NullReferenceException when using AsyncPool

[wassim-k]

I'm using FasterWrapper as suggested by #780 (comment)
But when running the following code, which checkpoints from multiple threads, I'm receving a NullReferenceException at var status = session.Upsert(key, value);

Can you please explain why?

var faster = new FasterWrapper<long, long>(isRefType: false, useLargeLog: false, useOsReadBuffering: false);
await Task.WhenAll(Enumerable.Range(0, 2).Select(async i =>
{
    faster.Upsert(i, i);
    await faster.Store.TakeHybridLogCheckpointAsync(CheckpointType.FoldOver);
}));

I've also noticed in the synchronous variation of methods like Upsert in FasterWrapper, _sessionPool.GetAsync().GetAwaiter().GetResult() is being used instead of the synchronous _sessionPool.Get(), is there reason for that?

[tcsaddul]

Maybe the same problem here when I upgraded to the latest version that error appears in the Upsert in concurrent calls. After reverting to the previous version, error disappeared.

[wassim-k]

Worth mentioning I've had this issue happen in 2.2.0 & 2.3.0 versions.

[badrishc]

Can you summarize in detail what's the exact issue in the latest version. Where is the exception, what's the call stack, have your tried any fixes yourself etc.

For sync, you can check the code to see if there is a good reason. I think it's because Try might fail.

[wassim-k]

Reproduction:

• Checkout AsyncStress playground
• Expose public FasterKV<Key, Value> Store => _store; from FasterWrapper<Key, Value>
• Replace Program's Main method with above code as is.

Exception:

Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at FASTER.core.ClientSession`6.InternalFasterSession.TryLockEphemeralExclusive(RecordInfo& recordInfo)
   at AsyncStress.FasterWrapper`2.Upsert(Key key, Value value) in C:\Code\AsyncStress\FasterWrapper.cs:line 87
   at AsyncStress.Program.<>c__DisplayClass10_0.<<Main>b__0>d.MoveNext() in C:\Code\AsyncStress\Program.cs:line 70
--- End of stack trace from previous location ---
   at AsyncStress.Program.Main(String[] args) in C:\Code\AsyncStress\Program.cs:line 68
   at AsyncStress.Program.<Main>(String[] args)

The issue happens in v2.2.0 and v2.3.0 but not in v.2.1.0

[badrishc]

Fixed in #785