sql/schemachanger: improve performance of TestBackupSuccess

[fqazi]

Previously, the successful backup tests would for each stage of a schema
change would run the full schema change, taking a backup at the target
stage. This patch modifies the TestBackupSuccess variants of the schema
changer test to start the server once, take all the backups and then
restore the stages we should be testing.

Testing show around a ~30% improvement with this change in execution
time.

Fixes: #152076

Release note: None

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[Copilot]

Pull Request Overview

This PR refactors the backup success tests for the declarative schema changer to improve performance by ~30%. Instead of starting a server for each test stage and running the full schema change every time, it creates a single server once, takes all necessary backups during the schema change execution, and then reuses those backups across test cases.

• Introduces a shared server pattern for backup tests with a new TestServer struct
• Refactors backupSuccess into preparation and execution phases to enable backup reuse
• Updates the test framework to support optional preparation functions

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/sql/schemachanger/sctest/test_server_factory.go	Adds TestServer struct and Start method to enable server reuse across test cases
pkg/sql/schemachanger/sctest/framework.go	Updates framework to accept optional preparation function for test setup
pkg/sql/schemachanger/sctest/cumulative.go	Updates test calls to pass nil for preparation function where not needed
pkg/sql/schemachanger/sctest/backup.go	Refactors backup tests to use preparation phase for taking backups and execution phase for testing
pkg/ccl/schemachangerccl/multiregion_testcluster_factory.go	Implements Start method for multi-region test cluster factory

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[Copilot]

The error check is missing a return statement. If the backup fails, the function should return the error instead of continuing to update the backups map.

[rafiss]

nice work!

[fqazi]

@rafiss TFTR

bors r+

[craig]

Build succeeded:

• unit_tests
• docker_image_amd64
• local_roachtest_fips
• lint
• acceptance
• local_roachtest
• check_generated_code
• linux_amd64_fips_build
• linux_amd64_build
• examples_orms

[blathers-crl]

Based on the specified backports for this PR, I applied new labels to the following linked issue(s). Please adjust the labels as needed to match the branches actually affected by the issue(s), including adding any known older branches.

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Issue #152076: branch-release-25.2, branch-release-25.3, branch-release-25.4.

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[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from 09a15e4 to blathers/backport-release-25.2-152112: POST https://api.github.com/repos/fqazi/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 25.2.x failed. See errors above.

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