erlangelixiractorgen-server

Elixir/Erlang - Trace when a message arrives in the mailbox

Fairly straight forward question, is it possible to trace messages arriving in (the mailbox of) a Process/GenServer? Note, this is different from tracing when a message is received (which would be once it leaves the mailbox and is handled). I've not found a way of doing this until now.

Solution

  • In erlang you have flags for it in dbg:p/2, s for sending and r for receiving:

    1> dbg:tracer().
{ok,<0.82.0>}
2> dbg:p(self(), r).
(<0.80.0>) << {dbg,{ok,[{matched,nonode@nohost,1}]}}
(<0.80.0>) << {io_reply,#Ref<0.2586582558.1779957764.183997>,319}
{ok,[{matched,nonode@nohost,1}]}
(<0.80.0>) << {io_reply,#Ref<0.2586582558.1779957764.184000>,
                        [{expand_fun,#Fun<group.0.82824323>},
                         {echo,true},
                         {binary,false},
                         {encoding,latin1}]}
(<0.80.0>) << {io_reply,#Ref<0.2586582558.1779957764.184002>,ok}
3> self() ! trace_me.
(<0.80.0>) << {shell_cmd,<0.73.0>,
                         {eval,[{op,{1,8},
                                    '!',
                                    {call,{1,1},{atom,{1,1},self},[]},
                                    {atom,{1,10},trace_me}}]},
                         cmd}
(<0.80.0>) << trace_me
(<0.80.0>) << {io_reply,#Ref<0.2586582558.1779957764.184006>,319}
trace_me
(<0.80.0>) << {io_reply,#Ref<0.2586582558.1779957764.184008>,
                        [{expand_fun,#Fun<group.0.82824323>},
                         {echo,true},
                         {binary,false},
                         {encoding,latin1}]}
(<0.80.0>) << {io_reply,#Ref<0.2586582558.1779957764.184011>,ok}