PMM-14594 Backup compatibility fix.

[JiriCtvrtka]

PMM-14594

FB: Percona-Lab/pmm-submodules#4368

• Fixed MySQL backup compatibility checks for MySQL 8.4 LTS by allowing Percona XtraBackup 8.4.x with MySQL 8.4.x.
• Kept existing MySQL 8.0.22+ behavior: XtraBackup 8.0.x must have the same or newer core version and stay below 8.1.
• Improved incompatible XtraBackup error messages so they explain which XtraBackup family/version should be used.
• Added unit coverage for MySQL 8.4 positive/negative compatibility cases and equivalence tests to ensure the refactor preserves existing behavior.
• Updated MySQL backup prerequisites documentation with the MySQL-to-XtraBackup compatibility matrix.

[codecov]

Codecov Report

❌ Patch coverage is 75.30864% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 43.15%. Comparing base (7fccd1e) to head (97c5373).
⚠️ Report is 1 commits behind head on v3.

Files with missing lines	Patch %	Lines
managed/services/backup/compatibility_helpers.go	75.30%	19 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #5395      +/-   ##
==========================================
- Coverage   43.22%   43.15%   -0.08%     
==========================================
  Files         413      413              
  Lines       42279    42213      -66     
==========================================
- Hits        18277    18216      -61     
  Misses      22134    22134              
+ Partials     1868     1863       -5     

Flag	Coverage Δ
managed	42.31% <75.30%> (-0.03%)	⬇️

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[JiriCtvrtka]

@copilot review

[it-percona-cla]

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✅ JiriCtvrtka
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[Copilot]

@copilot review

Reviewed the changes. The core logic looks correct:

• maxAlignedXtrabackupVersion (9.0) properly replaced with bounded version constants (mysql81Version, mysql84Version, mysql85Version)
• Compatibility matrix entries for pre-8.0.22 MySQL now correctly use 8.1.0 as the XtraBackup upper bound instead of 9.0
• MySQL 8.4.x → XtraBackup 8.4.x mapping is correctly implemented
• Error messages are now actionable with specific guidance per version range
• Tests cover all new paths including MySQL 8.4 + XtraBackup 8.4 compatibility

One documentation formatting issue was found and fixed in commit 1d0cd0e: the version compatibility table and the XtraBackup 8.4 note were placed outside the caution admonition block. They are now properly indented inside it.

[maxkondr]

if later all those strings are converted into version.Version - isn't it better to declare them as version.Version already and avoid conversion on func calls?

[JiriCtvrtka]

Yes, it makes sense to change it this way. I only fixed the logic, but this refactor simplifies it nicely.