, MSL, Pid).

split_and_terminate(HandlerId, Args, MSL, SName, Handler2, Sup) ->
    case split(HandlerId, MSL) of
	{Mod, Handler, MSL1} ->
	    OldSup = Handler#handler.supervised,
	    NewSup = if
			 not Sup -> OldSup;
			 true    -> Sup
		     end,
	    {do_terminate(Mod, Handler, Args,
			  Handler#handler.state, swapped, SName,
			  {swapped, Handler2, NewSup}),
	     OldSup,
	     MSL1};
	error ->
            {error, false, MSL}
    end.

%% server_notify(Event, Func, MSL, SName) -> MSL'

server_notify(Event, Func, [Handler|T], SName) -> 
    case server_update(Handler, Func, Event, SName) of
	{ok, Handler1} ->
	    [Handler1|server_notify(Event, Func, T, SName)];
	no ->
	    server_notify(Event, Func, T, SName)
    end;
server_notify(_, _, [], _) ->
    [].

%% server_update(Handler, Func, Event, ServerName) -> Handler1 | no

server_update(Handler1, Func, Event, SName) ->
    Mod1 = Handler1#handler.module,
    State = Handler1#handler.state,
    case catch Mod1:Func(Event, State) of
	{ok, State1} -> 
	    {ok, Handler1#handler{state = State1}};
	{swap_handler, Args1, State1, Handler2, Args2} ->
	    do_swap(Mod1,Handler1,Args1,State1,Handler2,Args2,SName);
	remove_handler ->
	    do_terminate(Mod1, Handler1, remove_handler, State,
			 remove, SName, normal),
	    no;
	Other ->
	    do_terminate(Mod1, Handler1, {error, Other}, State,
			 Event, SName, crash),
	    no
    end.

do_swap(Mod1,Handler1,Args1,State1,Handler2,Args2,SName) ->
    %% finalise the existing handler
    State2 = do_terminate(Mod1, Handler1, Args1, State1,
			  swapped, SName,
			  {swapped, Handler2, Handler1#handler.supervised}),
    {Mod2,Handler} = new_handler(Handler2, Handler1),
    case catch Mod2:init({Args2, State2}) of
	{ok, State2a} ->
	    {ok, Handler#handler{state = State2a}};
	Other ->
	    report_terminate(Handler2, crash, {error, Other}, SName, false),
	    no
    end.

new_handler({Mod,Id}, Handler1) ->
    {Mod,#handler{module = Mod,
		  id = Id,
		  supervised = Handler1#handler.supervised}};
new_handler(Mod, Handler1) ->
    {Mod,#handler{module = Mod,
		  supervised = Handler1#handler.supervised}}.


%% split(Handler, [#handler]) ->
%%   {Mod, #handler, [#handler]} | error

split(Ha, MSL) -> split(Ha, MSL, []).

split({Mod,Id}, [Ha|T], L) when Ha#handler.module =:= Mod,
                                Ha#handler.id =:= Id ->
    {Mod, Ha, lists:reverse(L, T)};
split(Mod, [Ha|T], L) when Ha#handler.module =:= Mod,
                           not Ha#handler.id ->
    {Mod, Ha, lists:reverse(L, T)};
split(Ha, [H|T], L) ->
    split(Ha, T, [H|L]);
split(_, [], _) ->
    error.

%% server_call(Handler, Query, MSL, ServerName) ->
%%    {Reply, MSL1}

server_call(Handler, Query, MSL, SName) ->
    case search(Handler, MSL) of
	{ok, Ha} ->
	    case server_call_update(Ha, Query, SName) of
		{no, Reply} ->
		    {Reply, delete(Handler, MSL)};
		{{ok, Ha1}, Reply} ->
		    {Reply, replace(Handler, MSL, Ha1)}
	    end;
	false ->
	    {{error, bad_module}, MSL}
    end.

search({Mod, Id}, [Ha|_MSL]) when Ha#handler.module =:= Mod,
				  Ha#handler.id =:= Id ->
    {ok, Ha};
search(Mod, [Ha|_MSL]) when Ha#handler.module =:= Mod,
			    not Ha#handler.id ->
    {ok, Ha};
search(Handler, [_|MSL]) ->
    search(Handler, MSL);
search(_, []) ->
    false.

delete({Mod, Id}, [Ha|MSL]) when Ha#handler.module =:= Mod,
                                 Ha#handler.id =:= Id ->
    MSL;
delete(Mod, [Ha|MSL]) when Ha#handler.module =:= Mod,
                           not Ha#handler.id ->
    MSL;
delete(Handler, [Ha|MSL]) ->
    [Ha|delete(Handler, MSL)];
delete(_, []) ->
    [].

replace({Mod, Id}, [Ha|MSL], NewHa) when Ha#handler.module =:= Mod,
                                         Ha#handler.id =:= Id ->
    [NewHa|MSL];
replace(Mod, [Ha|MSL], NewHa) when Ha#handler.module =:= Mod,
                                   not Ha#handler.id ->
    [NewHa|MSL];
replace(Handler, [Ha|MSL], NewHa) ->
    [Ha|replace(Handler, MSL, NewHa)];
replace(_, [], NewHa) ->
    [NewHa].

