NativeQueue Count wrongly asks for previous job completion but TryDequeue doesn't.

EDIT: This was related to a system not getting input dependencies because it was not processing any of the same component types as the other. Read more here.
It was a freak race condition that allowed writing but not reading... Couldn't reproduce once fixed.



I guess it's maybe because Count calls AtomicSafetyHandle.CheckReadAndThrow and TryDequeue just AtomicSafetyHandle.CheckWriteAndThrow.

The weird thing is they DO complete. I've used

1. A ComponentSystem
2. A JobComponentSystem, with inputDeps.Complete()
3. A previous BarrierSystem

It doesn't matter it always has this trace:

Code (CSharp):

1. InvalidOperationException: The previously scheduled job Sys_EventQueue_Raise:Job writes to the NativeArray Job.Data._entsMT. You must call JobHandle.Complete() on the job Sys_EventQueue_Raise:Job, before you can read from the NativeArray safely.
2. Unity.Collections.LowLevel.Unsafe.AtomicSafetyHandle.CheckReadAndThrowNoEarlyOut (Unity.Collections.LowLevel.Unsafe.AtomicSafetyHandle handle) <0x30f963e0 + 0x00052> in <2f14d49edc704940aee557642343f344>:0
3. Unity.Collections.LowLevel.Unsafe.AtomicSafetyHandle.CheckReadAndThrow (Unity.Collections.LowLevel.Unsafe.AtomicSafetyHandle handle) (at C:/buildslave/unity/build/Runtime/Export/AtomicSafetyHandle.bindings.cs:143)
4. Unity.Collections.NativeQueue`1[T].get_Count () (at C:/Users/dosbam/AppData/Local/Unity/cache/packages/packages.unity.com/com.unity.collections@0.0.9-preview.10/Unity.Collections/NativeQueue.cs:226)
5.  

The offending job from Sys_EventQueue_Raise is managed by a JobComponentSystem, can't do a Complete manually besides inputDeps.

Dependencies seem OK (QueueSystem runs the offended job with _entsMT) :

Code (CSharp):

1.     [UpdateAfter(typeof(Sys_EventQueue_Raise))]
2.     [UpdateBefore(typeof(QueueSystem))]
3.     public class Sys_EventQueue_RaiseBarrier : BarrierSystem { }

I don't actually need a Count, just a safe way if it has anything pending. Could try-catch a Peek() but also has a CheckReadAndThrow, also why should I have to...
I can't "un-dequeue" after a TryDequeue check.

The

if(TryGet) do { } while(TryGet)

idiom from all NativeContainers is irritating...


Versions from package manager:

• com.unity.jobs@0.0.7-preview.5
• com.unity.entities@0.0.12-preview.21
• com.unity.collections@0.0.9-preview.10

EDIT: Workaround/use case, if anyone is interested...

Code (CSharp):

1. //Batch-create entities for each event.
2. if(_queueEvents.TryDequeue(out ev)) {
3.     //***NOTICE***: Count+1 because we just dequeued one.
4.     _entities = new NativeArray<Entity>(_queueEvents.Count+1, Allocator.TempJob);
5.     _emgr.CreateEntity(_archetype, _entities);
6.  
7.     var eventByEntArray = GetComponentDataFromEntity<Dat_EventWithSource>();
8.     int index = 0;
9.     do {
10.         eventByEntArray[_entities[index++]] = ev;
11.     } while(_queueEvents.TryDequeue(out ev));
12. }