InMemoryCacheProvider.TryGet can delete a fresh value that another thread just Set, when it sees an expired entry for the same key #47

What is wrong

InMemoryCacheProvider.TryGet (Essentials.CacheProviders.InMemory/InMemoryCacheProvider.cs, lines 26-35) reads an entry, sees that it has expired, and then removes whatever is currently stored under that key:

if (cache.TryGetValue(key, out CacheEntry? entry))
{
    if (entry.Expiration is null || entry.Expiration > DateTime.UtcNow) { ... return true; }
    // Entry has expired, remove it
    cache.TryRemove(key, out _);
}

Nothing prevents another thread from calling Set for the same key between the TryGetValue and the TryRemove, in which case the new, non-expired entry is deleted.

Concrete interleaving

1. The key holds entry E1, which has expired.
2. Thread A: TryGet(k) → TryGetValue returns E1; the expiration check fails.
3. Thread B: Set(k, v2) (no expiration) → cache[k] = E2.
4. Thread A: cache.TryRemove(k, out _) → removes E2.
5. Thread B: TryGet(k) returns false. The value it just set, which never expires, is gone, and GetOrAdd will call the factory again.

How it was verified

Stress program: one thread calls TryGet("k") in a tight loop; a second repeats Set("k", -1, TimeSpan.FromTicks(-1)); Set("k", i); if (!TryGet("k", out _)) lost++;. Runs against HEAD on a 4-core machine reported fresh non-expiring entries lost: 3 / 200000, then 16, 2 and 0 over three more runs. Correct code must always report 0.

Related failure in the same file

Set(key, value, TimeSpan.MaxValue) throws ArgumentOutOfRangeException from DateTime.UtcNow + expiration.Value (line 50) instead of storing an entry that never expires.

Suggested fix / acceptance criteria

• Remove only the entry that was observed to be expired. On .NET 5+ use cache.TryRemove(new KeyValuePair<TKey, CacheEntry>(key, entry)); on netstandard2.1 use ((ICollection<KeyValuePair<TKey, CacheEntry>>)cache).Remove(...). Both compare the value, so a concurrently set replacement is untouched.
• Saturate the expiration computation: treat overflow / TimeSpan.MaxValue as no expiration.
• A concurrency test like the one above reports zero lost entries.