Set LINELEN to 8k for clients that support long messages

This matches the value already negotiated for cap multiline. I think we
could go up to 16k without difficulty, but 8k is probably sufficient.

This also tells gen_tcp to use a 16k buffer; with the default settings,
long commands sometimes get split across multiple reads, with
unfortunate results.

lib/irc/handler.ex

@@ -607,6 +607,8 @@ defmodule M51.IrcConn.Handler do
       "CHANMODES=b,,,i",
       "CHANTYPES=#!",
       "CHATHISTORY=100",
+      # Matrix limit is 64k for the whole event, so this is fairly conservative.
+      "LINELEN=#{@multiline_max_bytes}",
       "MAXTARGETS=1",
       # https://github.com/ircv3/ircv3-specifications/pull/510
       "MSGREFTYPES=msgid",

lib/irc_server.ex

@@ -40,7 +40,14 @@ defmodule M51.IrcServer do
   end
 
   defp accept(port, retries_left \\ 10) do
-    case :gen_tcp.listen(port, [:binary, :inet6, packet: :line, active: false, reuseaddr: true]) do
+    opts = [
+      :binary, :inet6,
+      packet: :line,
+      active: false,
+      reuseaddr: true,
+      buffer: M51.IrcConn.Handler.multiline_max_bytes * 2
+    ]
+    case :gen_tcp.listen(port, opts) do
       {:ok, server_sock} ->
         Logger.info("Listening on port #{port}")
         loop_accept(server_sock)

test/irc/handler_test.exs

@@ -20,7 +20,7 @@ defmodule M51.IrcConn.HandlerTest do
 
   @cap_ls_302 ":server. CAP * LS :account-tag batch draft/account-registration=before-connect draft/channel-rename draft/chathistory draft/message-redaction draft/multiline=max-bytes=8192 draft/no-implicit-names draft/sasl-ir echo-message extended-join labeled-response message-tags sasl=PLAIN server-time soju.im/account-required standard-replies userhost-in-names\r\n"
   @cap_ls ":server. CAP * LS :account-tag batch draft/account-registration draft/channel-rename draft/chathistory draft/message-redaction draft/multiline draft/no-implicit-names draft/sasl-ir echo-message extended-join labeled-response message-tags sasl server-time soju.im/account-required standard-replies userhost-in-names\r\n"
-  @isupport "CASEMAPPING=rfc3454 CLIENTTAGDENY=*,-draft/react,-draft/reply CHANLIMIT= CHANMODES=b,,,i CHANTYPES=#! CHATHISTORY=100 MAXTARGETS=1 MSGREFTYPES=msgid PREFIX= TARGMAX=JOIN:1,PART:1 UTF8ONLY :are supported by this server\r\n"
+  @isupport "CASEMAPPING=rfc3454 CLIENTTAGDENY=*,-draft/react,-draft/reply CHANLIMIT= CHANMODES=b,,,i CHANTYPES=#! CHATHISTORY=100 LINELEN=8192 MAXTARGETS=1 MSGREFTYPES=msgid PREFIX= TARGMAX=JOIN:1,PART:1 UTF8ONLY :are supported by this server\r\n"
 
   setup do
     start_supervised!({MockMatrixClient, {self()}})