ben-manes / caffeine / 2258

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Fix variable expiration overflowing with the maximum duration (fixes #217)

When the duration is set to the maximum length, Long.MAX_VALUE nanoseconds,
the calcuation of expirationTime - currentTime > 0 may overflow and be
negative. This will not occur if the same thread calculates both timestamps.
It may occur across threads when the expirationTime is concurrently updated
using a later base time than t1's reading of the currentTime. This can
occur whenever the maintenance work is triggered to sweep expired entries
and a user thread accesses the entry. The later timestamp plus the maximum
duration results in an overflow, causing the remaining time to be negative,
and therefore causes the cache to expire the entry.

The internal maximum is now capped at Long.MAX_VALUE / 2 or ~150 years. This
should give a broad safety net to avoid these concurrency-inducing overflows
in normal code.

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