Bind the Locust master to IPv4 loopback

[Amaury Chamayou (achamayou)]

Summary

Follow-up to #8301, fixing the Benchmark Virtual failure on its merge commit, 28d40d248.

localhost can resolve to both IPv4 and IPv6. Locust 2.46.5 then enables IPv6 for its ZeroMQ listener, while the port probe and forked workers use IPv4. If ::1 is unavailable, the master fails with:

Socket bind failure: Cannot assign requested address (addr='tcp://localhost:43749')

This affected all five Locust benchmarks; their workers subsequently spent about five minutes retrying connections.

Use 127.0.0.1 explicitly for the port probe and master binding. This preserves #8301's loopback-only binding and port-collision protection without depending on hostname resolution or IPv6 availability.

Validation

• Reproduced the exact failure with Locust 2.46.5 in an isolated Linux network/mount namespace where localhost resolves to both address families but IPv6 loopback is unavailable.
• Confirmed that explicit IPv4 binding succeeds and a worker/master RPC round trip completes in the same environment.
• scripts/ci-checks.sh -f passed under WSL.

[Copilot]

🟢 Approval recommended

The change is minimal, consistent with existing IPv4-only port probing (AF_INET), and directly addresses the documented Locust bind failure scenario without introducing new behavioral risk.

Pull request overview

This PR updates the Locust benchmark harness to bind the Locust master RPC listener explicitly to the IPv4 loopback address, avoiding failures in environments where localhost resolves to IPv6 but ::1 is unavailable.

Changes:

• Switch Locust master binding host from localhost to 127.0.0.1.
• Add an inline comment clarifying the IPv4/IPv6 rationale and matching of probe/worker behavior.

Custom instructions used: None (no repository instruction files were separately loaded from .github/copilot-instructions.md or .github/instructions/ during this review).

File summaries

File	Description
tests/infra/locust_benchmark.py	Bind Locust master explicitly to 127.0.0.1 to avoid localhost IPv6 resolution issues and keep port probing/binding consistent.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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