Move unsetting _childSession to Close method. Add tests. To be squashed.

Author: hazzik

src/NHibernate.Test/NHSpecificTest/NH3985/Fixture.cs

@@ -24,5 +24,20 @@ public void GetChildSession_ShouldReturnNonDisposedInstance()
 				}
 			}
 		}
+
+		[Test]
+		public void GetChildSession_ShouldReturnNonClosedInstance()
+		{
+			using (var rootSession = OpenSession())
+			{
+				var childSession1 = rootSession.GetChildSession();
+				childSession1.Close();
+
+				using (var childSession2 = rootSession.GetChildSession())
+				{
+					Assert.DoesNotThrow(() => { childSession2.Get<Process>(Guid.NewGuid()); });
+				}
+			}
+		}
 	}
 }

src/NHibernate/Impl/SessionImpl.cs

@@ -6,7 +6,6 @@
 using System.Linq.Expressions;
 using System.Runtime.Serialization;
 using System.Security;
-using System.Threading;
 using NHibernate.AdoNet;
 using NHibernate.Collection;
 using NHibernate.Criterion;
@@ -382,6 +381,7 @@ public DbConnection Close()
 				{
 					SetClosed();
 					Cleanup();
+					if (rootSession != null) rootSession._childSession = null;
 				}
 			}
 		}
@@ -1683,8 +1683,6 @@ private void Dispose(bool isDisposing)
 
 				IsAlreadyDisposed = true;
 
-				rootSession?.RemoveChildSession(this);
-
 				// nothing for Finalizer to do - so tell the GC to ignore it
 				GC.SuppressFinalize(this);
 			}
@@ -2608,10 +2606,5 @@ public override IEntityPersister GetEntityPersister(string entityName, object ob
 				}
 			}
 		}
-
-		void RemoveChildSession(ISession session)
-		{
-			Interlocked.CompareExchange(ref _childSession, null, session);
-		}
 	}
 }