⚡ perf: Optimize proxy balancer round-robin

[james-yusuke]

Description

Removes the lock from BalancerForward server selection to reduce contention on the round-robin hot path.

The selector now uses an atomic bounded counter while preserving the existing cyclic selection behavior, including counter rollover. Public APIs, proxy validation, and SSRF protections are unchanged.

Changes introduced

• Benchmarks: Added parallel benchmarks comparing the atomic selector with the previous mutex-based approach. Both paths perform zero allocations.
• API Longevity: No public API or configuration changes.
• Added rollover coverage to preserve round-robin ordering when the counter reaches the uint64 boundary.
• Added concurrent selection coverage to validate race-safe distribution.

Type of change

• Performance improvement (non-breaking change which improves efficiency)

Checklist

• Conducted a self-review of the code and provided comments for complex or critical parts.
• Added or updated unit tests to validate the effectiveness of the changes or new features.
• Ensured that new and existing unit tests pass locally with the changes.
• Aimed for optimal performance with minimal allocations in the new code.
• Provided benchmarks for the new code to analyze and improve upon.

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Walkthrough

The proxy round-robin selector now uses an atomic CAS-based counter instead of a mutex. Tests cover counter wrap-around and concurrent distribution, while benchmarks compare atomic and mutex-protected implementations.

Changes

Proxy round-robin selection

Layer / File(s)	Summary
Atomic round-robin implementation middleware/proxy/proxy.go	urlRoundrobin uses an atomic.Uint64 counter and CAS-based advancement with pool-size wrap-around.
Selection validation and benchmarking middleware/proxy/security_test.go, middleware/proxy/bench_test.go	Tests cover modulo and uint64 wrap-around plus concurrent selection distribution; benchmarks compare atomic and mutex-based selectors.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• gofiber/fiber#4405: Reworks proxy round-robin usage and introduces the related selector implementation.

Suggested labels: ⚡️ Performance

Suggested reviewers: sixcolors, renewerner87, efectn

Poem

A rabbit hops through upstream lanes,
While atoms spin instead of chains.
Round and round the URLs flow,
Past the max and back they go.
“Hop!” says Bun, “the tests all glow!”

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 40.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately describes the proxy round-robin performance change.
Description check	✅ Passed	The description covers the purpose, changes, benchmarks, type of change, and checklist; only optional template items like issue links are omitted.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.29%. Comparing base (48b5ea3) to head (58c6313).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4549   +/-   ##
=======================================
  Coverage   93.29%   93.29%           
=======================================
  Files         140      140           
  Lines       14854    14856    +2     
=======================================
+ Hits        13858    13860    +2     
  Misses        620      620           
  Partials      376      376           

Flag	Coverage Δ
unittests	93.29% <100.00%> (+<0.01%)	⬆️

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[gaby]

Why was this removed?

[james-yusuke]

You're right—there was no reason to remove this coverage. I've restored it and expanded it to cover both the normal round-robin sequence and the counter-limit case.

[gaby]

@james-yusuke Why you closed it?

[james-yusuke]

Sorry, I closed it by mistake while handling the benchmark failure. The checks are now green, so I’ve reopened it. Could you please take another look?