Implement Broker Redirects for Session and Messages (#3103)

src/Sdk/DTWebApi/WebApi/BrokerMigrationMessage.cs

@@ -0,0 +1,38 @@
+using System;
+using System.Runtime.Serialization;
+
+namespace GitHub.DistributedTask.WebApi
+{
+    /// <summary>
+    /// Message that tells the runner to redirect itself to BrokerListener for messages.
+    /// (Note that we use a special Message instead of a simple 302. This is because 
+    /// the runner will need to apply the runner's token to the request, and it is
+    /// a security best practice to *not* blindly add sensitive data to redirects
+    /// 302s.)
+    /// </summary>
+    [DataContract]
+    public class BrokerMigrationMessage
+    {
+        public static readonly string MessageType = "BrokerMigration";
+
+        public BrokerMigrationMessage()
+        {
+        }
+
+        public BrokerMigrationMessage(
+            Uri brokerUrl)
+        {
+            this.BrokerBaseUrl = brokerUrl;
+        }
+
+        /// <summary>
+        /// The base url for the broker listener
+        /// </summary>
+        [DataMember]
+        public Uri BrokerBaseUrl
+        {
+            get;
+            internal set;
+        }
+    }
+}