tsdb: fix grow/shrink nextIndex calculation#17863
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krajorama merged 1 commit intoprometheus:mainfrom Jan 14, 2026
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juliusmh
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Jan 14, 2026
juliusmh
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Jan 14, 2026
Signed-off-by: Julius Hinze <julius.hinze@grafana.com>
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As was pointed out by AI on a mimir PR, there is a bug when calculating the nextIndex during exemplar buffer grow/shrink operations. When the buffer is not full, nextIndex is calculated based on the non-empty entries only, which is incorrect. We have to include all entries to get the valid position in the buffer.
This PR changes copyExemplarRanges to return two values: totalCopied and migratedEntries (non-empty). It also adds two new test cases that grow and shrink non-full buffers (which fail without the fix).
The original comment: grafana/mimir#14009 (comment)
Which issue(s) does the PR fix:
n/a
Does this PR introduce a user-facing change?