Refactor hybrid accessors for async

src/Umbraco.Web/HybridAccessorBase.cs

@@ -1,16 +1,41 @@
-using System;
+using System.Runtime.Remoting.Messaging;
 
 namespace Umbraco.Web
 {
-    // fixme - must ensure that the ThreadStatic value is properly cleared!
     internal abstract class HybridAccessorBase<T>
     {
         private readonly IHttpContextAccessor _httpContextAccessor;
 
-        protected abstract string HttpContextItemKey { get; }
+        protected abstract string ItemKey { get; }
 
-        [ThreadStatic]
-        private static T _value;
+        // read
+        // http://blog.stephencleary.com/2013/04/implicit-async-context-asynclocal.html
+        // http://stackoverflow.com/questions/14176028/why-does-logicalcallcontext-not-work-with-async
+        // http://stackoverflow.com/questions/854976/will-values-in-my-threadstatic-variables-still-be-there-when-cycled-via-threadpo
+        // https://msdn.microsoft.com/en-us/library/dd642243.aspx?f=255&MSPPError=-2147217396 ThreadLocal<T>
+        // http://stackoverflow.com/questions/29001266/cleaning-up-callcontext-in-tpl clearing call context
+        //
+        // anything that is ThreadStatic will stay with the thread and NOT flow in async threads
+        // the only thing that flows is the logical call context (safe in 4.5+)
+        // now...
+        // tests seem to show that either newing Thread or enqueuing in the ThreadPool both produce a thread
+        // with a clear logical call context, which would mean that it is somewhat safe to "fire and forget"
+        // because whatever is in the call context will be gone when the thread returns to the pool
+
+        // no!
+        //[ThreadStatic]
+        //private static T _value;
+
+        // yes! flows with async!
+        private T NonContextValue
+        {
+            get { return (T) CallContext.LogicalGetData(ItemKey); }
+            set
+            {
+                if (value == null) CallContext.FreeNamedDataSlot(ItemKey);
+                else CallContext.LogicalSetData(ItemKey, value);
+            }
+        }
 
         protected HybridAccessorBase(IHttpContextAccessor httpContextAccessor)
         {
@@ -22,8 +47,8 @@ namespace Umbraco.Web
             get
             {
                 var httpContext = _httpContextAccessor.HttpContext;
-                if (httpContext == null) return _value;
-                return (T) httpContext.Items[HttpContextItemKey];
+                if (httpContext == null) return NonContextValue;
+                return (T) httpContext.Items[ItemKey];
             }
 
             set
@@ -31,10 +56,10 @@ namespace Umbraco.Web
                 var httpContext = _httpContextAccessor.HttpContext;
                 if (httpContext == null)
                 {
-                    _value = value;
+                    NonContextValue = value;
                     return;
                 }
-                httpContext.Items[HttpContextItemKey] = value;
+                httpContext.Items[ItemKey] = value;
             }
         }
     }

src/Umbraco.Web/HybridEventMessagesAccessor.cs

@@ -4,7 +4,7 @@ namespace Umbraco.Web
 {
     internal class HybridEventMessagesAccessor : HybridAccessorBase<EventMessages>, IEventMessagesAccessor
     {
-        protected override string HttpContextItemKey => "Umbraco.Core.Events.EventMessages";
+        protected override string ItemKey => "Umbraco.Core.Events.EventMessages";
 
         public HybridEventMessagesAccessor(IHttpContextAccessor httpContextAccessor)
             : base(httpContextAccessor)

src/Umbraco.Web/HybridUmbracoContextAccessor.cs

@@ -2,7 +2,7 @@
 {
     internal class HybridUmbracoContextAccessor : HybridAccessorBase<UmbracoContext>, IUmbracoContextAccessor
     {
-        protected override string HttpContextItemKey => "Umbraco.Web.UmbracoContext";
+        protected override string ItemKey => "Umbraco.Web.UmbracoContext";
 
         public HybridUmbracoContextAccessor(IHttpContextAccessor httpContextAccessor)
             : base(httpContextAccessor)

src/Umbraco.Web/HybridUmbracoDatabaseAccessor.cs

@@ -4,7 +4,7 @@ namespace Umbraco.Web
 {
     internal class HybridUmbracoDatabaseAccessor : HybridAccessorBase<UmbracoDatabase>, IUmbracoDatabaseAccessor
     {
-        protected override string HttpContextItemKey => "Umbraco.Core.Persistence.UmbracoDatabase";
+        protected override string ItemKey => "Umbraco.Core.Persistence.UmbracoDatabase";
 
         public HybridUmbracoDatabaseAccessor(IHttpContextAccessor httpContextAccessor)
             : base(httpContextAccessor)