KILL TIDB or global KILL should cancel DDL

[morgo]

Bug Report

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This is forked from #24031

Currently DDL tasks must be killed with ADMIN CANCEL DDL x. The MySQL compatible way should be to use the KILL statement.

It should be possible for KILL TIDB / KILL (with global kill) to understand it is a DDL statement and execute ADMIN CANCEL DDL.

[morgo]

I chatted with some colleagues on this - some of this might be historical (before global kill). There is also the case that some DDL jobs can not actually be canceled, so having a different statement may be useful since it can retrun an error to the caller etc.

[cosven]

/severity moderate

[dveeden]

With MySQL if you kill a query/connection that somehow can't be killed it will still be in the processlist with a state of 'Killed' until it is really gone. So TiDB could do something similar. A warning/info message about this would be good indeed.

See also: https://dev.mysql.com/doc/refman/8.0/en/general-thread-states.html