Restore: make loading compressor commands from MANIFEST opt-in

[timvaillancourt]

Description

This PR addresses a security concern with external compressors at restore-time by requiring the user to opt-in to the potentially-dangerous behaviour of trusting the backup MANIFEST file as a source of compressor configuration

To opt-in to the pre-v24 behaviour of trusting the MANIFEST, add the new flag to VTTablet:

--external-decompressor-use-manifest
allows the decompressor command stored in the backup manifest to be used at restore time. Enabling this is a security risk: an attacker with write access to the backup storage could modify the manifest to execute arbitrary commands on the tablet as the Vitess user. NOT RECOMMENDED.

From v24 an on, we won't trust the MANIFEST for compressor configuration by default 🔒

This attack vector is possible when no --external-decompressor flag is provided to the tablet, but the MANIFEST contains a configuration for an external compressor

In a follow-up feature request, a longer-term solution for securing external compressors will be proposed and implemented. This PR is a short term fix so that default-users are not at risk, and users aware of the risks can decide if they'd like this behaviour

Related Issue(s)

Resolves: #19459

Partially-resolves: GHSA-8g8j-r87h-p36x

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

AI Disclosure

Claude Code w/Opus 4.6 wrote the unit tests 🙇

[vitess-bot]

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[promptless]

📝 Documentation updates detected!

New suggestion: Document external-decompressor-use-manifest flag for backup/restore security

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[codecov]

Codecov Report

❌ Patch coverage is 77.77778% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.74%. Comparing base (70c7a72) to head (3f1473f).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/mysqlctl/compression.go	85.71%	1 Missing ⚠️
go/vt/mysqlctl/xtrabackupengine.go	0.00%	1 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (70c7a72) and HEAD (3f1473f). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (70c7a72)	HEAD (3f1473f)
	1	0

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #19460       +/-   ##
===========================================
- Coverage   69.67%   48.74%   -20.94%     
===========================================
  Files        1614       46     -1568     
  Lines      216793     6988   -209805     
===========================================
- Hits       151044     3406   -147638     
+ Misses      65749     3582    -62167     

Flag	Coverage Δ
partial	48.74% <77.77%> (?)

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[mhamza15]

LGTM!