Scripting: expire keys in scripts only at first access.
Keys expiring in the middle of the execution of Lua scripts are to create inconsistencies in masters and / or AOF files. See the following example: if redis.call("exists",KEYS[1]) == 1 then redis.call("incr","mycounter") end if redis.call("exists",KEYS[1]) == 1 then return redis.call("incr","mycounter") end The script executes two times the same *if key exists then incrementcounter* logic. However the two executions will work differently in the master and the slaves, provided some unlucky timing happens. In the master the first time the key may still exist, while the second time the key may no longer exist. This will result in the key incremented just one time. However as a side effect the master will generate a synthetic `DEL` command in the replication channel in order to force the slaves to expire the key (given that key expiration is master-driven). When the same script will run in the slave, the key will no longer be there, so the script will not increment the key. The key idea used to implement the expire-at-first-lookup semantics was provided by Marc Gravell.
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