fix(api): prevent panic on 204 No Content with Go 1.26

[sylr]

Summary

• Fix responseWriter.finalize() in OTLP middleware to skip Write() when buffered data is empty, preventing panic with Go 1.26 which strictly enforces that 204 No Content responses must not have a body
• Fix NoContent() to call WriteHeader(204) directly instead of routing through writeJSON() which unnecessarily sets Content-Type: application/json

Note: This fix is based on the v4.1.1 tag. The same files were moved to pkg/transport/ in v5 and need the same fix there.

Context

Go 1.26 now panics with http: request method or response status code does not allow body when any write (even empty) is attempted on a 204 response. This breaks formancehq/membership-api unit tests (PR #700) which upgrade to Go 1.26.

Test plan

• Verified the panic reproduces in membership-api CI with Go 1.26
• Run go-libs unit tests
• Verify membership-api tests pass after bumping to the fixed version

🤖 Generated with Claude Code

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 2b6715cf-f015-45ce-bc17-5c12997b3f2c

📥 Commits

Reviewing files that changed from the base of the PR and between 2844b0a and 5df80b3.

📒 Files selected for processing (2)

• pkg/transport/api/utils.go
• pkg/transport/httpserver/otlp_middleware.go

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Walkthrough

Two HTTP transport utilities were refined: NoContent responses now write only status headers without JSON serialization, and the OTLP response writer's finalization method early-exits when data buffer is empty, preventing unnecessary zero-length writes.

Changes

Cohort / File(s)	Summary
Response Handling Simplification pkg/transport/api/utils.go	Refactored NoContent() to use w.WriteHeader() directly instead of delegating to writeJSON(), eliminating JSON serialization overhead for 204 responses.
Response Writer Optimization pkg/transport/httpserver/otlp_middleware.go	Added early-exit condition in finalize() to skip writing when buffered data is empty, avoiding unnecessary zero-length writes to underlying response writer.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Poem

🐰 A NoContent hop, so swift and clean,
No JSON bloat in between,
Empty buffers skip their dance,
Headers fly with graceful glance,
HTTP responses, lean and keen! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the main fix: preventing panic on 204 No Content responses with Go 1.26, which is the primary change across both modified files.
Description check	✅ Passed	The description is directly related to the changeset, explaining the Go 1.26 panic issue, the two specific fixes made, and providing context about why these changes were necessary.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/otlp-middleware-go126-no-content-v4

📝 Coding Plan

• Generate coding plan for human review comments

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[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 25.82%. Comparing base (2844b0a) to head (5df80b3).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/transport/httpserver/otlp_middleware.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #575      +/-   ##
==========================================
+ Coverage   25.78%   25.82%   +0.03%     
==========================================
  Files         180      180              
  Lines        7065     7067       +2     
==========================================
+ Hits         1822     1825       +3     
+ Misses       5134     5132       -2     
- Partials      109      110       +1     

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