Support specifying expected primary in ERS

[timvaillancourt]

Description

This PR allows a EmergencyReparentShard request to contain the alias of the primary we expect to be the current primary for the action to succeed. If this alias is incorrect or stale, the reparent fails with an error indicating there is a failed precondition/mismatch.

This is useful to prevent races between external automation that runs EmergencyReparentShard and Vitess (vtorc and vtctld), which may reparent a shard in the time external automation is creating/processing it's own reparent request. With this support added, automation with a stale view of the world will encounter an error instead of triggering another reparent

In the future, Vitess could use this field internally to be more explicit but because everything uses shard locks, it should never encounter a mismatch

Related Issue(s)

Resolves https://github.com/vitessio/vitess/issues/16430

Checklist

• [x] "Backport to:" labels have been added if this change should be back-ported to release branches
• [x] If this change is to be back-ported to previous releases, a justification is included in the PR description
• [x] Tests were added or are not required
• [x] Did the new or modified tests pass consistently locally and on CI?
• [x] Documentation was added or is not required

Deployment Notes

[vitess-bot[bot]]

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 53.84615% with 6 lines in your changes missing coverage. Please review.

Project coverage is 69.43%. Comparing base (2e2b223) to head (c0abfdc). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
go/cmd/vtctldclient/command/reparents.go	14.28%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #16852 +/- ## ======================================= Coverage 69.43% 69.43% ======================================= Files 1571 1571 Lines 203021 203080 +59 ======================================= + Hits 140970 141013 +43 - Misses 62051 62067 +16 ```

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[mattlord]

You'll want to add support for this in the client command, no? Here: go/cmd/vtctldclient/command/reparents.go

[timvaillancourt]

You'll want to add support for this in the client command, no? Here: go/cmd/vtctldclient/command/reparents.go

@mattlord oh thanks, yes I'll add support for the CLI binary 👍👀

[timvaillancourt]

Added:

tvaillancourt@tvailla-ltmxctu vitess % ./bin/vtctldclient EmergencyReparentShard --help 2>&1 | grep expected-primary
      --expected-primary string          Alias of a tablet that must be the current primary in order for the reparent to be processed.

👍