ApplySchema: deprecate '--skip_preflight' flag

[shlomi-noach]

Description

This PR deprecates the flag --skip_preflight in vtctlclient ApplySchema. The new behavior is as if --skip_preflight=true. That is, to always skip preflight. In fact, the preflight code is completely removed in this PR.

The Vitess team discussed deprecating --skip_preflight a long time ago, with the advent of Online DDL. Moreover, we have noticed that in production environments we always set the flag. And furthermore, the logic used by --skip_preflight is not on par with new schema change logic: Online DDL and schemadiff, and we've encountered scenarios where the logic was flawed.

As Vitess now recommends running migrations via Online DDL, we should move away from preflight checks.

Related Issue(s)

#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[bot]]

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[mattlord]

I feel that deprecating the flag would be setting the default to true and adding a deprecation warning in the CLI and docs while removal would be removing the code done here along the the preflight tablet manager client code. The PR seems to be in the middle. Do we need to do a deprecation cycle? What's the value of keeping the flag if you can't set it?

Please keep in mind that I am glad we're getting rid of this and appreciate you working on that. 🙂

[shlomi-noach]

Do we need to do a deprecation cycle? What's the value of keeping the flag if you can't set it?

@mattlord we have to have a deprecation cycle. So in v15 the user should get a warning (you're right! I did not add a warning log -- will fix), and in v16 we can remove the flag altogether.

You're also right that the easiest way forward would be to just take care of the flag, force it to true, then keep the rest of internal logic. However, we're doing some refactoring in tablet_executor.go to reduce footprint, and this would help in simplifying these efforts as it would remove some bulk of code.

---

UPDATE v16 is already released, so this work applies to v17 & v18.

[shlomi-noach]

Added deprecation warning

[shlomi-noach]

Cluster (vtgate_schema) seems to be consistently failing. Unsure yet how this PR affects it -- but it looks like it does.

[github-actions[bot]]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

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• Remove the stale label.
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If no action is taken within 7 days, this PR will be closed.

[shlomi-noach]

This is not stale. I'm just stuck. Can't solve the CI issue.

[shlomi-noach]

Still stuck here.

[github-actions[bot]]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.

[github-actions[bot]]

This PR is being marked as stale because it has been open for 30 days with no activity. To rectify, you may do any of the following:

• Push additional commits to the associated branch.
• Remove the stale label.
• Add a comment indicating why it is not stale.

If no action is taken within 7 days, this PR will be closed.

[github-actions[bot]]

This PR was closed because it has been stale for 7 days with no activity.

[shlomi-noach]

Fashionably late, this PR is ready for review!

[shlomi-noach]

Current error in Upgrade Downgrade Testing Query Serving (Schema):

2023-02-27T15:36:44.2867788Z ERROR: Go version reported: go version go1.19.4 linux/amd64. Version 1.20.1+ required. See https://vitess.io/contributing/build-from-source for install instructions.
2023-02-27T15:36:44.2873075Z ##[error]Process completed with exit code 1.

[shlomi-noach]

Upgrade Downgrade Testing Query Serving (Schema) failure is a known issue to be fixed imminently; meanwhile merging this PR.