Intermittent null reference exception on CloseConnection

[kriewall]

Describe the bug

Intermittent NullReferenceException thrown at NHibernate.Connection.ConnectionProvider.CloseConnection(DbConnection conn)

Steps to reproduce

I unfortunately don't have a simple standalone project to reproduce this issue. I see it from time to time when running regression tests.

Expected behavior

• No null reference exception should occur.

Environment

• Windows 10
• NHibernate 5.2.7
• Oracle.ManagedDataAccess 18.6.0

AdditionalContext

Full stack trace below, together with my (condensed) code for UnitOfWork - credits to Vladimir Khorikov - and SessionFactory construction. Mine is a multi-threaded application. The SessionFactory is created one and used across the app. Multiple sessions may be opened simultaneously using the same connection string, but session usage always occurs within a single unit of work, and a new unit of work is always newed up for a given command (i.e. I don't believe the same unit of work can be used across multiple threads). If I've misunderstood the documentation regarding session usage in a multithreaded environment, please help me understand my error.

Message: System.Exception : Exception=Object reference not set to an instance of an object.
StackTrace=
   at NHibernate.Connection.ConnectionProvider.CloseConnection(DbConnection conn)
   at NHibernate.Connection.DriverConnectionProvider.CloseConnection(DbConnection conn)
   at NHibernate.Transaction.AdoNetTransactionFactory.ExecuteWorkInIsolation(ISessionImplementor session, IIsolatedWork work, Boolean transacted)
   at NHibernate.Transaction.AdoNetWithSystemTransactionFactory.ExecuteWorkInIsolation(ISessionImplementor session, IIsolatedWork work, Boolean transacted)
   at NHibernate.Engine.Transaction.Isolater.DoIsolatedWork(IIsolatedWork work, ISessionImplementor session)
   at NHibernate.Engine.TransactionHelper.DoWorkInNewTransaction(ISessionImplementor session)
   at NHibernate.Id.TableGenerator.Generate(ISessionImplementor session, Object obj)
   at NHibernate.Id.TableHiLoGenerator.Generate(ISessionImplementor session, Object obj)
   at NHibernate.Event.Default.AbstractSaveEventListener.SaveWithGeneratedId(Object entity, String entityName, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.SaveWithGeneratedOrRequestedId(SaveOrUpdateEvent event)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.EntityIsTransient(SaveOrUpdateEvent event)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.PerformSaveOrUpdate(SaveOrUpdateEvent event)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.OnSaveOrUpdate(SaveOrUpdateEvent event)
   at NHibernate.Impl.SessionImpl.FireSaveOrUpdate(SaveOrUpdateEvent event)
   at NHibernate.Impl.SessionImpl.SaveOrUpdate(String entityName, Object obj)
   at NHibernate.Engine.CascadingAction.SaveUpdateCascadingAction.Cascade(IEventSource session, Object child, String entityName, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeToOne(Object parent, Object child, IType type, CascadeStyle style, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeAssociation(Object parent, Object child, IType type, CascadeStyle style, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeProperty(Object parent, Object child, IType type, CascadeStyle style, String propertyName, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeCollectionElements(Object parent, Object child, CollectionType collectionType, CascadeStyle style, IType elemType, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeCollection(Object parent, Object child, CascadeStyle style, Object anything, CollectionType type)
   at NHibernate.Engine.Cascade.CascadeAssociation(Object parent, Object child, IType type, CascadeStyle style, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeProperty(Object parent, Object child, IType type, CascadeStyle style, String propertyName, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeOn(IEntityPersister persister, Object parent, Object anything)
   at NHibernate.Event.Default.AbstractSaveEventListener.CascadeAfterSave(IEventSource source, IEntityPersister persister, Object entity, Object anything)
   at NHibernate.Event.Default.AbstractSaveEventListener.PerformSaveOrReplicate(Object entity, EntityKey key, IEntityPersister persister, Boolean useIdentityColumn, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
   at NHibernate.Event.Default.AbstractSaveEventListener.PerformSave(Object entity, Object id, IEntityPersister persister, Boolean useIdentityColumn, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
   at NHibernate.Event.Default.AbstractSaveEventListener.SaveWithGeneratedId(Object entity, String entityName, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.SaveWithGeneratedOrRequestedId(SaveOrUpdateEvent event)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.EntityIsTransient(SaveOrUpdateEvent event)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.PerformSaveOrUpdate(SaveOrUpdateEvent event)
   at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.OnSaveOrUpdate(SaveOrUpdateEvent event)
   at NHibernate.Impl.SessionImpl.FireSaveOrUpdate(SaveOrUpdateEvent event)
