Add option to forcefully put a 5.6 slave below a 5.7 machine

[dveeden]

This should not be allowed by default but should be possible with a -force flag.

The reason is:

1. a 5.6 chain with a 5.7 branch.
2. you want to promote the 5.7 branch to be the master
3. This would result in 5.7 (new master) replicating to a 5.6 machine (old master)

But reversing replication might not work. So it's best to put a 5.6 machine in the 5.7 branch before switching the master to ensure it is replicating.

Replication from 5.7 to 5.6 only works if 5.7 is configured to create a 5.6 compatible binlog stream and is not really supported. However doing this for a short amount of time can be used to provide a rollback option in case things don't work out with the new version.

And of course 5.7→5.6 might also be 8.0→5.7 or 10.2→10.1..

[sjmudd]

This is actually 2 requests I think

• add a --force command line option applicable to a case like this
• in the GUI / http interface detect this issue and add a dialog explaining it is dangerous and asking you if you want to proceed.

In both cases while it might work it may well not and the user should be well aware that he's moving into potentially uncharted territory which may lead to breakage. That said I agree that having the option is useful as right now there's no way to proceed even when you know the operation you want to do will work.