Atomic_Add(key, -1) can “leak” keys with value 0 if you are not careful about foundationdb HOT 5 CLOSED

Resolved in #1105 which implements CompareAndClear op.

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This seems pretty useful to me. I also have used counters in this way and have had to manually garbage collect them to remove those that are no longer in use. There are probably several atomic operations that have not yet been implemented that would also be useful.

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It sounds like there were two options here, either to implement COMPARE_AND_CLEAR that clears key k if and only if the value at k is equal to the other parameter, or a more specific ADD_AND_CLEAR that would clear a key that hits zero. I think some of us on the team decided that COMPARE_AND_CLEAR, being more generic, would be more useful even if it means a few more client operations.

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Bindings could provide a ADD_AND_CLEAR overload that internally calls ADD and COMPARE_AND_CLEAR, so that we have both :)

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And more generally, layers can create functions that combine any of the atomic ops as they see fit. I suspect that will be how we initially approach the bindings we're currently publishing -- we probably won't provide any combinations in the bindings themselves, but instead defer that to layer writers as is appropriate to them.

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