NH-4031 - adding AsyncLocalSessionContext.

doc/reference/modules/architecture.xml

@@ -239,12 +239,21 @@
         </para>
 
         <itemizedlist>
+            <listitem>
+                <para>
+                    <literal>NHibernate.Context.AsyncLocalSessionContext</literal> - current sessions are tracked
+                    by current asynchronous flow. You are responsible to bind and unbind an
+                    <literal>ISession</literal> instance with static methods of class
+                    <literal>CurrentSessionContext</literal>. Binding operations from inner flows will not be
+                    propagated to outer or siblings flows.
+                </para>
+            </listitem>
             <listitem>
                 <para>
                     <literal>NHibernate.Context.CallSessionContext</literal> - current sessions are tracked
-                    by <literal>CallContext</literal>. You are responsible to bind and unbind an <literal>
-                    ISession</literal> instance with static methods of class <literal>CurrentSessionContext
-                    </literal>.
+                    by <literal>CallContext</literal>. You are responsible to bind and unbind an
+                    <literal>ISession</literal> instance with static methods of class
+                    <literal>CurrentSessionContext</literal>.
                 </para>
             </listitem>
             <listitem>
@@ -257,26 +266,26 @@
             </listitem>
             <listitem>
                 <para>
-                    <literal>NHibernate.Context.WebSessionContext</literal> - 
-					stores the current session in <literal>HttpContext</literal>. 
-					You are responsible to bind and unbind an <literal>ISession</literal> 
-					instance with static methods of class <literal>CurrentSessionContext</literal>.
+                    <literal>NHibernate.Context.WebSessionContext</literal> -
+                    stores the current session in <literal>HttpContext</literal>.
+                    You are responsible to bind and unbind an <literal>ISession</literal>
+                    instance with static methods of class <literal>CurrentSessionContext</literal>.
                 </para>
             </listitem>
             <listitem>
                 <para>
                     <literal>NHibernate.Context.WcfOperationSessionContext</literal> - current sessions are tracked
                     by WCF <literal>OperationContext</literal>. You need to register the <literal>WcfStateExtension</literal>
-                    extension in WCF. You are responsible to bind and unbind an <literal>ISession
-                    </literal> instance with static methods of class <literal>CurrentSessionContext</literal>.
+                    extension in WCF. You are responsible to bind and unbind an <literal>ISession</literal>
+                    instance with static methods of class <literal>CurrentSessionContext</literal>.
                 </para>
             </listitem>
             <listitem>
                 <para>
                     <literal>NHibernate.Context.ManagedWebSessionContext</literal> - current
-                    sessions are tracked by <literal>HttpContext</literal>. Removed in NHibernate 4.0 
-					- <literal>NHibernate.Context.WebSessionContext</literal> should be used instead.
-					You are responsible to bind and unbind an <literal>ISession</literal> instance with static methods
+                    sessions are tracked by <literal>HttpContext</literal>. Removed in NHibernate 4.0
+                    - <literal>NHibernate.Context.WebSessionContext</literal> should be used instead.
+                    You are responsible to bind and unbind an <literal>ISession</literal> instance with static methods
                     on this class, it never opens, flushes, or closes an <literal>ISession</literal> itself.
                 </para>
             </listitem>
@@ -287,7 +296,8 @@
             defines which <literal>NHibernate.Context.ICurrentSessionContext</literal> implementation
             should be used. Typically, the value of this parameter would just name the implementation
             class to use (including the assembly name); for the out-of-the-box implementations, however,
-            there are corresponding short names: "call", "thread_static", "web" and "wcf_operation",
+            there are corresponding short names: <literal>async_local</literal>, <literal>call</literal>,
+            <literal>thread_static</literal>, <literal>web</literal> and <literal>wcf_operation</literal>,
             respectively.
         </para>
     </sect1>

