EvictableCache race condition can temporarily revive old entry

[findepi]

TestEvictableLoadingCache.testInvalidateAndLoadConcurrently was found to be flaky (with probability ~0.000003 on my local machine).

found by @anusudarsan

[findepi]

I think this is indeed possible to get a stale value:

     Thread A                       |   Thread B                                        |   Thread C
----------------------------------------------------------------------------------------------------------------------------------------------
ENTER invalidate(42)
invalidations ++
                                                                                            change remote state
                                                                                            ENTER invalidate(42)
                                                                                            invalidations ++
                                        ENTER get(42, loader)
                                        i = invalidations
                                        t = tokens.computeIfAbsent(42, -> new) // old
t = tokens.remove(42) // found 
                                        val = dataCache.get(t, loader) // from cache
                                                                                            t = tokens.remove(42) // not found
                                                                                            EXIT invalidate(42)
                                        if i == invalidations // true
                                        tokens.putIfAbsent(42, t) // "Revive"
                                                                                            ENTER get(42, loader)
                                                                                            t = tokens.computeIfAbsent(42, -> new) // old
                                                                                            val = dataCache.get(t, loader) // from cache
                                                                                            return val                             // ‼️  old value returned despite explicit invalidation
                                                                                            EXIT get(42, loader)
                                        if !dataCache.containsKey(t) // false
                                        return val
                                        EXIT get(42, loader)
dataCache.invalidate(t) // this fixes the temporarily incorrect state
EXIT invalidate(42)

Indeed, if i revert the "Revive" code (17faae3), i can no longer reproduce the problem