idempotent value creation

[bitfaster]

Provide building blocks to synchronize cache value factories in order to mitigate a cache stampede.

In each case below, multiple threads calling GetValue equivalents will be synchronized such that only a single thread runs at a time and the others see the result of the winning thread:

var idempotent = new Idempotent<int, int>();
int i = idempotent.GetValue(1, k => k);

var idempotent = new AsyncIdempotent<int, int>();
int i = await idempotent.GetValueAsync(1, k => Task.FromResult(k));

var idempotent = new ScopedIdempotent<int, Disposable>();
bool r = idempotent.TryCreateLifetime(1, k => new Disposable(), out var lifetime);

var idempotent = new ScopedAsyncIdempotent<int, Disposable>();
(bool r, Lifetime<Disposable> l) result = await idempotent.TryCreateLifetimeAsync(1, k =>
            {
                return Task.FromResult(new Disposable);
            });

If an exception is thrown, it is not cached. Intent is to mimic behavior of ConcurrentDictionary.GetOrAdd. In the case of the async variants, the result of the single failing task can be seen by other threads if they attempt to create a value while it is running. Once it is finished, a new attempt can be made.

This would probably need to be extended to include some parameterization of the lock timeout. This would require switching to Monitor.TryEnter.