[erlang-questions] The Danger of gen:server timeout

Ben Murphy benmmurphy@REDACTED
Fri Jun 2 11:09:03 CEST 2017

So a bunch of gen_server:* methods allow you to return a timeout
value. So if you were thinking about how this would be implemented you
would expect if elapsed time >Timeout it would send a timeout message
to your process. This is true. However, if elapsed time > much greater
than Timeout then a timeout message could be sent to your process :/
How is this possible?

If your process receives a system message before the timeout then the
timeout is reset without considering how long you have waited before
receiving a system message. For example: you could be waiting 1 month
for a timeout then after 20 days someone did sys:get_state on your
process then you are waiting another 1 month for your timeout..


-export([init/1, code_change/3, handle_call/3, handle_cast/2,
handle_info/2, terminate/2]).

init(_Args) ->
  TS = erlang:timestamp(),
  io:format("init: ~p~n", [TS]),
  {ok, TS, 30 * 1000}.

handle_info(Message, TS) ->
  io:format(user, "handle info :~p ~p ~n", [Message, erlang:timestamp()]),
  Now = erlang:timestamp(),
  io:format(user, "difference: ~p ~n", [timer:now_diff(Now, TS)]),
  {noreply, TS}.

code_change(_1, _2, _3) ->
handle_call(_1, _2, _3) ->
handle_cast(_1, _2) ->
terminate(_1, _2) ->

Erlang/OTP 19 [erts-8.0] [source] [64-bit] [smp:4:4]
[async-threads:10] [hipe] [kernel-poll:false]

Eshell V8.0  (abort with ^G)
1> {ok, Pid} =  gen_server:start_link(wtf, [], []).
init: {1496,394234,956218}
2> sys:get_state(Pid).
3> handle info :timeout {1496,394279,244306}
difference: 44288466

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