[kernel] Use async_dist in net_kernel instead of nosuspend tricks

net_kernel used to avoid getting blocked when sending replies by using
send with nosuspend and spawn a process doing the reply if the send operation
would have suspended. Using async_dist we prevent the blocking, keep the
signal order, avoid one extra copying of the reply, avoid the overhead
of creating a process, and avoid the extra memory consumption due to the
extra process.

net_kernel also used noconnect in replies since it would deadlock when
connection setup was syncronous which is no longer needed.

Due to the two above options, it also did not use the gen_server:reply/2
API which caused it not to reply using aliases. Now all replies are made
using gen_server:reply/2.

1 files changed, 30 insertions, 31 deletions

diff --git a/lib/kernel/src/net_kernel.erl b/lib/kernel/src/net_kernel.erl
index 50a1ca4d0a..b95953d256 100644
--- a/lib/kernel/src/net_kernel.erl
+++ b/lib/kernel/src/net_kernel.erl
@@ -577,6 +577,10 @@ init(#{name := Name,
        supervisor := Supervisor,
        dist_listen := DistListen,
        hidden := Hidden}) ->
+    %% We enable async_dist so that we won't need to do the
+    %% nosuspend/spawn trick which just cause even larger
+    %% memory consumption...
+    _ = process_flag(async_dist, true),
     process_flag(trap_exit,true),
     persistent_term:put({?MODULE, publish_type},
                         if Hidden -> hidden;
@@ -794,12 +798,9 @@ handle_call({is_auth, _Node}, From, State) ->
 %%
 %% Not applicable any longer !?
 %%
-handle_call({apply,_Mod,_Fun,_Args}, {From,Tag}, State)
-  when is_pid(From), node(From) =:= node() ->
-    async_gen_server_reply({From,Tag}, not_implemented),
-%    Port = State#state.port,
-%    catch apply(Mod,Fun,[Port|Args]),
-    {noreply,State};
+handle_call({apply,_Mod,_Fun,_Args}, {Pid, _Tag} = From, State)
+  when is_pid(Pid), node(Pid) =:= node() ->
+    async_reply({reply, not_implemented, State}, From);
 
 handle_call(longnames, From, State) ->
     async_reply({reply, get(longnames), State}, From);
@@ -2285,7 +2286,7 @@ reply_waiting(_Node, Waiting, Rep) ->
     reply_waiting1(lists:reverse(Waiting), Rep).
 
 reply_waiting1([From|W], Rep) ->
-    async_gen_server_reply(From, Rep),
+    gen_server:reply(From, Rep),
     reply_waiting1(W, Rep);
 reply_waiting1([], _) ->
     ok.
@@ -2391,24 +2392,23 @@ return_call({noreply, _State}=R, _From) ->
 return_call(R, From) ->
     async_reply(R, From).
 
-async_reply({reply, Msg, State}, From) ->
-    async_gen_server_reply(From, Msg),
-    {noreply, State}.
-
-async_gen_server_reply(From, Msg) ->
-    {Pid, Tag} = From,
-    M = {Tag, Msg},
-    try erlang:send(Pid, M, [nosuspend, noconnect]) of
-        ok ->
-            ok;
-        nosuspend ->
-            _ = spawn(fun() -> catch erlang:send(Pid, M, [noconnect]) end),
-	    ok;
-        noconnect ->
-            ok % The gen module takes care of this case.
-    catch
-        _:_ -> ok
-    end.
+-compile({inline, [async_reply/2]}).
+async_reply({reply, _Msg, _State} = Res, _From) ->
+    %% This function call is kept in order to not unnecessarily create a huge diff
+    %% in the code.
+    %%
+    %% Here we used to send the reply explicitly using 'noconnect' and 'nosuspend'.
+    %%
+    %% * 'noconnect' since setting up a connection from net_kernel itself would
+    %%   deadlock when connects were synchronous. Since connects nowadays are
+    %%   asynchronous this is no longer an issue.
+    %% * 'nosuspend' and spawn a process taking care of the reply in case
+    %%   we would have suspended. This in order not to block net_kernel. We now
+    %%   use 'async_dist' enabled and by this prevent the blocking, keep the
+    %%   signal order, avoid one extra copying of the reply, avoid the overhead
+    %%   of creating a process, and avoid the extra memory consumption due to the
+    %%   extra process.
+    Res.
 
 handle_async_response(ResponseType, ReqId, Result, #state{req_map = ReqMap0} = S0) ->
     if ResponseType == down -> ok;
@@ -2422,20 +2422,19 @@ handle_async_response(ResponseType, ReqId, Result, #state{req_map = ReqMap0} = S
                         reply -> Result;
                         down -> {error, noconnection}
                     end,
-            S1 = S0#state{req_map = ReqMap1},
-            async_reply({reply, Reply, S1}, From);
+            gen_server:reply(From, Reply),
+            {noreply, S0#state{req_map = ReqMap1}};
 
         {{setopts_new, Op}, ReqMap1} ->
             case maps:get(Op, ReqMap1) of
                 {setopts_new, From, 1} ->
                     %% Last response for this operation...
+                    gen_server:reply(From, ok),
                     ReqMap2 = maps:remove(Op, ReqMap1),
-                    S1 = S0#state{req_map = ReqMap2},
-                    async_reply({reply, ok, S1}, From);
+                    {noreply, S0#state{req_map = ReqMap2}};
                 {setopts_new, From, N} ->
                     ReqMap2 = ReqMap1#{Op => {setopts_new, From, N-1}},
-                    S1 = S0#state{req_map = ReqMap2},
-                    {noreply, S1}
+                    {noreply, S0#state{req_map = ReqMap2}}
             end
     end.