LocalCache: Fix compute() to report COLLECTED removal cause for GC'd entries

[nickita-khylkouski]

Summary

When compute() returns null for an entry whose value has been garbage collected, the removal cause should be COLLECTED, not EXPLICIT.

Changes

• Fix: In LocalCache.Segment.compute(), before calling removeEntry(), check if the original value was null and the reference is still active. If so, use RemovalCause.COLLECTED instead of RemovalCause.EXPLICIT.

• Test: Added testComputeRemovalCause_collected() that simulates garbage collection of a cached value and verifies that compute() returning null reports COLLECTED cause.

Motivation

Reported by @ben-manes in #7985:

After the compute locks it runs preWriteCleanup() which performs an amortized drain of the pending work. However it does not perform all that might have built up since it is on a caller's thread. After this is performed, an existing entry may be found that is pending eviction but still present. In this test it returns null, so which leads to the mapping removal, and the listener reports this as an EXPLICIT removal cause with a null value.

The null value in the removal notification should only occur for a COLLECTED cause.

Testing

The new test:

1. Creates a cache with soft values
2. Puts an entry
3. Clears the value reference to simulate garbage collection
4. Calls compute() which returns null
5. Verifies the removal notification has COLLECTED cause

Fixes #7985