[erlang-questions] why does rpc:call do this?

[Garry Hodgson]

I noticed in lib/kernel/src/rpc.erl that got me curious,
as I'm implementing something related. It seems like
there's more work going on than needed, so I suspect
I'm missing some subtlety.

So, rpc:call() is a gen_server call to:

handle_call({call, Mod, Fun, Args, Gleader}, To, S) ->
     handle_call_call(Mod, Fun, Args, Gleader, To, S);

which just invokes:

handle_call_call(Mod, Fun, Args, Gleader, To, S) ->
     RpcServer = self(),
     %% Spawn not to block the rpc server.
     {Caller,_} =
     erlang:spawn_monitor(
       fun () ->
           set_group_leader(Gleader),
           Reply =
               %% in case some sucker rex'es
               %% something that throws
               case catch apply(Mod, Fun, Args) of
               {'EXIT', _} = Exit ->
                   {badrpc, Exit};
               Result ->
                   Result
               end,
           RpcServer ! {self(), {reply, Reply}}
       end),
     {noreply, gb_trees:insert(Caller, To, S)}.

which spawns a process to run the function, stash its results in S, and 
send results as an erlang message to rpc process.

That will then get handled as info message to rpc gen_server, handled here:

handle_info({Caller, {reply, Reply}}, S) ->
     case gb_trees:lookup(Caller, S) of
     {value, To} ->
         receive
         {'DOWN', _, process, Caller, _} ->
             gen_server:reply(To, Reply),
             {noreply, gb_trees:delete(Caller, S)}
         end;
     none ->
         {noreply, S}
     end;

which calls gen_server:reply() to send result to original caller.

So the question is, why do the extra overhead of stashing result in 
gb_tree S and sending it as erlang message,
vs. just having the fun in handle_call_call just use gen_server:reply() 
directly? is it to clean
up the DOWN message from the spawn_monitor? if so, why is the monitor 
part of that needed? What
benefit does it provide?

Any insights appreciated.

Thanks




-- 
Garry Hodgson
Lead Member of Technical Staff
AT&T Chief Security Office (CSO)

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