Cleanup hybrid accessors

src/Umbraco.Web/HybridAccessorBase.cs

@@ -7,6 +7,11 @@ namespace Umbraco.Web
     internal abstract class HybridAccessorBase<T>
         where T : class
     {
+        // ReSharper disable StaticMemberInGenericType
+        private static readonly object Locker = new object();
+        private static bool _registered;
+        // ReSharper restore StaticMemberInGenericType
+
         private readonly IHttpContextAccessor _httpContextAccessor;
 
         protected abstract string ItemKey { get; }
@@ -20,11 +25,6 @@ namespace Umbraco.Web
         //
         // 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...
-        // fixme - which tests?!
-        // 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]
@@ -41,16 +41,20 @@ namespace Umbraco.Web
             }
         }
 
-        // every class inheriting from this class *must* implement a static ctor
-        // and register itself against the SafeCallContext using this method.
-        //
-        // note: because the item key is not static, we cannot register here in the
-        // base class - unless we do it in the non-static Value property setter,
-        // with a static bool to keep track of registration - less error-prone but
-        // perfs impact - considering implementors should be careful.
-        //
-        protected static void SafeCallContextRegister(string itemKey)
+        protected HybridAccessorBase(IHttpContextAccessor httpContextAccessor)
         {
+            if (httpContextAccessor == null) throw new ArgumentNullException(nameof(httpContextAccessor));
+            _httpContextAccessor = httpContextAccessor;
+
+            lock (Locker)
+            {
+                // register the itemKey once with SafeCallContext
+                if (_registered) return;
+                _registered = true;
+            }
+
+            // ReSharper disable once VirtualMemberCallInConstructor
+            var itemKey = ItemKey; // virtual
             SafeCallContext.Register(() =>
             {
                 var value = CallContext.LogicalGetData(itemKey);
@@ -65,11 +69,6 @@ namespace Umbraco.Web
             });
         }
 
-        protected HybridAccessorBase(IHttpContextAccessor httpContextAccessor)
-        {
-            _httpContextAccessor = httpContextAccessor;
-        }
-
         protected T Value
         {
             get

src/Umbraco.Web/HybridEventMessagesAccessor.cs

@@ -4,14 +4,7 @@ namespace Umbraco.Web
 {
     internal class HybridEventMessagesAccessor : HybridAccessorBase<EventMessages>, IEventMessagesAccessor
     {
-        private const string ItemKeyConst = "Umbraco.Core.Events.HybridEventMessagesAccessor";
-
-        protected override string ItemKey => ItemKeyConst;
-
-        static HybridEventMessagesAccessor()
-        {
-            SafeCallContextRegister(ItemKeyConst);
-        }
+        protected override string ItemKey => "Umbraco.Core.Events.HybridEventMessagesAccessor";
 
         public HybridEventMessagesAccessor(IHttpContextAccessor httpContextAccessor)
             : base(httpContextAccessor)

src/Umbraco.Web/HybridUmbracoContextAccessor.cs

@@ -2,19 +2,12 @@
 {
     internal class HybridUmbracoContextAccessor : HybridAccessorBase<UmbracoContext>, IUmbracoContextAccessor
     {
-        private const string ItemKeyConst = "Umbraco.Web.HybridUmbracoContextAccessor";
-
-        protected override string ItemKey => ItemKeyConst;
-
-        static HybridUmbracoContextAccessor()
-        {
-            SafeCallContextRegister(ItemKeyConst);
-        }
-
         public HybridUmbracoContextAccessor(IHttpContextAccessor httpContextAccessor)
             : base(httpContextAccessor)
         { }
 
+        protected override string ItemKey => "Umbraco.Web.HybridUmbracoContextAccessor";
+
         public UmbracoContext UmbracoContext
         {
             get { return Value; }

src/Umbraco.Web/HybridUmbracoDatabaseAccessor.cs

@@ -4,14 +4,7 @@ namespace Umbraco.Web
 {
     internal class HybridUmbracoDatabaseAccessor : HybridAccessorBase<UmbracoDatabase>, IUmbracoDatabaseAccessor
     {
-        private const string ItemKeyConst = "Umbraco.Core.Persistence.HybridUmbracoDatabaseAccessor";
-
-        protected override string ItemKey => ItemKeyConst;
-
-        static HybridUmbracoDatabaseAccessor()
-        {
-            SafeCallContextRegister(ItemKeyConst);
-        }
+        protected override string ItemKey => "Umbraco.Core.Persistence.HybridUmbracoDatabaseAccessor";
 
         public HybridUmbracoDatabaseAccessor(IHttpContextAccessor httpContextAccessor)
             : base(httpContextAccessor)