Make stateful backend forwarding cancellation-safe

[mjc]

Summary

• complete a backend read after select chooses it before writing to the client
• preserve pooled-buffer forwarding and response ordering
• enforce the focused Clippy policy used by the companion Lific project

Branch-added tests

• A gated async client writer accepts a partial response, blocks, then receives another ready client command; the test verifies both backend replies are emitted in exact order without loss.
• Existing stateful EOF, oversized-command, authentication, and byte-accounting coverage remains green.

Branch-added checks

• Nix format checking and workspace Clippy run across all targets and features with warnings denied under the focused policy.

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📥 Commits

Reviewing files that changed from the base of the PR and between 6797ac4 and 7afa6cc.

📒 Files selected for processing (14)

• Cargo.toml
• benches/cache_metadata_payload.rs
• docs/reference/rfc3977-response-codes.md
• src/cache/availability_identity.rs
• src/cache/hybrid.rs
• src/cache/hybrid_codec.rs
• src/command/handler.rs
• src/pool/provider.rs
• src/protocol/request.rs
• src/session/backend.rs
• src/session/handlers/hybrid.rs
• src/session/handlers/stateful.rs
• src/session/multiline_framing.rs
• src/session/response_transfer.rs

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Reviewing files that changed from the base of the PR and between 351b828 and 6797ac4.

📒 Files selected for processing (8)

• Cargo.toml
• src/cache/availability_identity.rs
• src/cache/mock_hybrid.rs
• src/pool/deadpool_connection.rs
• src/proxy/builder.rs
• src/tui/app.rs
• src/tui/helpers.rs
• tests/test_helpers.rs

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📝 Walkthrough

Walkthrough

The stateful proxy now separates backend reads from client writes. Both forwarding paths complete writes after selection. A gated AsyncWrite test verifies that ready client input does not cancel backend output. The change also enables Clippy lints and applies related cleanup.

Changes

Stateful forwarding

Layer / File(s)	Summary
Separate backend reads and client writes src/session/handlers/stateful.rs	The combined forwarding method is split into read and write methods. The drain and tokio::select! paths call them in sequence.
Gated writer cancellation test src/session/handlers/stateful.rs	A gated writer verifies that backend responses are fully written while client input is ready.

Clippy cleanup

Layer / File(s)	Summary
Lint configuration and code cleanup Cargo.toml, src/cache/availability_identity.rs, src/pool/deadpool_connection.rs, src/proxy/builder.rs, src/tui/helpers.rs	The project denies 18 Clippy lints. Production code removes redundant clones, simplifies mappings, and uses integer sparkline scaling.
Test signature and ownership cleanup src/cache/mock_hybrid.rs, src/tui/app.rs, tests/test_helpers.rs	Tests remove unnecessary Result returns, simplify dispatch, update a rate assertion, and move the proxy into the spawned task.

Estimated code review effort: 4 (Complex) | ~45 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Backend
  participant StatefulProxyLoop
  participant GateWriter
  participant ClientInput
  Backend->>StatefulProxyLoop: Send two responses
  StatefulProxyLoop->>GateWriter: Write backend output
  GateWriter-->>StatefulProxyLoop: Block after partial write
  ClientInput->>StatefulProxyLoop: Send ready DATE command
  GateWriter-->>StatefulProxyLoop: Complete after release
  StatefulProxyLoop->>GateWriter: Write remaining output and flush

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Merge Risk: 🔵 Low · up to 6797a

Stateful backend replies are now completed after selection to prevent cancellation during client writes. The forwarding regression is covered, but the helper interface still permits an inconsistent buffer length and should be addressed before stricter lint enforcement.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 56.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 8 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: making stateful backend forwarding cancellation-safe.

Full details: Docstring Coverage

Explanation

Docstring coverage is 56.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 30 functions across 8 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch nntpp-27-cancel-safe-stateful-forwarding

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[Copilot]

🟢 Approval recommended

The refactor cleanly isolates backend reads from client writes in the select! loop and includes a regression test that directly covers the prior cancellation hazard while preserving pooled-buffer forwarding behavior.

Pull request overview

This PR makes stateful-mode backend→client forwarding cancellation-safe by separating the backend read from the client write so that once a backend read wins tokio::select!, its corresponding client output is completed without being dropped mid-write, preserving ordered forwarding while still using pooled buffers.

Changes:

• Split backend forwarding into read_stateful_backend_bytes (read-only, select!-safe) and write_stateful_backend_bytes (completes output after selection).
• Update the drain-backend-replies path to use the new read/write split consistently.
• Add a regression test that uses a gated AsyncWrite to force a partial write, then introduces ready client input to ensure backend output is not cancelled and ordering is preserved.

File summaries

File	Description
src/session/handlers/stateful.rs	Refactors stateful backend forwarding to avoid select! cancellation mid-write; adds a targeted async test that exercises the cancellation-safety guarantee.

Review details

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

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Actionable comments posted: 1

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Inline comments:
In `@src/session/handlers/stateful.rs`:
- Around line 211-221: Update the stateful backend response path around
write_stateful_backend_bytes to accept the response bytes as a slice rather than
separate buffer and len parameters, remove the unused self receiver, and pass
the correctly bounded slice at every call site. Preserve the existing
client_writes_for_backend_read processing and write behavior.

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Configuration used: Repository UI (base), Organization UI (inherited)

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• src/session/handlers/stateful.rs

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