%% server_call_update(Handler, Query, ServerName) ->
%%    {{Handler1, State1} | no, Reply}

server_call_update(Handler1, Query, SName) ->
    Mod1 = Handler1#handler.module,
    State = Handler1#handler.state,
    case catch Mod1:handle_call(Query, State) of
	{ok, Reply, State1} -> 
	    {{ok, Handler1#handler{state = State1}}, Reply};
	{swap_handler, Reply, Args1, State1, Handler2, Args2} ->
	    {do_swap(Mod1,Handler1,Args1,State1,Handler2,Args2,SName), Reply};
	{remove_handler, Reply} -> 
	    do_terminate(Mod1, Handler1, remove_handler, State,
			 remove, SName, normal),
	    {no, Reply};
	Other ->
	    do_terminate(Mod1, Handler1, {error, Other}, State,
			 Query, SName, crash),
	    {no, {error, Other}}
end.

do_terminate(Mod, Handler, Args, State, LastIn, SName, Reason) ->
    Res = (catch Mod:terminate(Args, State)),
    report_terminate(Handler, Reason, Args, State, LastIn, SName, Res),
    Res.

report_terminate(Handler, crash, {error, Why}, State, LastIn, SName, _) ->
    report_terminate(Handler, Why, State, LastIn, SName);
report_terminate(Handler, How, _, State, LastIn, SName, _) ->
    %% How == normal | shutdown | {swapped, NewHandler, NewSupervisor}
    report_terminate(Handler, How, State, LastIn, SName).

report_terminate(Handler, Reason, State, LastIn, SName) ->
    report_error(Handler, Reason, State, LastIn, SName),
    case Handler#handler.supervised of
	false ->
	    ok;
	Pid ->
	    Pid ! {gen_event_EXIT,handler(Handler),Reason},
	    ok
    end.

report_error(_Handler, normal, _, _, _)               -> ok;
report_error(_Handler, shutdown, _, _, _)             -> ok;
report_error(_Handler, {swapped,_,_}, _, _, _)        -> ok;
report_error(Handler, Reason, State, LastIn, SName)   ->
    Reason1 = 
	case Reason of
	    {'EXIT',{undef,[{M,F,A}|MFAs]}} ->
		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 ->
				{undef,[{M,F,A}|MFAs]};
			    false ->
				{'function not exported',[{M,F,A}|MFAs]}
			end
		end;
	    {'EXIT',Why} -> 
		Why;
	    _ ->            
		Reason
	end,
    error_msg("** gen_event handler ~p crashed.~n"
	      "** Was installed in ~p~n"
	      "** Last event was: ~p~n"
	      "** When handler state == ~p~n"
	      "** Reason == ~p~n",
	      [handler(Handler),SName,LastIn,State,Reason1]).

handler(Handler) when not Handler#handler.id ->
    Handler#handler.module;
handler(Handler) ->
    {Handler#handler.module, Handler#handler.id}.

%% stop_handlers(MSL, ServerName) -> []

stop_handlers([Handler|T], SName) ->
    Mod = Handler#handler.module,
    do_terminate(Mod, Handler, stop, Handler#handler.state,
		 stop, SName, shutdown),
    stop_handlers(T, SName);
stop_handlers([], _) ->
    [].

the_handlers(MSL) ->
    lists:map(fun(Handler) when not Handler#handler.id ->
		      Handler#handler.module;
		 (Handler) ->
		      {Handler#handler.module, Handler#handler.id}
	      end,
	      MSL).

%% Message from the release_handler.
%% The list of modules got to be a set !
get_modules(MSL) ->
    Mods = lists:map(fun(Handler) -> Handler#handler.module end,
		     MSL),
    ordsets:to_list(ordsets:from_list(Mods)).

%%-----------------------------------------------------------------
%% Status information
%%-----------------------------------------------------------------
format_status(_Opt, StatusData) ->
    [_PDict, SysState, Parent, _Debug, [ServerName, MSL]] = StatusData,
    Header = lists:concat(["Status for event handler ", ServerName]),
    [{header, Header},
     {data, [{"Status", SysState},
	     {"Parent", Parent}]},
     {items, {"Installed handlers", MSL}}].