Add kv txn get-or-empty operation.

[hashi-derek]

Description

Adds a get-or-empty operation to the kv txn api. There is currently no operation for fetching the value of a key that may not yet exist. The current get command will cause the entire transaction to roll-back if the key doesn't exist. The new op will return a nil value for a key if-not-found.

https://www.consul.io/api-docs/txn#kv-operations

PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern

[banks]

Amazing job adding this small enhancement @hashi-derek 👏

Overall this is exactly what I imagined too when thinking about this problem.

I have a couple of minor nits inline which I don't think block merging this.

My biggest bit of feedback would be on the name chosen. I think get-if-exists is slightly more clear/descriptive of the behaviour vs. get-not-exists. I see how there is a consistency argument with check-not-exists but I think that makes more sense in context "Check that key X does not exist" is naturally expressed by check-not-exists whereas "Get the value of key X if it exists or empty otherwise" requires a double-negative to map to get-not-exists and maps more naturally to get-if-exists in my mind anyway 😄.

If you disagree though it'd not a mountain I feel the need to die on! Should be a quick enough update if you do!

[kisunji]

Amazing job adding this small enhancement @hashi-derek 👏

Overall this is exactly what I imagined too when thinking about this problem.

I have a couple of minor nits inline which I don't think block merging this.

My biggest bit of feedback would be on the name chosen. I think get-if-exists is slightly more clear/descriptive of the behaviour vs. get-not-exists. I see how there is a consistency argument with check-not-exists but I think that makes more sense in context "Check that key X does not exist" is naturally expressed by check-not-exists whereas "Get the value of key X if it exists or empty otherwise" requires a double-negative to map to get-not-exists and maps more naturally to get-if-exists in my mind anyway 😄.

If you disagree though it'd not a mountain I feel the need to die on! Should be a quick enough update if you do!

How about get-or-nil or get-or-empty? Semantically, get-if-exists isn't descriptive of the "if not exists" case (it could very well describe get)

[hashi-derek]

@kisunji Yeah, I wasn't a fan of any of the names I had come up with. get-or-empty seems like a reasonable choice. Since the action will always return a result, it's probably more clear than get-if-exists as well.

[kisunji]

👍