Feature Request: Add support for stopping `OnlineDDL complete`

[stankevich]

Feature Description

Discussed with @shlomi-noach and @rohit-nayak-ps.

Currently, once an OnlineDDL complete operation is submitted, it isn’t possible to stop or pause it without fully OnlineDDL cancel-ing the migration (which stops the copy and removes the data). Considering that complete might not immediately succeed on a busy table and will keep retrying indefinitely, it would be helpful to be able to “un-complete” it to have more control over the cutover timing.

Use Case(s)

Stopping an OnlineDDL complete operation that can’t succeed (for example, can’t get a lock on the table) and retrying at a later time without restarting the migration from scratch.

[shlomi-noach]

Request accepted as discussed in person. We will introduce a statement such as ALTER VITESS_MIGRATION ... POSTPONE_COMPLETION. It will essentially mostly just set postpone_completion=1 but further validations required.

[shlomi-noach]

Addressed by #18118