compactv2: propagate series set error in RelabelModifier

[aeron-gh]

thanos tools bucket rewrite --rewrite.to-relabel-config can write a block that is missing data and report success while doing it.

RelabelModifier.Modify drains the source ChunkSeriesSet into maps and builds a new listChunkSeriesSet from what it collected, but it never checks set.Err(). Next() returns false on failure as well as on completion, so if reading the source block fails partway through, the loop simply ends and Modify returns a normal looking result holding only the series it read before the failure. The caller then writes that out as the new block.

Three things in the same package suggest this is an oversight rather than intentional:

• the same function already checks the inner chunk iterator in two places, and returns errorOnlyStringIter when it fails
• DeletionModifier propagates the source error through delModifierSeriesSet.Err(), which forwards d.ChunkSeriesSet.Err(). RelabelModifier cannot rely on that because it returns a brand new set, so it has to check explicitly
• the dry run path in compactor.go does check set.Err(), so a dry run surfaces the failure today while the actual rewrite does not

Fixes #8933

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• pkg/compactv2/modifiers.go: return the series set error from RelabelModifier.Modify, reusing the errorOnlyStringIter path the function already uses for chunk iterator errors.

Verification

go build passes on pkg/compactv2. go vet reports only two pre-existing stdmethods warnings on lines this change does not touch.

[aeron-gh]

@fpetkovski
gentle ping on this one, when u have some time
thanks !

[aeron-gh]

@GiedriusS
ci is green, bump 🙂 No rush just flagging it whenever a reviewer can take a look. Thanks!

[GiedriusS]

Hi, there's no rush. Please be patient.

[aeron-gh]

Hi, there's no rush. Please be patient.

@GiedriusS
Understood, sorry for the noise. I'll leave it with you.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.50%. Comparing base (d567cbc) to head (1ae7847).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/compactv2/modifiers.go	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8937   +/-   ##
=======================================
  Coverage   64.50%   64.50%           
=======================================
  Files         289      289           
  Lines       37352    37354    +2     
=======================================
+ Hits        24094    24096    +2     
+ Misses      11160    11158    -2     
- Partials     2098     2100    +2     

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