fix(libsy): client disconnect logging

[messiaen]

What

Emit client disconnect metric and ensure processing stops on client disconnect.

This was only reproducible on macOS

Why

The motivation — what problem does this solve, or which ticket does it close?

Closes SWITCH-1218

How tested

• uv run ruff check . clean
• uv run mypy switchyard clean
• uv run pytest tests/ green
• Manual smoke (describe what was run)

Checklist

• One class per file; filename = snake_case of the primary class.
• New public symbols exported from switchyard/__init__.py.__all__ if intended for downstream use.
• Unit tests added for new components / bug fixes.
• README / --help updated if customer-facing surface changed.
• Commits signed off (Signed-off-by: Your Name <email>) per the DCO.

Notes for reviewers

Anything reviewers should pay extra attention to — risky paths, follow-up tickets, intentional trade-offs.

Summary by CodeRabbit

• Bug Fixes

  • Improved handling of client disconnections during in-progress requests.
  • Cancelled requests are now logged with a clear disconnect status and warning.
  • Prevented completed requests from being logged more than once.

• Monitoring

  • Added client-disconnect outcomes to request metrics for improved visibility into interrupted requests.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: f43614cd-7658-48da-89af-f015dc1af31e

📥 Commits

Reviewing files that changed from the base of the PR and between 21f6951 and 635ecb2.

📒 Files selected for processing (2)

• crates/switchyard-server/src/lib.rs
• crates/switchyard-server/src/metrics.rs

Included review availability: Your plan provides up to 12 included reviews per hour; 11 remain after this review.

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Walkthrough

The server now detects buffered handler cancellation after client disconnects. It emits a terminal warning event with status 499, records a dedicated Prometheus outcome, and prevents duplicate events after normal response logging. Tests cover these paths.

Changes

Client disconnect accounting

Layer / File(s)	Summary
Disconnect metric outcome crates/switchyard-server/src/metrics.rs, crates/switchyard-server/src/lib.rs	The metrics module defines and records the client_disconnected outcome. Request logging uses status 499 for client disconnects.
Request cancellation logging crates/switchyard-server/src/lib.rs	RequestLogGuard emits a warning cancellation event when a handler future is dropped before a response. Normal response logging disarms the guard. Tests cover cancellation and duplicate-event suppression.

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

Merge Risk: ⚪ Minimal · up to 635ec

No actionable merge-blocking risk remains based on the supplied evidence; the PR is merge-ready after normal checks and review.

Poem

A rabbit guards the request stream,

Drops leave footprints in the log,
Five-four-nine marks the disconnect,
Metrics count the vanished hop,
One bright event, then no more noise.

🚥 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%. Docstring coverage is scoped to functions touched by this diff. Analyzed 20 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies client disconnect logging, which is a central part of the changes. It is concise and specific.
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.

• Fix all pre-merge checks with AI

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