Let spawn_opt config replace erlang:spawn_opt/2 for request processes

By accepting an MFA that is applied to the fun that is otherwise spawned
for each incoming request, to allow handler processes to be reused. This
is not yet documented and may change, but the motivation is to let spawn
be replaced by process pool, from which the MFA selects. A list-valued
spawn_opt is equivalent to {erlang, spawn_opt, [Opts]}.

3 files changed, 44 insertions, 7 deletions

diff --git a/lib/diameter/src/base/diameter_config.erl b/lib/diameter/src/base/diameter_config.erl
index 1db9b52dfa..99d8c8c6ec 100644
--- a/lib/diameter/src/base/diameter_config.erl
+++ b/lib/diameter/src/base/diameter_config.erl
@@ -610,6 +610,9 @@ opt({watchdog_timer, Tmo}) ->
 opt({watchdog_config, L}) ->
     is_list(L) andalso lists:all(fun wdopt/1, L);
 
+opt({spawn_opt, {M,F,A}})
+  when is_atom(M), is_atom(F), is_list(A) ->
+    true;
 opt({spawn_opt = K, Opts}) ->
     if is_list(Opts) ->
             {value, {K, spawn_opts(Opts)}};
@@ -739,6 +742,10 @@ opt(incoming_maxlen, N)
   when 0 =< N, N < 1 bsl 24 ->
     N;
 
+opt(spawn_opt, {M,F,A} = T)
+  when is_atom(M), is_atom(F), is_list(A) ->
+    T;
+
 opt(spawn_opt, L)
   when is_list(L) ->
     spawn_opts(L);
diff --git a/lib/diameter/src/base/diameter_service.erl b/lib/diameter/src/base/diameter_service.erl
index be50e87179..8e383818ea 100644
--- a/lib/diameter/src/base/diameter_service.erl
+++ b/lib/diameter/src/base/diameter_service.erl
@@ -114,7 +114,7 @@
                       incoming_maxlen := diameter:message_length(),
                       strict_mbit := boolean(),
                       string_decode := boolean(),
-                      spawn_opt := list()}}).
+                      spawn_opt := list() | {module(), atom(), list()}}}).
 
 %% Record representing an RFC 3539 watchdog process implemented by
 %% diameter_watchdog.
diff --git a/lib/diameter/src/base/diameter_traffic.erl b/lib/diameter/src/base/diameter_traffic.erl
index 54f39afbf0..af7ac10f13 100644
--- a/lib/diameter/src/base/diameter_traffic.erl
+++ b/lib/diameter/src/base/diameter_traffic.erl
@@ -212,8 +212,9 @@ incr_rc(Dir, Pkt, TPid, Dict0) ->
 %% ---------------------------------------------------------------------------
 
 -spec receive_message(pid(), Route, #diameter_packet{}, module(), RecvData)
-   -> pid()
-    | boolean()
+   -> pid()     %% request handler
+    | boolean() %% answer, known request or not
+    | discard   %% request discarded by MFA
  when Route :: {Handler, RequestRef, Seqs}
              | Ack,
       RecvData :: {[SpawnOpt], #recvdata{}},
@@ -230,9 +231,10 @@ receive_message(TPid, Route, Pkt, Dict0, RecvData) ->
 %% recv/6
 
 %% Incoming request ...
-recv(true, Ack, TPid, Pkt, Dict0, RecvData)
+recv(true, Ack, TPid, Pkt, Dict0, T)
   when is_boolean(Ack) ->
-    spawn_request(Ack, TPid, Pkt, Dict0, RecvData);
+    {Opts, RecvData} = T,
+    spawn_request(Ack, TPid, Pkt, Dict0, RecvData, Opts);
 
 %% ... answer to known request ...
 recv(false, {Pid, Ref, TPid}, _, Pkt, Dict0, _) ->
@@ -254,18 +256,46 @@ recv(false, false, TPid, Pkt, _, _) ->
     incr(TPid, {{unknown, 0}, recv, discarded}),
     false.
 
-%% spawn_request/5
+%% spawn_request/6
 
-spawn_request(Ack, TPid, Pkt, Dict0, {Opts, RecvData}) ->
+%% An MFA should return a pid() or the atom 'discard'. The latter
+%% results in an acknowledgment back to the transport process when
+%% appropriate, to ensure that send/recv callbacks can count
+%% outstanding requests. Acknowledgement is implicit if the
+%% handler process dies (in a handle_request callback for example).
+spawn_request(Ack, TPid, Pkt, Dict0, RecvData, {M,F,A}) ->
+    ReqF = fun() ->
+                   ack(Ack, TPid, recv_request(Ack, TPid, Pkt, Dict0, RecvData))
+           end,
+    ack(Ack, TPid, apply(M, F, [ReqF | A]));
+
+%% A spawned process acks implicitly when it dies, so there's no need
+%% to handle 'discard'.
+spawn_request(Ack, TPid, Pkt, Dict0, RecvData, Opts) ->
     spawn_opt(fun() ->
                       recv_request(Ack, TPid, Pkt, Dict0, RecvData)
               end,
               Opts).
 
+%% ack/3
+
+ack(Ack, TPid, RC) ->
+    RC == discard andalso Ack andalso (TPid ! {send, false}),
+    RC.
+
 %% ---------------------------------------------------------------------------
 %% recv_request/5
 %% ---------------------------------------------------------------------------
 
+-spec recv_request(Ack :: boolean(),
+                   TPid :: pid(),
+                   #diameter_packet{},
+                   Dict0 :: module(),
+                   #recvdata{})
+   -> ok        %% answer was sent
+    | discard   %% or not
+    | false.    %% no transport
+
 recv_request(Ack,
              TPid,
              #diameter_packet{header = #diameter_header{application_id = Id}}