   at NHibernate.Impl.SessionImpl.SaveOrUpdate(String entityName, Object obj)
   at NHibernate.Engine.CascadingAction.SaveUpdateCascadingAction.Cascade(IEventSource session, Object child, String entityName, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeToOne(Object parent, Object child, IType type, CascadeStyle style, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeAssociation(Object parent, Object child, IType type, CascadeStyle style, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeProperty(Object parent, Object child, IType type, CascadeStyle style, String propertyName, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeCollectionElements(Object parent, Object child, CollectionType collectionType, CascadeStyle style, IType elemType, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeCollection(Object parent, Object child, CascadeStyle style, Object anything, CollectionType type)
   at NHibernate.Engine.Cascade.CascadeAssociation(Object parent, Object child, IType type, CascadeStyle style, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeProperty(Object parent, Object child, IType type, CascadeStyle style, String propertyName, Object anything, Boolean isCascadeDeleteEnabled)
   at NHibernate.Engine.Cascade.CascadeOn(IEntityPersister persister, Object parent, Object anything)
   at NHibernate.Event.Default.AbstractFlushingEventListener.CascadeOnFlush(IEventSource session, IEntityPersister persister, Object key, Object anything)
   at NHibernate.Event.Default.AbstractFlushingEventListener.PrepareEntityFlushes(IEventSource session)
   at NHibernate.Event.Default.AbstractFlushingEventListener.FlushEverythingToExecutions(FlushEvent event)
   at NHibernate.Event.Default.DefaultFlushEventListener.OnFlush(FlushEvent event)
   at NHibernate.Impl.SessionImpl.Flush()
   at NHibernate.Impl.SessionImpl.FlushBeforeTransactionCompletion()
   at NHibernate.Impl.SessionImpl.BeforeTransactionCompletion(ITransaction tx)
   at NHibernate.Transaction.AdoTransaction.Commit()
   at MyApp.Core.Domain.Core.CommandQueryHandling.UnitOfWork.Commit() in C:\src\NSG\MyApp\MyApp.Core\Domain\Core\CommandQueryHandling\UnitOfWork.cs:line 38
   at MyApp.Core.Domain.State.Commands.AdvanceStateHistoryLegacy.AdvanceStateHistoryHandler.<>c__DisplayClass3_1.<Handle>b__6() in C:\src\NSG\MyApp\MyApp.Core\Domain\State\Commands\AdvanceStateHistoryLegacy.cs:line 56
   at CSharpFunctionalExtensions.ResultExtensions.OnSuccess(Result result, Action action)
   at MyApp.Core.Domain.State.Commands.AdvanceStateHistoryLegacy.AdvanceStateHistoryHandler.Handle(AdvanceStateHistoryLegacy command) in C:\src\NSG\MyApp\MyApp.Core\Domain\State\Commands\AdvanceStateHistoryLegacy.cs:line 53
   at CallSite.Target(Closure , CallSite , Object , Object )
   at System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1)
   at CallSite.Target(Closure , CallSite , Object , Object )
   at MyApp.Core.Domain.Core.CommandQueryHandling.Messages.Dispatch(ICommand command) in C:\src\NSG\MyApp\MyApp.Core\Domain\Core\CommandQueryHandling\Messages.cs:line 22
   at MyApp.Core.CrossCutting.Extensions.ResultExtensions.Map[T](Result`1 result, Func`2 func) in C:\src\NSG\MyApp\MyApp.Core\CrossCutting\Extensions\ResultExtensions.cs:line 88
   at MyApp.Core.Application.Runjobs.StoreMeasurements.StoreMeasurementsRunjob.GetNormalReply(StoreMeasurementsContext runjobCollaterals) in C:\src\NSG\MyApp\MyApp.Core\Application\Runjobs\StoreMeasurements\StoreMeasurementsRunjob.cs:line 61
   at MyApp.Core.Application.Runjobs.StoreMeasurements.StoreMeasurementsRunjob.<>c__DisplayClass7_0.<Handle>b__3(IntroType introType) in C:\src\NSG\MyApp\MyApp.Core\Application\Runjobs\StoreMeasurements\StoreMeasurementsRunjob.cs:line 45
   at MyApp.Core.CrossCutting.Extensions.ResultExtensions.Map[T,K](Result`1 result, Func`2 func) in C:\src\NSG\MyApp\MyApp.Core\CrossCutting\Extensions\ResultExtensions.cs:line 82
   at MyApp.Core.Application.Runjobs.StoreMeasurements.StoreMeasurementsRunjob.Handle(XDocument runjob) in C:\src\NSG\MyApp\MyApp.Core\Application\Runjobs\StoreMeasurements\StoreMeasurementsRunjob.cs:line 45
   at MyApp.Core.Application.Runjobs.GenericRunjob.<>c__DisplayClass7_0.<Handle>b__3(IClassifiedRunjob runjob) in C:\src\NSG\MyApp\MyApp.Core\Application\Runjobs\GenericRunjob.cs:line 42
   at MyApp.Core.CrossCutting.Extensions.ResultExtensions.Map[T,K](Result`1 result, Func`2 func) in C:\src\NSG\MyApp\MyApp.Core\CrossCutting\Extensions\ResultExtensions.cs:line 82
   at MyApp.Core.Application.Runjobs.GenericRunjob.Handle(XDocument xDoc) in C:\src\NSG\MyApp\MyApp.Core\Application\Runjobs\GenericRunjob.cs:line 39
   at MyApp.Core.Application.Entry.NonComServiceRequest.<>c__DisplayClass4_1.<HandleRunjob>b__9(IRunjob`1 runjob) in C:\src\NSG\MyApp\MyApp.Core\Application\Entry\NonComServiceRequest.cs:line 40
   at MyApp.Core.CrossCutting.Extensions.ResultExtensions.Map[T,K](Result`1 result, Func`2 func) in C:\src\NSG\MyApp\MyApp.Core\CrossCutting\Extensions\ResultExtensions.cs:line 82
   at MyApp.Core.Application.Entry.NonComServiceRequest.HandleRunjob(String method, String requestXml) in C:\src\NSG\MyApp\MyApp.Core\Application\Entry\NonComServiceRequest.cs:line 37

