erlang:monitor can hang under certain conditions

Mickael Remond <>
Sun Nov 20 21:28:44 CET 2005


Hello,

The following problem has been brought to my attention:
If this happen to be really a bug, it could trigger deadlock when using
Erlang monitor for a disappered process.
The ProcessId is a local processid, but the node has changed in the
meantime. It seems to confuse the monitor.

Here is how to reproduce it.
************ ping.erl *******************
-module(ping).
-export([start/0,loop/1]).


start() ->
        register(ping,spawn(ping,loop,[initial])).

loop(X) ->
        receive
                {store, Y} -> loop(Y);
                {retrieve, Pid} when pid(Pid) -> Pid ! X, loop(X)
        end.
****************************************

    First time, the Erlang monitor is not hanging:
$ erl -sname ping -s ping -detached
$ erl -sname hanger
Erlang (BEAM) emulator version 5.4.4 [source] [threads:0]

Eshell V5.4.4  (abort with ^G)
()1> OldPid = spawn(io, format, ["ok",""]).
ok<0.37.0>
...
()4> {ping, ''} !
{store, OldPid}.
{store,<0.37.0>}
()5> {ping, ''} !
{retrieve, self()}.
{retrieve,<0.42.0>}
()6> RetPid = receive X -> X end.
<0.37.0>
()7> erlang:monitor(process,RetPid).
#Ref<0.0.0.68>
()8> halt().


Second time:
erl -sname hanger
Erlang (BEAM) emulator version 5.4.4 [source] [threads:0]
Eshell V5.4.4  (abort with ^G)
()1> {ping, ''} !
{retrieve, self()}.
{retrieve,<0.35.0>}
()2> RetPid = receive X -> X end.
<0.37.0>
()3> RetPid.
<0.37.0>
()4> erlang:monitor(process,RetPid).

The function never returns.

I have started looking at the Erlang code to analyze more deeply the
problem, but I thought that it might be interesting to share the
information.

Cheers,

-- 
Mickaël Rémond
 http://www.process-one.net/



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