`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>
* Consistent license message for all the go files
* Fixing the last set of unconsistencies with the license headers
* Addressing PR review comments
* Fixing busy.go and busy_test.go license header
* Add SetWithOptions
* Avoid passing two structs to the functions
* Rename ExpiryInSeconds -> ExpireInSeconds
* Use t.Run for the tests
* Fix build
* Address feedback
* Update log message
* Update docs and use KVSetWithOptions in KVCompareAndSetJSON
* Improve code style
* Use struct instead of pointer to struct
* Fix minimum server versions
* Update documentation
* Address feedback
* Revert new implemention of kv helpers
* Adress feedback
* Implement KVCompareAndDelete and KVCompareAndDeleteJSON
* Add tests for KVCompareAndDelete
* Update minimum server version
* Handle nil value on CompareAndSet so that it deletes it
* Fix comments
* Tweaks from PR comments
* Go back to deleted, err
* MM-14246 - Plugin framework: support transactional semantics with KV Store
Rename old, new variable names
Moving New function to the bottom
* Made CompareAndUpdate sync, updated tests
* Removed going through channel in CompareAndSetPluginKey
* Inserting new key when oldValue is nil to KVCompareAndSet
* Updated error text to include CompareAndSet
Expire K/V Values
Regenerate Code
pathfix
Update Expiry on Update
Check for Exit Signal
gofmt
Rewrote Go Routine
Remove tempoarily cleanup loop
fix expiretime
TEST: Expired Watchdog as GoRoutine
Check if Srv is nil
Use Scheduler/Worker for Expired Key CleanUp
add license
fix scheduler job type; DoJob Restructuring
Remove unused imports and constants
move db migration from 5.4 to 5.5
