WATCH command lacks the atomicity guarantees

[suxb201]

Discussed in #4015

^{Originally posted by suxb201 October 30, 2024}
How can we ensure the atomicity between WATCH and EXEC?

WATCH k1
MULTI
SET k2 v2
EXEC

When k1 is modified, the current conn context's watched_dirty flag is set to true. If watched_dirty is false, EXEC will begin execution. However, how can we ensure that the check of this flag and the execution of the transaction itself are atomic?

In other words, there is a potential issue where k1 could be modified during the EXEC execution, or k1 could be modified just before EXEC is executed and after the flag is checked.

You are right, the implementation of this feature is indeed lacks the atomicity guarantees. The right way to do it would be to check if something is modified during the schedule phase of MULTI transaction, when run locally on the shard thread.

#4015 (comment)