Can't read from NETMF socket

Question

This trivial adaptation of the Pachube sample for Netduino doesn't work. It doesn't barf, it just fails silently. I noticed that if you wait a little after the write, the socket reports 315 bytes in its Available property, but attempting to read the data simply doesn't work - cb contains zero and the unchanged buffer agrees.

Why can't I read this data? I suspect it's a server response that might well help me figure out what's wrong with the rest of the application.

Here's the code:

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
using Microsoft.SPOT;
using Microsoft.SPOT.Hardware;
using SecretLabs.NETMF.Hardware.NetduinoPlus;
using SecretLabs.NETMF.Hardware;

public class HelloPachubeSockets
{
    static string apiKey = "wggHejZTKTvE6ZplSc5feSs1rQLeD4cYmi7WPYiG0VLIx_TC-isp8CI9vkKq3nQxadhydl3gUiSK8vS5SBd9JCCoXQG-g0N8FXYlYcNjEfJVMQJA-usgijpE1LRT-xw5";
    static string feedId = "43974";
    static double maxVoltage = 3.3;
    static int maxAdcValue = 1023;
    static int interval = 20000;
    static Socket connection = null;
    static Timer timer;
    static AnalogInput voltagePort;
    static OutputPort lowPort, highPort;
    static IPEndPoint remoteEndPoint;
    static double V = 0;

    static void LogSensor(object state)
    {
        Debug.Print("time: " + DateTime.Now);
        Debug.Print("memory available: " + Debug.GC(true));
        try
        {
            connection = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            connection.SendTimeout = interval / 2;
            connection.Connect(remoteEndPoint);
            int rawValue = voltagePort.Read();
            double value = (rawValue * maxVoltage) / maxAdcValue;
            string sample = "voltage," + V.ToString("f");
            V += 0.1;
            Debug.Print("new message: " + sample);
            byte[] contentBuffer = Encoding.UTF8.GetBytes(sample);
            const string CRLF = "\r\n";
            var requestLine = "PUT /v2/feeds/" + feedId + ".csv HTTP/1.1\r\n";
            byte[] requestLineBuffer = Encoding.UTF8.GetBytes(requestLine);
            var headers =
                "Host: api.pachube.com\r\n" +
                "X-PachubeApiKey: " + apiKey + CRLF +
                "Content­Type: text/csv\r\n" +
                "Content­Length: " + contentBuffer.Length + CRLF +
                CRLF;
            byte[] headersBuffer = Encoding.UTF8.GetBytes(headers);
            connection.Send(requestLineBuffer);
            connection.Send(headersBuffer);
            connection.Send(contentBuffer);
            Thread.Sleep(400);
            int a = connection.Available;
            byte[] r = new byte[a];
            int cb = connection.Receive(r, 0, a, SocketFlags.None);
            connection.Close();
        }
        catch
        {
          Debug.Print("connection failed");
        }
    }

    public static void Main()
    {
        voltagePort = new AnalogInput(Pins.GPIO_PIN_A1);
        lowPort = new OutputPort(Pins.GPIO_PIN_A0, false);
        highPort = new OutputPort(Pins.GPIO_PIN_A2, true);
        remoteEndPoint = new IPEndPoint(Dns.GetHostEntry("api.pachube.com").AddressList[0], 80);
        timer = new Timer(LogSensor, null, 0, interval);
        Thread.Sleep(Timeout.Infinite);
    }
}

As regards validity of the Pachube key, I checked with cURL and the key and feed details are right.

Answer 1

Looking at http://msdn.microsoft.com/en-us/library/w3xtz6a5.aspx#Y0, you shouldn't need to sleep to wait for a response on the socket as Receive is a blocking operation if the buffer is empty.

What if you tried simplifying your Receive code to something like:

// Blocks until send returns.
connection.Send(...)

// Get reply from the server.
int byteCount = server.Receive(bytes, 0, connection.Available, 
                               SocketFlags.None);

if (byteCount > 0)
    Console.WriteLine(Encoding.UTF8.GetString(bytes));

Alternatively, you could explicitly attempt to read something like 256 bytes off of the socket, instead of the available number.