fix(proxy): cancel upstream before response headers

[ndizazzo]

Fixes #1406.

What changed

• Race local and remote upstream response probes against a non-consuming downstream socket watcher.
• Treat socket errors and resets as terminal client disconnects, canceling the active local TCP or remote QUIC upstream through the existing cancellation path.
• Preserve legal HTTP write-half-close behavior by leaving clean EOF pending, and preserve pipelined request bytes with peek.
• Bound the complete response-header and overflow-preview probe to one configured deadline, including continuation reads after partial data.

Disconnect semantics

A downstream reset or socket error counts as a disconnect and cancels upstream work immediately. A clean FIN does not: HTTP clients may legally half-close their write side after sending a request, so clean EOF remains pending while the response probe runs. If the upstream also stalls, that case is bounded by the probe deadline (5 minutes for remote routes and 10 minutes for local routes).

Validation

• cargo test -p mesh-llm-host-runtime --lib: 2666 passed, 8 ignored (initial change)
• Response probe tests: 6 passed
• Routing tests: 6 passed
• Concrete API proxy regression tests: 28 passed
• cargo test -p mesh-llm-host-runtime probe_partial_headers_share_one_timeout_budget --lib
• cargo clippy -p mesh-llm-host-runtime --lib --no-deps -- -D warnings
• cargo fmt --all -- --check
• git diff --check

The broader all-targets Clippy run is currently blocked by pre-existing Rust 1.98 chunks_exact_to_as_chunks warnings in skippy-protocol and network/openai/transport_tests/lifecycle.rs; neither is changed here.

The regression suite exercises real local sockets and the remote cancellation arm, including reset, pipelined-byte, half-close, partial-header timeout, and slow-drip deadline behavior. The original rc6 report has not been reproduced on a separate two-node deployment in this worktree.

Summary by CodeRabbit

• Bug Fixes
  • Upstream response probing now respects the remaining configured timeout, preventing stalled responses from extending request wait times.
  • Disconnected client connections now cancel upstream processing promptly, improving responsiveness and reducing unnecessary work.
  • Pipelined downstream data is preserved while checking connection status.
  • Improved handling of partially received responses, stalled continuation reads, and write-half-closed connections.
  • Requests now respond more reliably when clients disconnect or upstream services stop sending data.

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• crates/mesh-llm-host-runtime/src/network/openai/response/probe.rs

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📝 Walkthrough

Walkthrough

Response probing now uses one shared timeout budget for all upstream reads. Local and remote route attempts watch downstream disconnects without consuming pipelined bytes, cancel upstream work, and preserve responses after a downstream write-half close.

Changes

Response probe and route cancellation

Layer / File(s)	Summary
Bound continuation reads crates/mesh-llm-host-runtime/src/network/openai/response/probe.rs	Continuation and preview reads now use the remaining configured timeout. Tests cover stalled reads and partial headers.
Disconnect-aware local and remote routing crates/mesh-llm-host-runtime/src/network/openai/response/routing.rs	Both route arms race probing with a non-consuming disconnect watch. Client disconnects cancel upstream work. Tests cover cancellation, pipelined bytes, and write-half-closed clients.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: 🟡 Moderate · up to 93be9

Pipelined downstream data can still stop disconnect monitoring while response headers are pending, so a later client disconnect may leave upstream work running longer than intended and delay resource cleanup. This should be fixed or explicitly accepted before merging.

Sequence Diagram(s)

sequenceDiagram
  participant DownstreamClient
  participant RouteAttempt
  participant ResponseProbe
  participant Upstream
  DownstreamClient->>RouteAttempt: send request
  RouteAttempt->>ResponseProbe: probe upstream response
  RouteAttempt->>DownstreamClient: peek for disconnect
  DownstreamClient-->>RouteAttempt: report disconnect
  RouteAttempt->>Upstream: cancel upstream
  RouteAttempt-->>RouteAttempt: return ClientDisconnected

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🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 54.17% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 24 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue #1406. They monitor downstream disconnects during local and remote probes, preserve pipelined bytes and clean write-half-close behavior, cancel upstreams, prevent retries aft…
Out of Scope Changes check	✅ Passed	The changes are within the scope of issue #1406. The timeout-budget fix addresses the continuation-read gap identified in the issue, and the routing and test changes directly support downstream discon…
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: canceling the upstream before response headers arrive when the downstream client disconnects.

Full details: Linked Issues check

Explanation

The changes satisfy issue #1406. They monitor downstream disconnects during local and remote probes, preserve pipelined bytes and clean write-half-close behavior, cancel upstreams, prevent retries after disconnection, apply timeout budgets to continuation reads, and add coverage for the required cases.

Full details: Out of Scope Changes check

Explanation

The changes are within the scope of issue #1406. The timeout-budget fix addresses the continuation-read gap identified in the issue, and the routing and test changes directly support downstream disconnect detection and upstream cancellation.

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• Commit unit tests in branch codex/issue-1406-client-disconnect

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[ndizazzo]

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Actionable comments posted: 1

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Inline comments:
In `@crates/mesh-llm-host-runtime/src/network/openai/response/routing.rs`:
- Around line 152-156: Update the downstream handling in the response probe race
so detecting DownstreamWatchResult::PipelinedBytes preserves the pipelined bytes
in connection state and keeps monitoring the downstream disconnect while
probe.await completes. Ensure a subsequent disconnect returns
ProbeOutcome::ClientDisconnected and cancels or stops the upstream response
work, while retaining the existing response outcome when the probe finishes
first.

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• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

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Run ID: 28ffddc8-4035-4543-8f02-c572efc19316

📥 Commits

Reviewing files that changed from the base of the PR and between 4c97866 and 1cd58ab.

📒 Files selected for processing (2)

• crates/mesh-llm-host-runtime/src/network/openai/response/probe.rs
• crates/mesh-llm-host-runtime/src/network/openai/response/routing.rs

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[i386]

Solid fix — reviewed as agent on James's behalf. The race shape is right: non-consuming watcher (readable + peek), biased select so a reset wins over a probe completing, pipelined bytes left for the normal path, and half-close handled as pending rather than disconnect. The test suite upgrade (real sockets, zero-linger resets, the removed Notify choreography) is a real improvement — the new tests prove liveness of the actual route arms, not just that a helper returned.

Two non-blocking notes inline: (1) clarify that clean-FIN-without-RST still waits on the probe timeout before cancellation, (2) per-read (not per-deadline) continuation timeouts can stretch the header phase unboundedly under a slow-drip upstream.

Questions before this goes green:

• Lanes currently show skipping — PR is still a draft, so presumably intentional; ready-for-review will light up Linux/Quality/Website/Windows/macOS. Confirm that's the plan.
• Has this been exercised against the rc6 scenario in the description (worker kept generating after request_dropped)? The duplex test covers the remote arm; an end-to-end two-node confirmation would close the loop on the observed behavior.

— Jian Yang (agent review)