Move RequestIdentifierFeature to HttpContext to allow lighter caching

Expose TraceIdentifier on Httpcontext
Also resolves aspnet#412

src/Microsoft.AspNet.Http.Abstractions/HttpContext.cs

@@ -36,6 +36,8 @@ public abstract class HttpContext : IDisposable
 
         public abstract ISession Session { get; set; }
 
+        public abstract string TraceIdentifier { get; set; }
+
         public abstract void Abort();
 
         public abstract void Dispose();

src/Microsoft.AspNet.Http/DefaultHttpContext.cs

@@ -117,6 +117,19 @@ private ISessionFeature SessionFeature
                 _session = value;
             }
         }
+        private IHttpRequestIdentifierFeature RequestIdentifierFeature
+        {
+            get {
+                return FeatureHelpers.GetOrCreate(
+                  this,
+                  _features,
+                  () => new DefaultHttpRequestIdentifierFeature());
+            }
+            set
+            {
+                _features.Set(value);
+            }
+        }
 
         public override IFeatureCollection Features { get { return _features; } }
 
@@ -167,6 +180,12 @@ public override CancellationToken RequestAborted
             set { LifetimeFeature.RequestAborted = value; }
         }
 
+        public override string TraceIdentifier
+        {
+            get { return RequestIdentifierFeature.TraceIdentifier; }
+            set { RequestIdentifierFeature.TraceIdentifier = value; }
+        }
+
         public override ISession Session
         {
             get

src/Microsoft.AspNet.Http/DefaultHttpRequestIdentifierFeature.cs

@@ -0,0 +1,64 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using System.Threading;
+
+namespace Microsoft.AspNet.Http.Features.Internal
+{
+    public class DefaultHttpRequestIdentifierFeature : HttpRequestIdentifierFeature, IHttpRequestIdentifierFeature
+    {
+        // Base64 encoding - but in ascii sort order for easy text based sorting 
+        private static readonly string _encode32Chars = "0123456789ABCDEFGHIJKLMNOPQRSTUV";
+        // Seed the _requestId for this application instance with 
+        // the number of 100-nanosecond intervals that have elapsed since 12:00:00 midnight, January 1, 0001
+        // for a roughly increasing _requestId over restarts
+        private static long _requestId = DateTime.UtcNow.Ticks;
+
+        private string _id = null;
+
+        public override string TraceIdentifier
+        {
+            get
+            {
+                // Don't incur the cost of generating the request ID until it's asked for
+                if (_id == null)
+                {
+                    _id = GenerateRequestId(Interlocked.Increment(ref _requestId));
+                }
+                return _id;
+            }
+            set
+            {
+                _id = value;
+            }
+        }
+
+        private static unsafe string GenerateRequestId(long id)
+        {
+            // The following routine is ~310% faster than calling long.ToString() on x64
+            // and ~600% faster than calling long.ToString() on x86 in tight loops of 1 million+ iterations
+            // See: https://github.com/aspnet/Hosting/pull/385
+
+            // stackalloc to allocate array on stack rather than heap
+            char* charBuffer = stackalloc char[13];
+
+            charBuffer[0] = _encode32Chars[(int)(id >> 60) & 31];
+            charBuffer[1] = _encode32Chars[(int)(id >> 55) & 31];
+            charBuffer[2] = _encode32Chars[(int)(id >> 50) & 31];
+            charBuffer[3] = _encode32Chars[(int)(id >> 45) & 31];
+            charBuffer[4] = _encode32Chars[(int)(id >> 40) & 31];
+            charBuffer[5] = _encode32Chars[(int)(id >> 35) & 31];
+            charBuffer[6] = _encode32Chars[(int)(id >> 30) & 31];
+            charBuffer[7] = _encode32Chars[(int)(id >> 25) & 31];
+            charBuffer[8] = _encode32Chars[(int)(id >> 20) & 31];
+            charBuffer[9] = _encode32Chars[(int)(id >> 15) & 31];
+            charBuffer[10] = _encode32Chars[(int)(id >> 10) & 31];
+            charBuffer[11] = _encode32Chars[(int)(id >> 5) & 31];
+            charBuffer[12] = _encode32Chars[(int)id & 31];
+
+            // string ctor overload that takes char*
+            return new string(charBuffer, 0, 13);
+        }
+    }
+}

src/Microsoft.AspNet.Http/Features/FeatureHelpers.cs

@@ -23,6 +23,21 @@ public static T GetAndCache<T>(
             return obj;
         }
 
+        public static T GetOrCreate<T>(
+            IFeatureCache cache,
+            IFeatureCollection features,
+            Func<T> factory)
+        {
+            T obj = features.Get<T>();
+            if (obj == null)
+            {
+                obj = factory();
+                features.Set(obj);
+            }
+
+            return obj;
+        }
+
         public static T GetOrCreateAndCache<T>(
             IFeatureCache cache, 
             IFeatureCollection features,

src/Microsoft.AspNet.Http/Features/HttpRequestIdentifierFeature.cs

@@ -5,6 +5,6 @@ namespace Microsoft.AspNet.Http.Features.Internal
 {
     public class HttpRequestIdentifierFeature : IHttpRequestIdentifierFeature
     {
-        public string TraceIdentifier { get; set; }
+        public virtual string TraceIdentifier { get; set; }
     }
 }

src/Microsoft.AspNet.Http/project.json

@@ -6,7 +6,8 @@
         "url": "git://github.com/aspnet/httpabstractions"
     },
     "compilationOptions": {
-        "warningsAsErrors": true
+			"warningsAsErrors": true,
+			"allowUnsafe": true
     },
     "dependencies": {
         "Microsoft.AspNet.Http.Abstractions": "1.0.0-*",