From 78056e00410153952d5a553fa47c213635e106bb Mon Sep 17 00:00:00 2001
From: Hampus <hampus@fluxer.app>
Date: Sun, 30 Aug 2026 23:05:03 +0200
Subject: [PATCH] perf(gateway): pre-encode voice state update fanout (#2177)

---
 .../src/guild/voice/guild_voice_broadcast.erl | 16 ++++++-
 .../test/guild_voice_broadcast_tests.erl      | 48 ++++++++++++++++++-
 2 files changed, 62 insertions(+), 2 deletions(-)

diff --git a/fluxer_gateway/src/guild/voice/guild_voice_broadcast.erl b/fluxer_gateway/src/guild/voice/guild_voice_broadcast.erl
index 9b2ece042..e9998e73f 100644
--- a/fluxer_gateway/src/guild/voice/guild_voice_broadcast.erl
+++ b/fluxer_gateway/src/guild/voice/guild_voice_broadcast.erl
@@ -111,13 +111,27 @@ maybe_dispatch_voice_state_update(Pids, SanitizedVoiceState, State) ->
             gateway_dispatch_relay:dispatch_many(
                 [Pid || Pid <- Pids, is_pid(Pid)],
                 voice_state_update,
-                SanitizedVoiceState,
+                dispatch_payload(SanitizedVoiceState),
                 GuildId
             );
         undefined ->
             ok
     end.
 
+-spec dispatch_payload(voice_state()) -> voice_state() | {pre_encoded, binary()}.
+dispatch_payload(SanitizedVoiceState) ->
+    case voice_state_utils:voice_state_guild_id(SanitizedVoiceState) of
+        undefined ->
+            SanitizedVoiceState;
+        _GuildId ->
+            {pre_encoded,
+                iolist_to_binary(
+                    json:encode(
+                        guild_data_wire:payload(SanitizedVoiceState), fun json:encode_value/2
+                    )
+                )}
+    end.
+
 -spec state_guild_id(guild_state()) -> integer() | undefined.
 state_guild_id(State) ->
     case map_utils:get_integer(State, id, undefined) of
diff --git a/fluxer_gateway/test/guild_voice_broadcast_tests.erl b/fluxer_gateway/test/guild_voice_broadcast_tests.erl
index db399428a..e6b39205f 100644
--- a/fluxer_gateway/test/guild_voice_broadcast_tests.erl
+++ b/fluxer_gateway/test/guild_voice_broadcast_tests.erl
@@ -72,13 +72,22 @@ pump_relay_to_guild(GuildState) ->
     NewGuildState.
 
 collect_voice_state_dispatches() ->
+    [decoded_voice_state(Payload) || Payload <- collect_raw_voice_state_dispatches()].
+
+collect_raw_voice_state_dispatches() ->
     receive
         {'$gen_cast', {dispatch, voice_state_update, Payload}} ->
-            [Payload | collect_voice_state_dispatches()]
+            [Payload | collect_raw_voice_state_dispatches()]
     after 0 ->
         []
     end.
 
+decoded_voice_state({pre_encoded, Bin}) -> json:decode(Bin);
+decoded_voice_state(Payload) -> Payload.
+
+map_path_wire_bytes(VoiceState) ->
+    iolist_to_binary(json:encode(guild_data_wire:payload(VoiceState))).
+
 confirm_join(ConnId, GuildState) ->
     VoiceServerState = (voice_server_state(GuildState))#{
         pending_voice_connections => #{ConnId => pending_join(ConnId)}
@@ -158,3 +167,40 @@ broadcast_sanitizes_routing_metadata_test() ->
     ?assertEqual(
         [broadcast_payload(join_voice_state(<<"conn1">>))], collect_voice_state_dispatches()
     ).
+
+guild_fanout_pre_encodes_identical_wire_bytes_test() ->
+    VS = join_voice_state(<<"conn1">>),
+    ?assertEqual(
+        {pre_encoded, map_path_wire_bytes(broadcast_payload(VS))}, dispatch_payload_for(VS)
+    ).
+
+guild_fanout_payload_never_touches_dm_voice_states_test() ->
+    {pre_encoded, Bin} = dispatch_payload_for(join_voice_state(<<"conn1">>)),
+    State = #{dm_voice_states => #{}},
+    ?assertEqual(
+        State,
+        session_dispatch_guild:update_dm_voice_states_map(
+            voice_state_update, json:decode(Bin), State
+        )
+    ).
+
+dm_scoped_voice_state_stays_a_map_and_updates_dm_voice_states_test() ->
+    DmVS = (join_voice_state(<<"conn1">>))#{<<"guild_id">> => null},
+    Payload = dispatch_payload_for(DmVS),
+    ?assertEqual(broadcast_payload(DmVS), Payload),
+    State = session_dispatch_guild:update_dm_voice_states_map(
+        voice_state_update, Payload, #{dm_voice_states => #{}}
+    ),
+    ?assertEqual(
+        broadcast_payload(DmVS),
+        maps:get(<<"conn1">>, maps:get(dm_voice_states, State))
+    ).
+
+dispatch_payload_for(VoiceState) ->
+    GuildState = base_guild_state(),
+    ok = guild_voice_broadcast:broadcast_voice_state_update(
+        VoiceState, voice_server_state(GuildState), null
+    ),
+    _ = pump_relay_to_guild(GuildState),
+    [Payload] = collect_raw_voice_state_dispatches(),
+    Payload.