Online DDL: migration state transitions to 'cancelled' after CANCEL command

[shlomi-noach]

Description

Up till now, when a user would run ALTER VITESS_MIGRATION '...' CANCEL or ALTER VITESS_MIGRATION CANCEL ALL, an affected pending migration (whether queued, ready or running) would transition into failed state.

This is largely because all ALTER tools: pt-osc, gh-ost and even VReplication are "3rd party" executors, and not under direct control of the Online DDL scheduler. The scheduler sends these tools the "abort" signal, and they follow up by failing with error -- which then gets recorded as failed.

However, this is not what the user expects. Now, in this PR, if the migration was terminated by a CANCEL command, then the migration status transitions to cancelled instead of failed.

we add a cancelled_timestamp in _vt.schema_migrations table, which indicates if/when a migration was cancelled. The value is cleared (set to NULL) upon RETRY.

Related Issue(s)

• Tracking: Online DDL: tracking issue #6926

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• If this is a change that users need to know about, please apply the release notes (needs details) label so that merging is blocked unless the summary release notes document is included.
• If a new flag is being introduced, review whether it is really needed. The flag names should be clear and intuitive (as far as possible), and the flag's help should be descriptive.
• If a workflow is added or modified, each items in Jobs should be named in order to mark it as required. If the workflow should be required, the GitHub Admin should be notified.

Bug fixes

• There should be at least one unit or end-to-end test.
• The Pull Request description should either include a link to an issue that describes the bug OR an actual description of the bug and how to reproduce, along with a description of the fix.

Non-trivial changes

• There should be some code comments as to why things are implemented the way they are.

New/Existing features

• Should be documented, either by modifying the existing documentation or creating new documentation.
• New features should have a link to a feature request issue or an RFC that documents the use cases, corner cases and test cases.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• vtctl command output order should be stable and awk-able.