[erlang-questions] gen_server doesn't respond

Dmitry Kolesnikov <>
Tue Jun 4 10:45:40 CEST 2013


Hello,

> {current_function,{gen,do_call,4}}

Your process made a call to another process with infinite timeout.

The process do not respond because you got a deadlock. 
You have to check:
  * no circular dependencies on process call
  * the parent supervisor is not called from process init
  * no infinite gen_server:call is used at your api. 

- Dmitry

On Jun 4, 2013, at 4:54 AM, "Park, Sungjin" <> wrote:

> Shell input/output logs below.
> 
> ============================
> 4> process_info(pid(0,453,4757)).
> [{current_function,{gen,do_call,4}},
>  {initial_call,{proc_lib,init_p,5}},
>  {status,waiting},
>  {message_queue_len,1},
>  {messages,[{'$gen_call',{<0.26172.4841>,
>                           #Ref<0.0.2946.93178>},
>                          state}]},
>  {links,[<0.25337.4839>]},
>  {dictionary,[{'$ancestors',[<0.452.4757>,<0.424.0>,
>                              <0.423.0>,ranch_sup,<0.420.0>]},
>               {'$initial_call',{my_session,init,1}}]},
>  {trap_exit,true},
>  {error_handler,error_handler},
>  {priority,normal},
>  {group_leader,<0.419.0>},
>  {total_heap_size,1364},
>  {heap_size,987},
>  {stack_size,32},
>  {reductions,1487},
>  {garbage_collection,[{min_bin_vheap_size,46368},
>                       {min_heap_size,233},
>                       {fullsweep_after,65535},
>                       {minor_gcs,8}]},
>  {suspending,[]}]
> =============================
> 
> As you can see the process has a message {..., state} in it's message queue but still in waiting state. It doesn't respond to gen_server:call(Pid, state) from this point on.  The process_info shows that the process is now in gen:do_call/4 which is not under my control. The module has all the fallback handlers for handle_call/3, handle_cast/2, handle_info/2 etc, so that every message be matched. There are a couple of places in my project where gen_server:call/2 used against this process.  I suppose that the calls should timeout in 5 sec by default as is stated in the gen_server specification.  So there should be no such case that the process stuck indefinitely.
> 
> 
> 
> On Mon, Jun 3, 2013 at 7:39 PM, Aaron France <> wrote:
> Does it make any synchronous calls to other gen_servers?
> 
> 
> On Mon, Jun 3, 2013 at 12:34 PM, Tim Watson <> wrote:
> Does the callback module execute any selective receives inside handlers perhaps? If not, are you sure the message has actually arrived in that process' mailbox, can you see unhandled messages in the message queue (I.e., length > 0 whilst waiting)?
> 
> On 3 Jun 2013, at 10:34, "Park, Sungjin" <> wrote:
> 
> > Guys,
> >
> > I'm encountering cases when gen_server doesn't respond.
> > In such cases, erlang:process_info/1 shows the process state 'waiting' with some messages in the message_queue.  The messages have not been consumed for tens of minutes at least.  Anybody has any idean about this?
> >
> >
> > --
> > Park, Sungjin
> > -------------------------------------------------------------------------------------------------------------------
> > Peculiar travel suggestions are dancing lessons from god.
> >   -- The Books of Bokonon
> > -------------------------------------------------------------------------------------------------------------------
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> 
> 
> -- 
> Park, Sungjin
> -------------------------------------------------------------------------------------------------------------------
> Peculiar travel suggestions are dancing lessons from god.
>   -- The Books of Bokonon
> -------------------------------------------------------------------------------------------------------------------
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