Incremental restore: copy files under Vitess dataroot dir

[shlomi-noach]

Description

During an incremental restore process, the built-in engine copies backup files onto a temporary directory. Right now this temporary directory is rooted at the default OS temp dir (i.e. /tmp on Linux).

In k8s deployments, this directory is not shared among pods. This means that the files written by the build in engine on one pod, are not visible to mysqlctl on the mysqld pod, where mysqlbinlog binary expects to find and apply them.

This PR roots the temporary directory under the Vitess data root.

Related Issue(s)

Fixes #14765

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• 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.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 65.65%. Comparing base (4c70c7e) to head (fa7000e).
Report is 1 commits behind head on main.

Files	Patch %	Lines
go/vt/mysqlctl/builtinbackupengine.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15440      +/-   ##
==========================================
+ Coverage   65.64%   65.65%   +0.01%     
==========================================
  Files        1562     1563       +1     
  Lines      194318   194380      +62     
==========================================
+ Hits       127553   127619      +66     
+ Misses      66765    66761       -4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[mattlord]

Do we copy only the binary logs here or the full backup? If it's the full backup, this can have a significant effect on the storage needed and we might want to make this configurable (I'm assuming that the vtdataroot has another copy of the actual data that was restored prior to this step?). /tmp may also be direct attached storage or tmpfs which could cause a significant performance impact when using VTDATAROOT. It may make sense for this to be a flag with the default value being /tmp.

[mattlord]

To answer some of my own questions... 🙂

https://vitess.io/docs/user-guides/operating-vitess/backup-and-restore/creating-a-backup/#create-an-incremental-backup-with-vtctl

So we’re restoring an incremental backup here, which was only binary logs to begin with. And incremental is always done by the builtinbackupengine, regardless of which engine you’re using for full backups.

[shlomi-noach]

Converted to Draft because I'm not sure this is necessarily the solution we're looking for. Alternatively, as @mattlord suggests, we could have a configurable path via command line flag.

[shlomi-noach]

We chose #15451 as an alternative approach.