ben-manes / caffeine / #2600
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ben-manes 
Improve robustness in racy scenarios (fixes #568)

1. When an entry is updated then a concurrent reader should observe
either the old or new value. This operation replaces the j.l.Reference
instance stored on the entry and the old referent becomes eligible for
garbage collection. A reader holding the stale Reference may therefore
return a null value, which is more likely due to the cache proactively
clearing the referent to assist the garbage collector.

When a null value is read then an extra volatile read is used to
validate that the Reference instance is still held by the entry. This
retry loop has negligible cost.

2. When an entry is eligible for removal due to its value being garbage
collected, then during the eviction's atomic map operation this
eligibility must be verified. If concurrently the entry was resurrected
and a new value set, then the cache writer has already dispatched the
removal notification and established a live mapping. If the evictor does
not detect that the cause is no longer valid, then it would incorrectly
discard the mapping with a removal notification containing a non-null
key, non-null value, and collected removal cause.

Like expiration and size policies, the reference eviction policy will
now validate and no-op if the entry is no longer eligible.

3. When the fixed expiration setting is dynamically adjusted, an
expired entry may be resurrected as no longer eligible for removal.
While the map operation detected this case, stemming from the entry
itself being updated and its lifetime reset, the outer eviction loop
could retry indefinitely due to a stale read of the fixed duration.
This caused the loop to retry the ineligible entry, but instead it
can terminate when eviction fails because it scans a queue ordered
by the expiration timestamp.

Co-authored-by: Justin Horvitz <jhorvitz@google.com>

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