FASTER: (1.7.2) NotImplementedException thrown sometimes in VarLenBlittableAllocator.cs when deleting entry, using SpanByte as key and value

I’m using FasterKV<SpanByte, SpanByte> the newest release (1.7.2), having a fully functional project, but sometimes I get System.NotImplementedException in VarLenBlittableAllocator.cs:line 191 when deleting entries in bulk.

When I’m deleting them one by one manually everything works fine, but if I iterate through keys from a List<> or Dictionary<>, or just calling it many times during stress-tests I always get this exception.

Stacktrace:

System.NotImplementedException
  HResult=0x80004001
  Message=The method or operation is not implemented.
  Source=FASTER_Test2
  StackTrace:
   at FASTER.core.VariableLengthBlittableAllocator`2.ShallowCopy(Key& src, Key& dst) in ...\FASTER\core\Allocator\VarLenBlittableAllocator.cs:line 191
   at FASTER.core.FasterKV`2.InternalDelete[Input,Output,Context,FasterSession](Key& key, Context& userContext, PendingContext`3& pendingContext, FasterSession fasterSession, FasterExecutionContext`3 sessionCtx, Int64 lsn) in ...\FASTER\core\Index\FASTER\FASTERImpl.cs:line 1029
   at FASTER.core.FasterKV`2.ContextDelete[Input,Output,Context,FasterSession](Key& key, Context context, FasterSession fasterSession, Int64 serialNo, FasterExecutionContext`3 sessionCtx) in ...\FASTER\core\Index\FASTER\FASTER.cs:line 537
   at FASTER.core.ClientSession`6.Delete(Key& key, Context userContext, Int64 serialNo) in ...\FASTER\core\ClientSession\ClientSession.cs:line 415
   at KV.KVStore.WorkThread.Loop() in ...\FASTER\KVStore.cs:line 2370
   at System.Threading.ThreadHelper.ThreadStart_Context(Object state) in /_/src/System.Private.CoreLib/src/System/Threading/Thread.CoreCLR.cs:line 50
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) in /_/src/System.Private.CoreLib/shared/System/Threading/ExecutionContext.cs:line 172
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() in /_/src/System.Private.CoreLib/shared/System/Runtime/ExceptionServices/ExceptionDispatchInfo.cs:line 63
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) in /_/src/System.Private.CoreLib/shared/System/Threading/ExecutionContext.cs:line 200
   at System.Threading.ThreadHelper.ThreadStart() in /_/src/System.Private.CoreLib/src/System/Threading/Thread.CoreCLR.cs:line 100

  This exception was originally thrown at this call stack:
    FASTER.core.VariableLengthBlittableAllocator<Key, Value>.ShallowCopy(ref Key, ref Key) in VarLenBlittableAllocator.cs
    FASTER.core.FasterKV<Key, Value>.InternalDelete<Input, Output, Context, FasterSession>(ref Key, ref Context, ref FASTER.core.FasterKV<Key, Value>.PendingContext<Input, Output, Context>, FasterSession, FASTER.core.FasterKV<Key, Value>.FasterExecutionContext<Input, Output, Context>, long) in FASTERImpl.cs
    FASTER.core.FasterKV<Key, Value>.ContextDelete<Input, Output, Context, FasterSession>(ref Key, Context, FasterSession, long, FASTER.core.FasterKV<Key, Value>.FasterExecutionContext<Input, Output, Context>) in FASTER.cs
    FASTER.core.ClientSession<Key, Value, Input, Output, Context, Functions>.Delete(ref Key, Context, long) in ClientSession.cs
    KV.KVStore.WorkThread.Loop() in KVStore.cs
    System.Threading.ThreadHelper.ThreadStart_Context(object) in Thread.CoreCLR.cs
    System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, object) in ExecutionContext.cs
    System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() in ExceptionDispatchInfo.cs
    System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback, object) in ExecutionContext.cs
    System.Threading.ThreadHelper.ThreadStart() in Thread.CoreCLR.cs

As the simplest fix I tried to modify this two functions in the VarLenBlittableAllocator.cs:

        public override void ShallowCopy(ref Key src, ref Key dst)
        {
            dst = src;
            // throw new NotImplementedException();
        }

        public override void ShallowCopy(ref Value src, ref Value dst)
        {
            dst = src;
            // throw new NotImplementedException();
        }

…and well, it works. I’m pretty sure not this is the solution, but since the modification I never get the exceptions, and deleting entries works just fine.

I didn’t have this issue with 1.7.1, but I can’t just switch libraries to check if the previous version throws this exception in this test project, because there is a considerable difference between 1.7.1 and 1.7.2 in the SpanByte and SpanByte Functions class, I’m having custom Functions classes, not really interchangeable.

Why do I get this excetion? Is the solution I came up with ok? I can’t really figure out how and why are these functions called inside, that’d need more time for me to understand how faster works.