public sealed class UnitOfWork : IDisposable
    {
        private readonly ISession _session;
        private readonly ITransaction _transaction;
        private bool _isAlive = true;
        private bool _disposed = false;

        public UnitOfWork(SessionFactory sessionFactory)
        {
            _session = sessionFactory.OpenSession();
            _transaction = _session.BeginTransaction(IsolationLevel.ReadCommitted);
        }

        public void Commit()
        {
            if (!_isAlive)
                return;

            try
            {
                _transaction.Commit();
            }
            finally
            {
                _isAlive = false;
                Dispose();
            }
        }
 
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        private void Dispose(bool disposing)
        {
            if (_disposed) return;
 
            if (disposing)
            {
                _transaction.Dispose();
                _session.Dispose();
            }
 
            _disposed = true;
        }
    }

         private ISessionFactory DoBuildSessionFactory(ConnectionString connectionString)
        {
            _logger.LogWarning("Constructing session factory");
            lock (_lockObject)
            {
                var dbConfig = OracleDataClientConfiguration.Oracle10
                    .ConnectionString(c => c.Is(connectionString))
                    .Driver<NHibernate.Driver.OracleManagedDataClientDriver>();
                        //.ShowSql();
                dbConfig = ApplicationGlobals.DefaultSchema.Equals(ApplicationGlobals.WindowsUser) ? dbConfig : dbConfig.DefaultSchema(ApplicationGlobals.DefaultSchema);

                FluentConfiguration configuration = Fluently.Configure()
                    .Database(dbConfig)
                    .Mappings(m =>
                        {
                            m.FluentMappings.AddFromAssembly(Assembly.GetExecutingAssembly()).AddConventions()/*.ExportTo(@"c:\temp\mappings")*/;
                            m.HbmMappings.AddFromAssembly(Assembly.GetExecutingAssembly());
                        })
                    .ExposeConfiguration(x =>
                    {
                        x.EventListeners.PostCommitUpdateEventListeners = new IPostUpdateEventListener[] { new EventListener() };
                        x.EventListeners.PostCommitInsertEventListeners = new IPostInsertEventListener[] { new EventListener() };
                        x.EventListeners.PostCommitDeleteEventListeners = new IPostDeleteEventListener[] { new EventListener() };
                        x.EventListeners.PostCollectionUpdateEventListeners = new IPostCollectionUpdateEventListener[] { new EventListener() };
                        x.DataBaseIntegration(dbi =>
                        {
                            dbi.Batcher<OracleDataClientBatchingBatcherFactory>();
                            dbi.Dialect<Oracle10gDialect>();
                        });
                        //x.SetInterceptor(new SqlStatementInterceptor());
                    });
   

                try
                {
                    var factory = configuration.BuildSessionFactory();
                    return factory;
                }
                catch (FluentConfigurationException e)
                {
                    var sb = new StringBuilder();
                    var fullMessage = e.FullMessage();
                    sb.AppendLine($"Fatal error building sessionFactory: '{fullMessage}'.");
                    sb.AppendLine($"Potential reasons:");
                    foreach (var reason in e.PotentialReasons) sb.AppendLine(reason);
                    sb.AppendLine($"Executing assembly path = {Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location)}");
                    _logger.LogCritical(e, $"Fatal error building session factory: {fullMessage}");
                    throw new System.Exception(sb.ToString());
                }
            }

[fredericDelaporte]

Checking NHibernate code, two other failures must have happened before that NRE but were swallowed by later failures in error handling code...

So we can fix those later failures, but that will not solve the root cause for your application. It will help diagnose it instead.

That is just a guess but I think the root cause is a connection opening failure, so some external cause, like your db being too busy.

[fredericDelaporte]

The linked PR will fix the NRE cause, which should help you see what is the trouble for your application.
The linked PR CI checks will contain a "NHibernate Release" build, from which you may download the built binaries, if you want to check what is the actual failure without having to wait for the 5.3 release.

[kriewall]

Thank you! I'll look into this and let you know what I find. Feel free to close it out as the error is most probably on my end; we can always reopen later.

[fredericDelaporte]

It will be closed with the PR. I consider this issue as reporting those error handling failures. The root trouble for your application is just something else, but failures in error handling have to be fixed.
Thanks for the report by the way.