GPRS or Edge Howto?

Ok, thanks! that's really handy. So it's actually not the GSM code at all, it's the SIM900 that's sending the extra newlines:

"\r\n+RECEIVE"
",0,289:\r\nHTTP/1.1 200 OK\r\nDate: Tue, 09 Jun 2015 11:07:06"
//      ^^^^

Now I can't find any info on +RECEIVE at all, so I don't know whether it's actually meant to do that after every RECEIVE or not... Maybe if you request a bigger file (>1.5kb?) we'll get two RECEIVEs, and can check... But for now let's assume that it always goes: +RECEIVE,X,Y:\r\n<data>

I've had to add a second handler called receiveHandler2, because unfortunately the newline after the +RECEIVE would totally confuse the AT handler if we left it in the buffer.

function receiveHandler(line) {
  var colon = line.indexOf(":\r\n");
  if (colon<0) return line; // not enough data here at the moment
  var parms = line.substring(9,colon).split(",");
  parms[1] = 0|parms[1];
  var len = line.length-(colon+3);
  if (len>=parms[1]) {
   // we have everything
   sockData[parms[0]] += line.substr(colon+1,parms[1]);
   return line.substr(colon+parms[1]+3); // return anything else
  } else { 
   // still some to get
   sockData[parms[0]] += line.substr(colon+3,len);
   return "+D,"+parms[0]+","+(parms[1]-len)+":"; // return +D so receiveHandler2 gets called next time    
  }
}

function receiveHandler2(line) {
  var colon = line.indexOf(":");
  if (colon<0) return line; // not enough data here at the moment
  var parms = line.substring(3,colon).split(",");
  parms[1] = 0|parms[1];
  var len = line.length-(colon+1);
  if (len>=parms[1]) {
   // we have everything
   sockData[parms[0]] += line.substr(colon+1,parms[1]);
   return line.substr(colon+parms[1]+1); // return anything else
  } else { 
   // still some to get
   sockData[parms[0]] += line.substr(colon+1,len);
   return "+D,"+parms[0]+","+(parms[1]-len)+":"; // return +D so receiveHandler2 gets called next time    
  }
}

// ....

exports.connect = function(usart, resetPin, connectedCallback) {
  rst = resetPin;
  gprsFuncs.at = at = require("AT").connect(usart);
  require("NetworkJS").create(netCallbacks);
  at.register("+RECEIVE", receiveHandler);
  at.register("+D", receiveHandler2);
  gprsFuncs.reset(connectedCallback);
  return gprsFuncs;
};