Add GetOrBuild on the session factory factory

src/NHibernate.Test/NHSpecificTest/GH3657/Entity.cs

@@ -0,0 +1,10 @@
+using System;
+
+namespace NHibernate.Test.NHSpecificTest.GH3657
+{
+	class Entity
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH3657/Fixture.cs

@@ -0,0 +1,54 @@
+using log4net;
+using NHibernate.Cfg;
+using NHibernate.Impl;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH3657
+{
+	[TestFixture]
+	public class Fixture
+	{
+		private static readonly ILog _log = LogManager.GetLogger(typeof(Fixture));
+		private const string TestSessionFactoryName = "TestName";
+
+		private Configuration _cfg;
+		private ISessionFactory _builtSessionFactory;
+
+		[OneTimeSetUp]
+		public void TestFixtureSetUp()
+		{
+			_cfg = TestConfigurationHelper.GetDefaultConfiguration();
+			var type = GetType();
+			_cfg.AddResource(type.Namespace + ".Mappings.hbm.xml", type.Assembly);
+			_cfg.SetProperty(Environment.SessionFactoryName, TestSessionFactoryName);
+		}
+
+		[TearDown]
+		public void TearDown()
+		{
+			_builtSessionFactory?.Dispose();
+			_builtSessionFactory = null;
+		}
+
+		private ISessionFactory SessionFactoryBuilder()
+		{
+			Assert.That(_builtSessionFactory, Is.Null, "SessionFactory was already built");
+
+			_builtSessionFactory = _cfg.BuildSessionFactory();
+			_log.Info("Successfully built session factory");
+
+			return _builtSessionFactory;
+		}
+
+		[Test]
+		public void GetOrAddTwice()
+		{
+			var factory = SessionFactoryObjectFactory.GetOrBuildNamedInstance(TestSessionFactoryName, SessionFactoryBuilder);
+			Assert.That(factory, Is.Not.Null, "Failed to get the factory once");
+
+			var factory2 = SessionFactoryObjectFactory.GetOrBuildNamedInstance(TestSessionFactoryName, SessionFactoryBuilder);
+			Assert.That(factory2, Is.Not.Null, "Failed to get the factory twice");
+			Assert.That(factory, Is.SameAs(factory2), "The two factories should be the same");
+		}
+	}
+}

src/NHibernate.Test/NHSpecificTest/GH3657/Mappings.hbm.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernate.Test"
+                   namespace="NHibernate.Test.NHSpecificTest.GH3657">
+
+  <class name="Entity">
+    <id name="Id" generator="guid.comb"/>
+    <property name="Name"/>
+  </class>
+
+</hibernate-mapping>

src/NHibernate/Impl/SessionFactoryObjectFactory.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Runtime.CompilerServices;
 
@@ -78,16 +79,46 @@ public static void RemoveInstance(string uid, string name, IDictionary<string, s
 		}
 
 		/// <summary>
-		/// Returns a Named Instance of the SessionFactory from the local "cache" identified by name.
+		/// Get an instance of the SessionFactory from the local "cache" identified by name if it
+		/// exists, otherwise run the provided factory and return its result.
 		/// </summary>
 		/// <param name="name">The name of the ISessionFactory.</param>
+		/// <param name="instanceBuilder">The ISessionFactory factory to use if the instance is not
+		/// found.</param>
 		/// <returns>An instantiated ISessionFactory.</returns>
+		/// <remarks>
+		/// <para>It is the caller responsibility to ensure <paramref name="instanceBuilder"/>
+		/// will add and yield a session factory of the requested <paramref name="name"/>.</para>
+		/// <para>If the session factory instantiation is performed concurrently outside of a
+		/// <c>GetOrAddNamedInstance</c> call, this method may yield an instance of it still being
+		/// built, which may lead to threading issues.</para>
+		/// </remarks>
+		[MethodImpl(MethodImplOptions.Synchronized)]
+		public static ISessionFactory GetOrBuildNamedInstance(string name, Func<ISessionFactory> instanceBuilder)
+		{
+			if (instanceBuilder == null)
+				throw new ArgumentNullException(nameof(instanceBuilder));
+
+			if (NamedInstances.TryGetValue(name, out var factory))
+				return factory;
+			return instanceBuilder();
+		}
+
+		/// <summary>
+		/// Returns a Named Instance of the SessionFactory from the local "cache" identified by name.
+		/// </summary>
+		/// <param name="name">The name of the ISessionFactory.</param>
+		/// <returns>An ISessionFactory if found, <see langword="null" /> otherwise.</returns>
+		/// <remarks>If the session factory instantiation is performed concurrently, this method
+		/// may yield an instance of it still being built, which may lead to threading issues.
+		/// Use <see cref="GetOrBuildNamedInstance(string, Func{ISessionFactory})" /> to get or
+		/// built the session factory in such case.</remarks>
 		[MethodImpl(MethodImplOptions.Synchronized)]
 		public static ISessionFactory GetNamedInstance(string name)
 		{
 			log.Debug("lookup: name={0}", name);
 			ISessionFactory factory;
-			bool found=NamedInstances.TryGetValue(name, out factory);
+			bool found = NamedInstances.TryGetValue(name, out factory);
 			if (!found)
 			{
 				log.Warn("Not found: {0}", name);
@@ -99,7 +130,7 @@ public static ISessionFactory GetNamedInstance(string name)
 		/// Returns an Instance of the SessionFactory from the local "cache" identified by UUID.
 		/// </summary>
 		/// <param name="uid">The identifier of the ISessionFactory.</param>
-		/// <returns>An instantiated ISessionFactory.</returns>
+		/// <returns>An ISessionFactory if found, <see langword="null" /> otherwise.</returns>
 		[MethodImpl(MethodImplOptions.Synchronized)]
 		public static ISessionFactory GetInstance(string uid)
 		{