Relax race condition between subkey compaction thread

[dongdongwcpp]

lazy delete to best effort relax race condition between command Expire and subkey Compaction.

It closes #1298.

[tisonkun]

Could you elaborate a bit how you fix this race condition logically? Generally, "wait for a while" is not logically correct but best effort.

[PragmaTwice]

Could you elaborate a bit how you fix this race condition logically? Generally, "wait for a while" is not logically correct but best effort.

Hi @tisonkun , this race is triggered by modifying the expire time of a key while compacting. Rare in real-world but indeed a race.

In my view, since it is hard and expensive to add locking to both compacting and command executing, waiting is the currently best way to avoid the race.

[tisonkun]

Could you elaborate a bit how you fix this race condition logically? Generally, "wait for a while" is not logically correct but best effort.

Hi @tisonkun , this race is triggered by modifying the expire time of a key while compacting. Rare in real-world but indeed a race.

In my view, since it is hard and expensive to add locking to both compacting and command executing, waiting is the currently best way to avoid the race.

Thank you. Then let's update the PR title as "Relax ..." instead of "Fix ...".

[git-hulk]

Thanks all, merging...

[git-hulk]

Thanks for your contribution again @dongdongwcpp