Add a hook to launch a debugger from the worker process. (#150)

src/csharp/Microsoft.Spark.Worker/TaskRunner.cs

@@ -3,11 +3,13 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Diagnostics;
 using System.IO;
 using System.Threading;
 using Microsoft.Spark.Interop.Ipc;
 using Microsoft.Spark.Network;
 using Microsoft.Spark.Services;
+using Microsoft.Spark.Utils;
 using Microsoft.Spark.Worker.Command;
 using Microsoft.Spark.Worker.Processor;
 using Microsoft.Spark.Worker.Utils;
@@ -47,6 +49,11 @@ namespace Microsoft.Spark.Worker
         {
             s_logger.LogInfo($"[{TaskId}] Starting with ReuseSocket[{_reuseSocket}].");
 
+            if (EnvironmentUtils.GetEnvironmentVariableAsBool("DOTNET_WORKER_DEBUG"))
+            {
+                Debugger.Launch();
+            }
+
             _isRunning = true;
             Stream inputStream = _socket.InputStream;
             Stream outputStream = _socket.OutputStream;