server.js

// test with:
// curl -X POST -d '{ "to" : "isaacs", "from" : "arlolra", "body" : "emoji 🍨 🍩 🍪"}' http://127.0.0.1:1337

var http = require("http")

// messages should be relatively short.
var maxLength = 1024

var server = http.createServer(function (req, res) {
  // requests come in as json
  // need to parse the json to get from, to, and body fields.
  // JSON is probably generated by some native C JSON encoder
  // on a device, or in some other language, or perhaps from
  // another node program (in which case, it'll already be broken).

  var chunks = []
  var msgLen = 0
  req.on("data", onData)
  req.on("end", onEnd)

  function onData (chunk) {
    chunks.push(chunk)
    msgLen += chunk.length
    if (msgLen > maxLength) {
      // DoS protection.
      res.writeHead(413, {"content-type":"application/json"})
      res.end(JSON.stringify({ok: false, error: "Request entity too large"}))
      req.removeListener("data", onData)
      req.removeListener("end", onEnd)
    }
  }

  function onEnd () {
    var message
    switch (chunks.length) {
      case 0: message = new Buffer(0); break
      case 1: message = chunks[0]; break
      default:
        message = new Buffer(msgLen)
        var i = 0
        chunks.forEach(function (c) {
          c.copy(message, i)
          i += c.length
        })
        break
    }
    
    var i = 0, code = ''
    for (; i < msgLen; i++) {
      code += String.fromCharCode(message[i])
    }

    // so, now message is the full message
    // it's a buffer, but we need to interpret it as JSON
    // this is where the Bad Thing happens.
    // if there were any characters outside the 2-byte plane,
    // then they're going to now get turned into 2 garbage characters.

    try {
      message = JSON.parse(code)
    } catch (ex) {
      console.error(ex)
      res.writeHead(400, {"content-type":"application/json"})
      res.end(JSON.stringify({ok:false, error:"Bad JSON"}))
      return
    }

    // in real life, this console.error would be replaced with
    // some nifty routing message passing thingie
    // console.error("From: %s\nTo: %s\n\n%s",
    //                   message.from, message.to, message.body);

    var mb = message.body
      , len = mb.length
      , buf = new Buffer(len + 1)

    for (i = 0; i < len; i++) {
      buf[i] = mb.charCodeAt(i)
    }
    buf[i] = 10
    
    process.stdout.write(buf)

    res.writeHead(200, { "content-type": "application/json" })
    res.end(JSON.stringify({ ok: true }) + "\n")
  }
})

server.listen(1337)