unset child session as soon as it is disposed.

Author: hazzik

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

@@ -20,7 +20,7 @@ public void GetChildSession_ShouldReturnNonDisposedInstance()
 
 				using (var childSession2 = rootSession.GetChildSession())
 				{
-					Assert.DoesNotThrow(new TestDelegate(() => { childSession2.Get<Process>(Guid.NewGuid()); }));
+					Assert.DoesNotThrow(() => { childSession2.Get<Process>(Guid.NewGuid()); });
 				}
 			}
 		}

src/NHibernate/Impl/SessionImpl.cs

@@ -6,6 +6,7 @@
 using System.Linq.Expressions;
 using System.Runtime.Serialization;
 using System.Security;
+using System.Threading;
 using NHibernate.AdoNet;
 using NHibernate.Collection;
 using NHibernate.Criterion;
@@ -73,7 +74,7 @@ public sealed class SessionImpl : AbstractSessionImpl, IEventSource, ISerializab
 		private readonly StatefulPersistenceContext persistenceContext;
 
 		[NonSerialized]
-		private readonly ISession rootSession;
+		private readonly SessionImpl rootSession;
 
 		[NonSerialized]
 		ISession _childSession;
@@ -1681,6 +1682,9 @@ private void Dispose(bool isDisposing)
 				// free unmanaged resources here
 
 				IsAlreadyDisposed = true;
+
+				rootSession?.RemoveChildSession(this);
+
 				// nothing for Finalizer to do - so tell the GC to ignore it
 				GC.SuppressFinalize(this);
 			}
@@ -2221,10 +2225,8 @@ public ISession GetChildSession()
 
 				CheckAndUpdateSessionStatus();
 
-				var childImpl = _childSession as SessionImpl;
-
 				// if child session never existed or has already been disposed, create new
-				if (childImpl == null || childImpl.IsAlreadyDisposed)
+				if (_childSession == null)
 				{
 					log.Debug("Creating child session.");
 					_childSession = new SessionImpl(this);
@@ -2607,5 +2609,10 @@ public override IEntityPersister GetEntityPersister(string entityName, object ob
 				}
 			}
 		}
+
+		void RemoveChildSession(ISession session)
+		{
+			Interlocked.CompareExchange(ref _childSession, null, session);
+		}
 	}
 }