Support PING / PONG messages

[endel]

Hey @britzl,

I'm trying to allow your library to respond to PING messages. It seems some PING messages aren't received sometimes.

So far, I've found that when the PING comes, it can be received here, and sometimes it doesn't:

defold-websocket/websocket/sync.lua

Line 25 in b480623

local chunk,err = self:sock_receive(bytes)

I've added a check for opcode == 0x9 here to automatically send the PONG messages:

if opcode == 0x9 then
    -- reply PING opcode with PONG
    print("REPLY WITH PONG")
    self.send(self, message, 0xA)
elseif message then
    on_message(message)
end

My server logs are looking like this (Colyseus automatically disconnects users after 3 PING's without response 😢 )

SEND PING
SEND PING
PONG RECEIVED!
SEND PING
SEND PING
SEND PING

Do you have any clue why? Cheers!

[endel]

FYI this problem doesn't happen in emscripten environment since the browser handles PING/PONG messages automatically

[britzl]

In the official LuaWebsocket project there's an issue discussing PING/PONG: lipp/lua-websockets#94