Bug Report: MySQL Shell backup engine doesn't release global read lock when backup fails

[rvrangel]

Overview of the Issue

As we roll out the new mysqlshell backup engine, we noticed that when a backup fails (usually at the beginning) because of creds/wrong args/issues with the binary/etc, then the global read lock will not be released.

This means replication will be blocked on the tablet and also new attempts to take a new backup will fail because the lock already exists. We need to ensure that when completing the execution of ExecuteBackup() for the engine, the lock will always be released, no matter if the backup is successful or not.

Reproduction Steps

1. change --mysql-shell-flags to something invalid
2. take a backup, which will fail
3. global lock will still be held after the backup
4. a new backup attempt will fail trying to acquire a global read lock since it already exists

Binary Version

happens on v21 after #16295 was merged

Operating System and Environment details

N/A

Log Fragments

No response