Redis/Locks lab

Locks Lab: Wrong-Owner DEL

This lab shows a subtle Redis lock bug: a worker whose lock expired can wake up and delete a lock now owned by another worker.

Break modeAcquire with SET NX PX, release with plain DEL.

Fix modeRelease with Lua compare-and-delete.

InvariantNo worker should delete a successor's lock.

TTL expires

plain DEL

EVAL

mismatch

Redis Commands

`SET key value NX PX ttl`

Acquires the lock only when missing. NX means do not overwrite an existing owner. PX sets TTL in milliseconds.

`GET key`

Reads the current lock owner token. The token is the proof of ownership.

`DEL key`

Deletes the lock. Plain DEL is unsafe because it does not verify ownership.

`EVAL script keys args`

Runs compare-and-delete atomically so no other command interleaves between GET and DEL.

Lua Release Script

if redis.call('GET', KEYS[1]) == ARGV[1] then
  return redis.call('DEL', KEYS[1])
else
  return 0
end

KEYS[1]

The lock key, lock:inventory.

ARGV[1]

The caller's ownership token, such as w3-i7.

redis.call('GET', KEYS[1])

Reads the current owner inside Redis.

== ARGV[1]

Checks whether the caller still owns the lock.

DEL

Deletes only on token match. Token mismatch returns 0 and preserves the successor's lock.

Run Targets

The runnable lab is labs/redis/locks, backed by labs/redis/cmd/locks.

`make break`

Naive release. Look for WRONG-OWNER DEL lines.

`make test`

Lua release. Look for expired holders skipping DEL.

`make load`

More workers and iterations to stress the release invariant.

Important Limit

What this fix does not solve The lab makes work duration longer than the lock TTL. That means concurrent critical-section occupancy can still happen. Lua compare-and-delete only prevents deleting another worker's lock. Correct writes often also need lock extension, idempotency, or fencing tokens.

Code Pointers

Code	Why it matters
`cmd/locks/main.go`	The expired-owner scenario, unsafe `DEL`, and Lua compare-and-delete release.
`locks/Makefile`	Targets for unsafe release, safe release, and stress runs.
`locks/compose.yaml`	Redis container used by the lab.
`locks/README.md`	Expected output and the limit of compare-and-delete.

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