Feature Request: hsetnx variant that also NOPs/fails if the hash ID does not exist

[jdavisclark]

There are a lot of times when I want to set the value of a hash field if-and-only-if the hash field doesn't already exist (covered by HSETNX) and the hash actually exists. This is not currently possible to do with a single command so far as I can tell, and requires either 1) a lua script, 2) multiple non-atomic round trips to redis, or maybe 3) some weird watch/multi/exec combo.

Proposal

Add support for an HESETNX or HSETENX command (not sure on the name) implemented such that it follows the existing behavior of HSETNX, but also returns a failure code if the hash ID does not exist rather than creating the hash.

Potential Issues

There are effectively three states that have to be represented in the response, and I am not sure if any existing commands with a success/fail response return anything more than 0 or 1.

Implementing HESETNX would require that either:

1. 0 was returned for both failure cases, which I think is too ambiguous
2. it returns 1 for sucess, 0 for failure in the case that the hash field already existing, and 2 (or something else) in the case that the hash ID did not exist.

Thoughts?