fix: trim whitespace-only Region in error responses before header fallback

[cipherprofessor]

Problem

httpRespToErrorResponse() only falls back to the x-amz-bucket-region header when the XML error body's <Region> is exactly "". A whitespace-only value (e.g. a single space) isn't caught by that check, so it gets used verbatim as the SigV4 signing region — corrupting the Authorization header on every subsequent request to that bucket with net/http: invalid header field value.

This is the error-response-path sibling of the whitespace-region bug already being fixed on the success-response path by #2274. While reviewing that PR, @allanrogerr pointed out the identical issue exists on this path too and asked for it to be handled in its own PR, since #2274 doesn't touch httpRespToErrorResponse(). This fix has no issue number of its own — it was found via that review comment, not filed as a separate issue.

Fix

One line: errResp.Region = strings.TrimSpace(errResp.Region) immediately before the existing empty-string check, so a whitespace-only region is treated the same as a genuinely empty one and correctly falls back to the header.

Scoped to Region only — not RequestID/HostID just above it, which are populated by the identical "if empty, fall back to header" pattern. A stray space in a diagnostic ID field has no functional consequence, while a corrupted Region breaks every subsequent signed request to the bucket. Happy to extend to those two as well if maintainers would rather have all three handled consistently.

Testing

• Added TestHttpRespToErrorResponseWhitespaceRegion, modeling a whitespace-only <Region> in the XML body alongside a real x-amz-bucket-region header, confirming the result now falls back to the header value instead of using the whitespace verbatim. Confirmed this test fails against the pre-fix code and passes after the fix (TDD red/green).
• go build ./..., go vet ./..., gofmt -l all clean.
• go test -short -race ./... (full repo) green.
• golangci-lint run --config ./.golangci.yml currently fails locally with unknown linters: 'gomodguard_v2' — verified this is pre-existing and unrelated to this change (identical failure on a clean checkout with this diff stashed out).

Summary by CodeRabbit

• Bug Fixes
  • Improved error response handling by ignoring whitespace-only region values and using the region provided in the response header instead.
  • Region comparisons now consistently use trimmed values.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 76fc4561-fead-4a36-a8c2-a1f0273af61c

📥 Commits

Reviewing files that changed from the base of the PR and between 32e1f32 and 7debd58.

📒 Files selected for processing (2)

• api-error-response.go
• api-error-response_test.go

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📝 Walkthrough

Walkthrough

The error response parser trims the XML Region value before checking for an empty value. A new test confirms that whitespace-only XML content falls back to the x-amz-bucket-region header.

Changes

Region normalization

Layer / File(s)	Summary
Normalize region and validate fallback api-error-response.go, api-error-response_test.go	httpRespToErrorResponse trims the parsed region. The new test verifies header fallback for a whitespace-only <Region> value.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Bug fix

Suggested reviewers: klauspost

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: trimming whitespace-only Region values before falling back to the header.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 2 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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_{A rabbit trims the region neat
Whitespace hops away from sight
The header points the parser right
Tests thump softly, green and bright
One small fix makes errors fleet}

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