My supervisor crashes when I try to start it from eshell?

Question

I'm very new to OTP, I'm trying to create simple example to understand supervisor behaviour: Here is simple increment server ``` -module( inc_serv ). -behaviour( gen_server ). -export( [ start/0, inc/1, stop/0 ] ). -export( [ init/1, handle_call/3, terminate/2 ] ). start() -> gen_server:start_link( { local, ?MODULE }, ?MODULE, no_args, [] ). stop() -> gen_server:call( ?MODULE, stop ). inc( Num ) -> gen_server:call( ?MODULE, { num, Num } ). init( no_args ) -> io:format( "~p~n", [ "Increment server started :)" ] ), { ok, no_state }. handle_call( { num, Num }, _From, no_state ) -> { reply, Num + 1, no_state }; handle_call( stop, _From, no_state ) -> { stop, normal, ok, no_state }. terminate( Reason, no_state ) -> io:format( "~p~n", [ "Increment server stopped" ] ). ``` And I'd like to make it supervised by this module: ``` -module( supervisor_inc ). -behaviour( supervisor ). -export( [ start/0 ] ). -export( [ init/1 ] ). start() -> supervisor:start_link( { local, ?MODULE }, ?MODULE, no_a…

Accepted Answer

This is actually a kind of race condition.

As you might know, the Erlang shell itself is a normal Erlang process. When you start your supervisor from the shell, the supervisor is linked to the shell (because you use `supervisor:start_link/3`).

When you call your gen_server process, that process crashes (and is correctly restarted by the supervisor, as you can see by the subsequent `"Increment server started :)"` output).

However, at the same time, your call to `gen_server:call/2` will result in the same crash (a gen_server crashing during the call will emit the same crash through the `gen_server:call/2` function). This then crashes the shell process, which is linked to your supervisor, which in turn crashes with the same reason (`badarith`).

Basically, your supervisor is backstabbed by your shell process, after it loyally restarted your gen_server. Like so:

```
       +---------(6)exit----------+    +---------(5)restart---------+
       |                          |    |                            |
       |                          v    |                            v
     Shell ---(1)start_link---> supervisor ---(2)start_link---> gen_server
     |  ^                         ^    |                         ^  |   ^
     |  |                         |    |                         |  |   |
     |  |                         |    +---------(7)exit---------+  |   |
     |  |                         |                                 |   |
     |  +-------------------------+--------------(4)exit------------+   |
     |                                                                  |
     +---------------------------(3)call--------------------------------+
```

You can avoid this by calling `catch inc_serv:inc(bad_arg).` in your shell:

```
90> inc_serv:inc(7).        
8
91> catch inc_serv:inc(bad_arg).
"Increment server stopped"

=ERROR REPORT==== 23-Aug-2012::22:10:02 ===
** Generic server inc_serv terminating 
** Last message in was {num,bad_arg}
** When Server state == no_state
** Reason for termination == 
** {badarith,[{inc_serv,handle_call,3,[{file,"inc_serv.erl"},{line,20}]},
              {gen_server,handle_msg,5,[{file,"gen_server.erl"},{line,588}]},
              {proc_lib,init_p_do_apply,3,
                        [{file,"proc_lib.erl"},{line,227}]}]}
"Increment server started :)"
{'EXIT',{{badarith,[{inc_serv,handle_call,3,
                              [{file,"inc_serv.erl"},{line,20}]},
                    {gen_server,handle_msg,5,
                              [{file,"gen_server.erl"},{line,588}]},
                    {proc_lib,init_p_do_apply,3,
                              [{file,"proc_lib.erl"},{line,227}]}]},
                    {gen_server,call,[inc_serv,{num,bad_arg}]}}}
92> inc_serv:inc(7).            
8
```

My supervisor crashes when I try to start it from eshell?

Solution

Problem