Bug: process unexpectedly exits loop

Doug Edmunds (gmail) <>
Mon May 3 22:51:28 CEST 2010


Hello,

I'm posting a module (conn3.erl) below.
This module builds a hierarchical tree of PIDs.
There are two loops, one for entries, and another
for the position in the tree(called 'me').

Each 'entry' runs a copy of the entry_loop.
Each entry keeps track of its parent (one pid)
and its children (list of 0 or more pids). Entries
are not called directly.

The me process runs in the me_loop. It manages the
entries, and moves up and down in the tree, via:
me ! add, me ! up, me ! down, me ! delete, me ! show.

The bug I've encountered is when trying to move down the
tree when there are multiple children. Here's the basic
scenario:
After running conn3:start(),
type:
	me ! add.
	me ! add.
to create two children.

Now type:
	me ! down.

Because there are more than one child, the code calls
indexlist/3, which returns a list of tuples:
[{1, PID1}, {2, PID2}, ...}.
Then the next line in the 'down' message prints
that list.

After that the user is supposed to pick the child
by using the integer:

    Input = get_user_input("Enter key: "),

But the pid exits the loop before it reaches
that line. The 'me' pid is still alive, but exits
the loop.  I get no error message.

It fails both in Windows XP and in a linux os.

If someone can figure out what the problem is, much appreciated.

Doug Edmunds

--------------------

-module(conn3_full).
-compile(export_all).

%% usage conn:start().
%% then send messages to 'me' (see me_loop)

start()     ->
%    process_flag(trap_exit,  true),
     Me = spawn(?MODULE, me_loop,[[],[],[]]),
     register(me, Me),
     Top = spawn(?MODULE, entry_loop,[[],[],[]]),
     register(top, Top),
     me ! {first_time},
     top ! {first_time},
% uncomment this next line to get to the problem faster
%    me ! add, me ! add, me! show,
     ok.

me_loop(M,K,P) ->
%    io:format("--me_loop self(): ~p M: ~p K:~p P: ~p~n",[self(),M,K,P]),
     receive

	{first_time} ->
         NM = whereis(top),
	    io:format("--setting me to top: self(): ~p M: ~p K: ~p P: ~p ~n",
                 [self(), whereis(top), K, P]),
	    NM = whereis(top), NK = K, NP = P;

	show ->
	    io:format("--show self():~p M: ~p K: ~p P: ~p~n",[self(),M,K,P]),
	    NM = M, NK = K, NP = P;

	add ->
	    %%create an entry
	    Pid = spawn(?MODULE,entry_loop,[[],[],M]),
	    Pid ! {set_pid, Pid},
	    %%update the entry that 'me' is copying
	    M ! {p_add_kid, Pid},
	    %%update 'me'
	    K2 = [Pid|K],
	    NM = M, NK = K2, NP = P;


	del ->
	    case P of
		[] -> io:format("--At the top~n");
		_  -> P ! {p_update_kids, M, K},
		      ok = connect_kids_to_P(K,P),
		      M ! die,
		      me ! up
	    end,
	    NM = M, NK = K, NP = P;



	down ->
	    case length(K) of
		0 -> io:format("--No kids~n");
		1 -> [Head |_] = K,
		    Head ! {self(), info_request};
		_ -> Out = indexlist(1, K, []),
		    ok = io:format("~p~n", [Out]),
%%%%%% When more than one 'kid',
%%%%%% process drops out of loop here.  BUG?

             Input = get_user_input("Enter key: "),
		    {Int,Rest} = string:to_integer(Input),
		    case is_integer(Int) andalso Rest == [] of
			true ->
			     Pick = pick_pid(Out,Int),
			     case is_pid(Pick) of
				 true ->
				     Pick ! {self(), info_request};
				 _ -> io:format("that number is not on the list~n")
			     end;
			_ ->
			     io:format("must enter an integer~n")
		    end
	    end,
	    NM = M, NK = K, NP = P;

	up ->
	    case P of
		[] -> io:format("--At the top~n");
		_  -> P ! {self(),info_request}
	    end,
	    NM = M, NK = K, NP = P;

	{info_requested, M2, K2, P2} ->
	    NM = M2, NK = K2, NP = P2;

	die ->
	    exit("killed"),
	    io:format("~p died~n", [self()]),
	    NM = M, NK = K, NP = P;

	Anything ->
	    io:format("--me_loop got this:~p~n", [Anything]), NM = M, NK = K, 
NP = P
     end,
     me_loop (NM,NK,NP).



entry_loop(M,K,P) ->
%    io:format("--entry_loop self(): ~p M: ~p K:~p P: ~p~n",[self(),M,K,P]),
     receive

	{first_time} ->
	    io:format("--setting top self(): ~p M: ~p K: ~p P ~p ~n",
            [self(), whereis(top), K, P]),
	    NM = whereis(top), NK = K, NP = P;

	show ->
	    io:format("--show self():~p M: ~p K: ~p P: ~p~n",[self(),M,K,P]),
	    NM = M, NK = K, NP = P;

	{set_pid, Pid} ->
	    NM = Pid, NK = K, NP = P;

	{From,info_request} ->
	    From ! {info_requested, M, K, P},
	    NM = M, NK = K, NP = P;

	{p_update_kids, Kid, GrandKidsList} ->
	    K2 = lists:delete(Kid, K),
	    K3 = lists:append(GrandKidsList,K2),
	    %%still have to move me
	    NM = M, NK = K3, NP = P;

	{kid_change_p, GrandP} ->
	    P2 = GrandP,
	    NM = M, NK = K, NP = P2;

	{p_add_kid, Pid} ->
	    K2 = [Pid|K],
	    NM = M, NK = K2, NP = P;

%	{tell_kids_about_Pid, Pid, Msg} ->
%	    Pidlist = [Pidx || Pidx <- K, is_pid(Pid), Pid /= Pidx],
%	    %%% exclude Pid
%	    %%  io:format("--Pid list: ~p~n",[Pidlist]),
%	    ok = tell_list(Pidlist, Pid, Msg),
%	    NM = M, NK = K, NP = P;

	die ->
	    exit("killed"),
	    io:format("~p died~n", [self()]),
	    NM = M, NK = K, NP = P;

	Anything ->
	    io:format("--entry_loop Got this:~p~n", [Anything]), NM = M, NK = 
K, NP = P
     end,
     %%  io:format("here i am~n"),
     entry_loop (NM,NK,NP).

indexlist(Start, [H|T],Out) ->
     NewOut = lists: append ([{Start, H}], Out),
     Start2 = Start+1,
     indexlist(Start2,  T, NewOut);
indexlist(_, [], Out) -> lists:reverse(Out).

pick_pid(Out, Key) ->
     NewDict = dict:from_list(Out),
     case dict:is_key(Key,NewDict) of
         true -> dict:fetch(Key,NewDict);
         false -> "no such key"
     end.

get_user_input( Prompt ) ->
   string:strip(   % remove spaces from front and back
     string:strip( % remove line-feed from the end
       io:get_line( Prompt), right, $\n)).


connect_kids_to_P([],_) -> ok;
connect_kids_to_P(K,P) ->
     [H|T] = K, H ! {kid_change_p,P},
     connect_kids_to_P(T,P).

%%%not implemented
% tell_list([],_,_) ->  ok;
% tell_list([H|T],X,Msg) -> H ! {Msg, X}, tell_list(T,X, Msg).


%%%macro-ish utility
b_alive(String) ->  % ie b_alive("<0.35.0>")
     is_process_alive(list_to_pid(String)).


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