• Oran Agra's avatar
    Fix LRU blue moon bug in RESTORE, RDB loading, module API (#9279) · b458b299
    Oran Agra authored
    The `lru_clock` and `lru` bits in `robj` save the least significant 24 bits of the unixtime (seconds since 1/1/1970),
    and wrap around every 194 days.
    The `objectSetLRUOrLFU` function, which is used in RESTORE with IDLETIME argument, and also in replica
    or master loading an RDB that contains LRU, and by a module API had a bug that's triggered when that happens.
    
    The scenario was that the idle time that came from the user, let's say RESTORE command is about 1000 seconds
    (e.g. in the `RESTORE can set LRU` test we have), and the current `lru_clock` just wrapped around and is less than
    1000 (i.e. a period of 1000 seconds once in some 6 months), the expression in that function would produce a negative
    value and the code (and comment) specified that the best way to solve that is push the idle time backwards into the
    past by 3 months. i.e. an idle time of 3 months instead of 1000 seconds.
    
    instead, the right thing to do is to unwrap it, and put it near LRU_CLOCK_MAX. since now `lru_clock` is smaller than
    `obj->lru` it will be unwrapped again by `estimateObjectIdleTime`.
    
    bug was introduced by 052e0349, but the code before it also seemed wrong.
    b458b299
object.c 54.5 KB