feat(commands): list/hash/set convenience variants (LPUSHX/RPUSHX/LMPOP/HRANDFIELD/SMOVE/SINTERCARD)

[TinDang97]

Summary

A grab-bag of missing convenience commands that client libraries call directly. None are load-bearing individually, but together they're a common source of "unknown command" errors during migration.

Missing commands

• LPUSHX key element [element ...] — push only if key exists as list
• RPUSHX key element [element ...] — push only if key exists as list
• LMPOP numkeys key [key ...] LEFT|RIGHT [COUNT count] — multi-key non-blocking pop
• HRANDFIELD key [count [WITHVALUES]] — random hash field sampling
• SMOVE src dst member — atomic move between sets
• SINTERCARD numkeys key [key ...] [LIMIT limit] — cardinality of intersection without materializing

Design notes

• All are thin layers over existing operations. LPUSHX/RPUSHX are one-line guards on the existing push paths.
• LMPOP needs the same multi-key iteration infrastructure that blocking ops will need — worth implementing before feat(commands): blocking list operations (BLPOP/BRPOP/BLMOVE/BLMPOP/BRPOPLPUSH) #56.
• SMOVE must be atomic across two set operations on the same shard; cross-shard SMOVE requires the existing cross-shard dispatch plumbing.
• HRANDFIELD sampling distribution must match Redis exactly for consistency tests.
• SINTERCARD with LIMIT can short-circuit iteration — the whole point is avoiding the full materialization.

Acceptance criteria

• Consistency tests for each command including the LIMIT/COUNT variants and the cross-shard SMOVE case.
• No hot-path allocations.

References

• Redis 6.2 / 7.0 changelog

[pilotspacex-byte]

Closing — implemented in commit fff223b9 (PR #66, closing #62):
LPUSHX/RPUSHX/LMPOP (src/command/list/list_write.rs), HRANDFIELD
(src/command/hash/hash_read.rs), SMOVE/SINTERCARD (src/command/set/), all
wired via dispatch arms in src/command/mod.rs and metadata entries.

Reopen if a specific subcommand or edge case is found broken.

Author: Tin Dang tindang.ht97@gmail.com