Add Clear() to MemoryCache

[adamsitnik]

fixes #45593

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Issue Details

---

fixes #45593

Author:	adamsitnik
Assignees:	-
Labels:	area-Extensions-Caching
Milestone:	7.0.0

[eerhardt]

Could we get away with this by introducing a new class that contained both the dictionary and the size? Then we can atomically replace the pointer with a new instance?

[adamsitnik]

In this particular case, yes. In other, where we add/remove item from the cache and update the size afterwards, not.

runtime/src/libraries/Microsoft.Extensions.Caching.Memory/src/MemoryCache.cs

Lines 265 to 270 in 40cbe82

	if (_entries.TryRemove(key, out CacheEntry entry))
	{
	if (_options.SizeLimit.HasValue)
	{
	Interlocked.Add(ref _cacheSize, -entry.Size.Value);
	}

runtime/src/libraries/Microsoft.Extensions.Caching.Memory/src/MemoryCache.cs

Lines 151 to 158 in 40cbe82

	entryAdded = _entries.TryUpdate(entry.Key, entry, priorEntry);
	if (entryAdded)
	{
	if (_options.SizeLimit.HasValue)
	{
	// The prior entry was removed, decrease the by the prior entry's size
	Interlocked.Add(ref _cacheSize, -priorEntry.Size.Value);

I am not sure if it's worth it.

[eerhardt]

In those other cases you can atomically read the pointer, store it in a local variable and modify that. If Clear gets called concurrently, it will either update the pointer before or after the other mutation. Thus, we should always have a coherent set of entries and cache size.

[adamsitnik]

@eerhardt well, it took me a while to get back to this PR. PTAL at my recent commit.

[eerhardt]

Looks good to me. Thanks for the contribution here @adamsitnik.

I just had some minor comments and questions.

[eerhardt]

Is this a case where we need volatile? My intuition tells me "no" because we only ever read the field once at the beginning of a method, and then use the local variable. But I wanted to ask the question, since I'm not an expert on when to use volatile.

@stephentoub ?