perfom full rpc tracker handshake (#1500)

jvm/core/src/main/java/ml/dmlc/tvm/rpc/ConnectTrackerServerProcessor.java

@@ -201,6 +201,12 @@ public class ConnectTrackerServerProcessor implements ServerProcessor {
     if (trackerMagic != RPC.RPC_TRACKER_MAGIC) {
       throw new SocketException("failed to connect to tracker (WRONG MAGIC)");
     }
+    String infoJSON = generateCinfo(key);
+    Utils.sendString(trackerOut, infoJSON);
+    int recvCode = Integer.parseInt(Utils.recvString(trackerIn));
+    if (recvCode != RPC.TrackerCode.SUCCESS) {
+      throw new SocketException("failed to connect to tracker (not SUCCESS)");
+    }
     return trackerSocket;
   }
 
@@ -239,6 +245,12 @@ public class ConnectTrackerServerProcessor implements ServerProcessor {
   }
 
   // handcrafted JSON
+  private String generateCinfo(String key) {
+    String cinfo = "{\"key\" : " + "\"server:" + key + "\"}";
+    return "[" + RPC.TrackerCode.UPDATE_INFO + ", " + cinfo + "]";
+  }
+
+  // handcrafted JSON
   private String generatePut(int code, String key, int port, String matchKey) {
     return "[" + code + ", " + "\"" + key + "\"" + ", " + "[" + port + ", "
             + "\"" + matchKey +  "\"" + "]" + ", " + "null" + "]";

jvm/core/src/main/java/ml/dmlc/tvm/rpc/RPC.java

@@ -32,6 +32,7 @@ public class RPC {
 
   public class TrackerCode {
     public static final int PUT = 3;
+    public static final int UPDATE_INFO = 5;
     public static final int GET_PENDING_MATCHKEYS = 7;
     public static final int SUCCESS = 0;
   }