HybridCache: LocalCacheExpiration is not relative to Expiration

[zmj]

Description

The docstring for HybridCacheEntryOptions.LocalCacheExpiration says:

When retrieving a cached value from an external cache store, this value will be used to calculate the local
cache expiration, not exceeding the remaining overall cache lifetime.

That doesn't seem to match the behavior in DefaultHybridCache.SetL1: https://github.com/dotnet/extensions/blob/main/src/Libraries/Microsoft.Extensions.Caching.Hybrid/Internal/DefaultHybridCache.L2.cs#L108 I don't see anything here that respects the overall lifetime.

My scenario is that the value I'm caching has a constant valid lifetime, which I set as HybridCacheEntryOptions.Expiration. LocalCacheExpiration is a lower value, to free up memory that we suspect won't be used based on usage patterns. However, I see some errors due to usage of an expired value out of GetOrCreateAsync.

I suspect what's happening is:

1. First GetOrCreateAsync is an L1 cache miss, and L2 hit. Could be any of: this instance never having the value in L1, or the value expiring due to the lower LocalCacheExpiration, or memory pressure. The retrieved value's true relative expiration (createdAt + options.Expiration - Now) is less than LocalCacheExpiration. L1 cache TTL is set to LocalCacheExpiration.
2. Second GetOrCreateAsync is an L1 cache hit, but it's in the time window after the value has expired. Using the value fails.

I could work around this by including my value's expiration in my cache data and invalidating appropriately, but I'm unclear whether that's intended usage. The docstring for LocalCacheExpiration suggests otherwise - that the L2 serialized payload would include the creation time, and the L1 TTL would be set relative to it.

Reproduction Steps

can provide if description is unclear

Expected behavior

HybridCacheEntryOptions.Expiration is an upper bound on L1 TTL

Actual behavior

L1 TTL is only equal to HybridCacheEntryOptions.LocalCacheExpiration

Regression?

No response

Known Workarounds

No response

Configuration

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Other information

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