src/NHibernate.Test/ConnectionTest/AsyncLocalSessionContextFixture.cs

@@ -0,0 +1,66 @@
+using System.Threading.Tasks;
+using NHibernate.Cfg;
+using NHibernate.Context;
+using NUnit.Framework;
+
+namespace NHibernate.Test.ConnectionTest
+{
+	[TestFixture]
+	public class AsyncLocalSessionContextFixture : ConnectionManagementTestCase
+	{
+		protected override ISession GetSessionUnderTest()
+		{
+			var session = OpenSession();
+			session.BeginTransaction();
+			return session;
+		}
+
+		protected override void Configure(Configuration configuration)
+		{
+			base.Configure(cfg);
+			cfg.SetProperty(Environment.CurrentSessionContextClass, "async_local");
+		}
+
+		[Test]
+		public async Task AsyncLocalIsolation()
+		{
+			using (var session = OpenSession())
+			{
+				CurrentSessionContext.Bind(session);
+				AssertCurrentSession(session, "Unexpected session after outer bind.");
+
+				await SubBind(session);
+				AssertCurrentSession(session, "Unexpected session after end of SubBind call.");
+			}
+		}
+
+		private async Task SubBind(ISession firstSession)
+		{
+			AssertCurrentSession(firstSession, "Unexpected session at start of SubBind.");
+
+			using (var session = OpenSession())
+			{
+				CurrentSessionContext.Bind(session);
+				AssertCurrentSession(session, "Unexpected session after inner bind.");
+
+				await Dummy();
+				AssertCurrentSession(session, "Unexpected session after dummy await.");
+			}
+		}
+
+		private Task Dummy()
+		{
+			return Task.FromResult(0);
+		}
+
+		private void AssertCurrentSession(ISession session, string message)
+		{
+			Assert.That(
+				Sfi.GetCurrentSession(),
+				Is.EqualTo(session),
+				"{0} {1} instead of {2}.", message,
+				Sfi.GetCurrentSession().GetSessionImplementation().SessionId,
+				session.GetSessionImplementation().SessionId);
+		}
+	}
+}

src/NHibernate.Test/NHibernate.Test.csproj

@@ -208,6 +208,7 @@
     <Compile Include="ConnectionTest\BatcherFixture.cs" />
     <Compile Include="ConnectionTest\AggressiveReleaseTest.cs" />
     <Compile Include="ConnectionTest\ConnectionManagementTestCase.cs" />
+    <Compile Include="ConnectionTest\AsyncLocalSessionContextFixture.cs" />
     <Compile Include="ConnectionTest\YetAnother.cs" />
     <Compile Include="ConnectionTest\Other.cs" />
     <Compile Include="ConnectionTest\Silly.cs" />

src/NHibernate/Context/AsyncLocalSessionContext.cs

@@ -0,0 +1,33 @@
+using System;
+using System.Threading;
+using NHibernate.Engine;
+
+namespace NHibernate.Context
+{
+	// Session contextes are serializable while not actually serializing any data. Others contextes just retrieve it back
+	// from their context, if it does still live when/where they are de-serialized. For having that with AsyncLocal,
+	// we would need to store it as static, and then we need to use a MapBasedSessionContext.
+	// But this would cause bindings operations done in inner flows to be potentially propagated to outer flows, depending
+	// on which flow has initialized the map. This is undesirable.
+	// So current implementation just loses its context in case of serialization, since AsyncLocal is not serializable.
+	// Another option would be to change MapBasedSessionContext for recreating a new dictionary from the
+	// previous one at each change, essentially using those dictionaries as immutable objects.
+	/// <summary>
+	/// Provides a <see cref="ISessionFactory.GetCurrentSession()">current session</see>
+	/// for current asynchronous flow.
+	/// </summary>
+	[Serializable]
+	public class AsyncLocalSessionContext : CurrentSessionContext
+	{
+		private readonly AsyncLocal<ISession> _session = new AsyncLocal<ISession>();
+
+		// Constructor signature required for dynamic invocation code.
+		public AsyncLocalSessionContext(ISessionFactoryImplementor factory) { }
+
+		protected override ISession Session
+		{
+			get => _session.Value;
+			set => _session.Value = value;
+		}
+	}
+}

src/NHibernate/Impl/SessionFactoryImpl.cs

@@ -1224,6 +1224,8 @@ private ICurrentSessionContext BuildCurrentSessionContext()
 			{
 				case null:
 					return null;
+				case "async_local":
+					return new AsyncLocalSessionContext(this);
 				case "call":
 					return new CallSessionContext(this);
 				case "thread_static":

src/NHibernate/NHibernate.csproj

@@ -144,6 +144,7 @@
     <Compile Include="Connection\DriverConnectionProvider.cs" />
     <Compile Include="Connection\IConnectionProvider.cs" />
     <Compile Include="Connection\UserSuppliedConnectionProvider.cs" />
+    <Compile Include="Context\AsyncLocalSessionContext.cs" />
     <Compile Include="Dialect\PostgreSQL83Dialect.cs" />
     <Compile Include="Impl\ISharedSessionCreationOptions.cs" />
     <Compile Include="ISharedSessionBuilder.cs" />