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`KVCompareAndSet(key, sameValue, sameValue)` can fail spuriously on MySQL if the underlying `UPDATE` requires no actual changes. As per the [MySQL documentation](https://dev.mysql.com/doc/refman/8.0/en/mysql-affected-rows.html), we can't rely on rows affected in this case: > For UPDATE statements, the affected-rows value by default is the number of rows actually changed. If you specify the CLIENT_FOUND_ROWS flag to mysql_real_connect() when connecting to mysqld, the affected-rows value is the number of rows “found”; that is, matched by the WHERE clause. It's not tenable to change `CLIENT_FOUND_ROWS` for the all connection, so handle this case in the code instead by running a `SELECT` after the fact. Note that `KVCompareAndSet` has no guarantee of atomicity in this case, but neither would `CompareAndSwap` on which this is method was inspired. Finally, note that no changes are required for Postgres, which has sane semantics as the default. Fixes: https://mattermost.atlassian.net/browse/MM-21328 Co-authored-by: mattermod <mattermod@users.noreply.github.com>