Fix lock contention in FluidValue.Create

[lahma]

While doing performance checkup in Orchard (load test profiling agency theme) I noticed that there was quite big lock contention problem in Fluid, it originates from #387 where the logic never assigns anything on cache miss thus making the locking and looping happen every time for types like ContentItem (non-dictionary type).

Fixed issue by using Interlocked.CompareExchange (no locks needed) and also storing null as result for given type. The dictionary was public, don't know why (Orchard doesn't seem to need it), made it private.

Main

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
CreateFromList	287.9 ns	1.22 ns	1.08 ns	0.0110	-	-	184 B

This PR

Method	Mean	Error	StdDev	Gen 0	Gen 1	Gen 2	Allocated
CreateFromList	46.98 ns	0.979 ns	1.048 ns	0.0029	-	-	48 B

[sebastienros]

There is a risk with the swap technique that multiple threads can never get to an agreement with all types in the dictionary. Example with two types, A and B.
Thread 1 checks for A, Thread 2 checks for B, none is there, T1 adds A replaces dictionary, T2 adds B, replaces dictionary, A is not there anymore, and this could go on. This is theory, but my previous lock was preventing this. Thread 2 would be blocked until Thread 1 was done.

So ideally the only thing to fix would be the null in the dictionary. In terms of perf no lock would be hit once the cache is warm.

[lahma]

https://docs.microsoft.com/en-us/dotnet/api/system.threading.interlocked.compareexchange?view=net-6.0#System_Threading_Interlocked_CompareExchange_System_Object__System_Object_System_Object_ - exchange only happens if field has same reference as in start, if not, someone did successful update and the current race loser will try again. But can also be changed back to double-checked locking.

[sebastienros]

I understand now, thanks for the explanation. There might be other places I could use that pattern then, I have used the "switch" technique in some places knowing the limitation I was explaining. This technique solves this quite nicely.