Fix LRU blue moon bug in RESTORE, RDB loading, module API (#9279)

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 052e03495f, but the code before it also seemed wrong.

1 files changed, 6 insertions, 6 deletions

diff --git a/src/object.c b/src/object.c
index 59ae47f2a..05831156c 100644
--- a/src/object.c
+++ b/src/object.c
@@ -1201,13 +1201,13 @@ int objectSetLRUOrLFU(robj *val, long long lfu_freq, long long lru_idle,
          * below statement will expand to lru_idle*1000/1000. */
         lru_idle = lru_idle*lru_multiplier/LRU_CLOCK_RESOLUTION;
         long lru_abs = lru_clock - lru_idle; /* Absolute access time. */
-        /* If the LRU field underflow (since LRU it is a wrapping
-         * clock), the best we can do is to provide a large enough LRU
-         * that is half-way in the circular LRU clock we use: this way
-         * the computed idle time for this object will stay high for quite
-         * some time. */
+        /* If the LRU field underflows (since lru_clock is a wrapping clock),
+         * we need to make it positive again. This be handled by the unwrapping
+         * code in estimateObjectIdleTime. I.e. imagine a day when lru_clock
+         * wrap arounds (happens once in some 6 months), and becomes a low
+         * value, like 10, an lru_idle of 1000 should be near LRU_CLOCK_MAX. */
         if (lru_abs < 0)
-            lru_abs = (lru_clock+(LRU_CLOCK_MAX/2)) % LRU_CLOCK_MAX;
+            lru_abs += LRU_CLOCK_MAX;
         val->lru = lru_abs;
         return 1;
     }