Fix: VTOrc forgetting old instances

[GuptaManan100]

Description

This PR fixes the bug described in #12088.
The problem was introduced in #11881 which changed the forgetting logic. Earlier we would compare the hostname and port of the tablets already stored with the ones read from the tablet records and remove the ones not available. A peculiar situation arises when we have a tablet whose hostname and port information is cleared out due to restart/reschedule on a different pod. We don't want to remove the tablet record entirely as this can change the quorum that VTOrc sees. This was fixed in #11881 by changing the forgetting logic to compare the tablet aliases instead of the hostname and port.

This however created a different issue wherein we don't delete the old records corresponding to the same tablet! When a tablet gets rescheduled, we don't delete the old record since the alias still matches the ones we read from the tablet records. The new record is also inserted and we end up with 2 records for the same tablet in the vitess_tablet information, one old and one new.

This problem is fixed by this PR by adding alias as an explicit column and making it a unique key. So now when we insert into the table the new record, the old one automatically disappears.

I would actually like to go a step further wherein we don't just make alias a unique key, but also the primary key. We should also rework the InstanceKey field to be the alias instead of identifying the tablets by their hostname and port. However, this isn't in the scope of the bug-fix and I'll do these changes in a separate PR.

Related Issue(s)

• Fixes Bug Report: VTOrc doesn't forget old instances #12088

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[GuptaManan100]

Prior to these changes, the newly added test would fail saying that the vitess_tablet table had 4 tablets instead of the expected 3.