Add IsAvailable property to ISession #634

Author: JunTaoLuo

.gitignore

@@ -27,3 +27,4 @@ nuget.exe
 project.lock.json
 .build/
 .testPublish/
+/.vs/

src/Microsoft.AspNetCore.Http.Features/ISession.cs

@@ -8,6 +8,11 @@ namespace Microsoft.AspNetCore.Http
 {
     public interface ISession
     {
+        /// <summary>
+        /// Indicate whether the current session has loaded correctly.
+        /// </summary>
+        bool IsAvailable { get; }
+
         /// <summary>
         /// A unique identifier for the current session. This is not the same as the session cookie
         /// since the cookie lifetime may not be the same as the session entry lifetime in the data store.

test/Microsoft.AspNetCore.Http.Tests/DefaultHttpContextTests.cs

@@ -288,6 +288,8 @@ private Dictionary<string, byte[]> _store
 
             public string Id { get; set; }
 
+            public bool IsAvailable { get; } = true;
+
             public IEnumerable<string> Keys { get { return _store.Keys; } }
 
             public void Clear()