Redis [NEW] Define NX in combination with GET on the SET command

The problem/use-case that the feature addresses

With Redis 6.2 the SET command allows the option flags XX and GET to be used together (getting the old value, but only writing the new one if the key already exists), but not NX and GET. Peeking and creating a key - but not overwriting it if it exists - would be a handy command, e.g. for atomically setting an expiring "lock" value on a key if it doesn't exist.

Description of the feature

The command SET key value NX GET should behave as follows: - if key exists and holds value oldValue - don't store value for key - reply with oldValue - if key doesn't exist - store value for key - reply with (nil)

The client can determine if the value was set by checking the reply for (nil) (yes) or another value (no).

Alternatives you've considered

Other string commands don't seem to support this case either, so alternatives would be issuing multiple commands or scripting.

Additional information

The limitation is not explicitly mentioned in the command documentation and the "Return value" section is ambiguous about it. Another user has also tripped over it: https://github.com/redis/redis/issues/8838

Comment From: itamarhaber

Makes sense to me.

Comment From: jperezr21

Would be nice to document the return behavior here: https://redis.io/docs/latest/commands/set/#return-information