Null reference at NHibernate.Util.AsyncReaderWriterLock.ReadLock()

[michalstepien]

Hi,
after upgrade NHibernate 5.2.7 -> 5.3.1, I have this error:

[NullReferenceException]
Object reference not set to an instance of an object.
   at NHibernate.Util.AsyncReaderWriterLock.ReadLock()
   at NHibernate.Cache.ReadWriteCache.Get(CacheKey key, Int64 txTimestamp)
   at NHibernate.Event.Default.DefaultLoadEventListener.LoadFromSecondLevelCache(LoadEvent event, IEntityPersister persister, LoadType options)
   at NHibernate.Event.Default.DefaultLoadEventListener.DoLoad(LoadEvent event, IEntityPersister persister, EntityKey keyToLoad, LoadType options)
   at NHibernate.Event.Default.DefaultLoadEventListener.Load(LoadEvent event, IEntityPersister persister, EntityKey keyToLoad, LoadType options)
   at NHibernate.Event.Default.DefaultLoadEventListener.ProxyOrLoad(LoadEvent event, IEntityPersister persister, EntityKey keyToLoad, LoadType options)
   at NHibernate.Event.Default.DefaultLoadEventListener.OnLoad(LoadEvent event, LoadType loadType)
   at NHibernate.Impl.SessionImpl.FireLoad(LoadEvent event, LoadType loadType)
   at NHibernate.Impl.SessionImpl.InternalLoad(String entityName, Object id, Boolean eager, Boolean isNullable)
   at NHibernate.Type.EntityType.ResolveIdentifier(Object id, ISessionImplementor session)
   at NHibernate.Collection.Generic.PersistentGenericSet`1.InitializeFromCache(ICollectionPersister persister, Object disassembled, Object owner)
   at NHibernate.Cache.Entry.CollectionCacheEntry.Assemble(IPersistentCollection collection, ICollectionPersister persister, Object owner)
   at NHibernate.Event.Default.DefaultInitializeCollectionEventListener.Assemble(CacheKey ck, Object ce, ICollectionPersister persister, ISessionImplementor source, IPersistentCollection collection, Object collectionKey, Boolean alterStatistics)
   at NHibernate.Event.Default.DefaultInitializeCollectionEventListener.InitializeCollectionFromCache(Object collectionKey, ICollectionPersister persister, IPersistentCollection collection, ISessionImplementor source)
   at NHibernate.Event.Default.DefaultInitializeCollectionEventListener.OnInitializeCollection(InitializeCollectionEvent event)
   at NHibernate.Impl.SessionImpl.InitializeCollection(IPersistentCollection collection, Boolean writing)
   at NHibernate.Collection.AbstractPersistentCollection.Initialize(Boolean writing)
   at NHibernate.Collection.AbstractPersistentCollection.ReadElementExistence[T](T element, Nullable`1& existsInDb)
   at NHibernate.Collection.Generic.PersistentGenericSet`1.Contains(T item)

The error occurs during high load applications.

[fredericDelaporte]

The offending line of code can only be:

nhibernate-core/src/NHibernate/Util/AsyncReaderWriterLock.cs

Line 90 in c3ac6a0

_waitingReadLockSemaphore.Wait();

I think it should just be switched to:

_waitingReadLockSemaphore?.Wait();

If the waiting lock has already been switched out, it means the read can go on.

[maca88]

The offending line of code can only be:
_waitingReadLockSemaphore.Wait();

Correct, what happend in this scenario:

public Releaser ReadLock()
{
	if (CanEnterReadLock())
	{
		return _readerReleaser;
	}

	// Before the below line is executed ExitWriteLock is called, which set _waitingDisposalSemaphore to null
	// and increases the _currentReaders by at least one as the above CanEnterReadLock returned false. If WriteLock
	// is called again it will wait due to _currentReaders is greater than zero. So there are two solutions, either
	// skip calling _waitingReadLockSemaphore when is null or add an output parameter for CanEnterReadLock in order
	// to obtain _waitingReadLockSemaphore
	_waitingReadLockSemaphore.Wait();

	return _readerReleaser;
}

to me adding an output parameter is more correct, which I did in #2469.

[bahusoid]

to me adding an output parameter is more correct

Just a question: are you sure when Wait method is called it can't be already disposed?

[maca88]

are you sure when Wait method is called it can't be already disposed?

The semaphore is disposed (DisposeWaitingSemaphore) in WriteLock only when _currentReaders is zero, which in this case won't be:

nhibernate-core/src/NHibernate/Util/AsyncReaderWriterLock.cs

Lines 43 to 59 in c3ac6a0

	if (!CanEnterWriteLock(out var waitForReadLocks))
	{
	_writeLockSemaphore.Wait();
	lock (_writeLockSemaphore)
	{
	_writersWaiting--;
	}
	}
	if (waitForReadLocks)
	{
	_readLockSemaphore.Wait();
	}
	DisposeWaitingSemaphore();
	return _writerReleaser;

In this case CanEnterWriteLock will return true, but waitForReadLocks will also be true due to _currentReaders being greater than zero, so the disposal will wait until the read lock is released.
So yes, I am pretty sure it can't be disposed when called.

[hazzik]

Fixed in #2469

[Gonnagle]

I run into bunch of my integration tests still failing on following error with 5.3.3 (ObjectDisposedException instead of NRE)

System.ObjectDisposedException : Cannot access a disposed object.
Object name: 'AsyncReaderWriterLock'.
   at NHibernate.Util.AsyncReaderWriterLock.AssertNotDisposed()
   at NHibernate.Util.AsyncReaderWriterLock.CanEnterWriteLock(Boolean& waitForReadLocks)
   at NHibernate.Util.AsyncReaderWriterLock.WriteLock()
   at NHibernate.Cache.ReadWriteCache.Lock(CacheKey key, Object version)
   at NHibernate.Action.CollectionAction.BeforeExecutions()
   at NHibernate.Engine.ActionQueue.PrepareActions[T](List`1 queue)
   at NHibernate.Event.Default.AbstractFlushingEventListener.PerformExecutions(IEventSource session)
   at NHibernate.Event.Default.DefaultFlushEventListener.OnFlush(FlushEvent event)
   at NHibernate.Impl.SessionImpl.Flush()
   at NHibernate.Impl.SessionImpl.BeforeTransactionCompletion(ITransaction tx)
   at NHibernate.Transaction.AdoTransaction.Commit()

Reverting to 5.2.7 seemed to fix the issue so will go with that as a workaround for now. Could definitely be that there is just something fishy going on my code causing this, but unfortunately can't dive much deeper right now at least.