fix: preserve SSH forwarding half-close semantics

[skevetter]

Summary

• preserve bidirectional forwarding after normal EOF by half-closing only the destination write side
• fully close both endpoints on context cancellation or copy/half-close errors
• add TCP regression coverage for request/response half-close behavior

Verification

• go vet ./...
• go test ./...
• go test -race ./...
• git diff --check

CodeRabbit local review completed; its read-deadline finding was addressed in the follow-up test commit.

Summary by CodeRabbit

• Bug Fixes
  • Improved bidirectional TCP connection handling during half-close and EOF scenarios.
  • Connections now remain available for the opposite direction until both data transfers complete.
  • Improved cleanup when transfers are canceled or encounter errors.
• Tests
  • Added coverage for bidirectional transfers, half-close propagation, EOF handling, and completion behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: ae249bed-93cf-41f7-bc6f-7e609cf0d3b9

📥 Commits

Reviewing files that changed from the base of the PR and between 4a5658e and a680563.

📒 Files selected for processing (2)

• tcpip.go
• tcpip_test.go

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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📝 Walkthrough

Walkthrough

bicopy now coordinates two directional copies that preserve TCP half-close behavior. The tests create loopback TCP pairs and verify bidirectional data transfer, EOF propagation, and completion.

Changes

TCP half-close forwarding

Layer / File(s)	Summary
Bicopy half-close coordination tcpip.go	bicopy runs two copyAndHalfClose operations, handles cancellation and copy errors, and closes both connections after both directions finish. CloseWrite errors from io.EOF are treated as success.
Half-close integration validation tcpip_test.go	The tests create loopback TCP connection pairs and verify bidirectional transfer, half-close propagation, EOF handling, and bicopy completion.

Priority: ⬇️ Low

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

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant bicopy
  participant copyAndHalfClose
  participant TCPConnections
  bicopy->>copyAndHalfClose: Start two directional copies
  copyAndHalfClose->>TCPConnections: Copy data
  copyAndHalfClose->>TCPConnections: CloseWrite after EOF
  copyAndHalfClose-->>bicopy: Return copyResult
  bicopy->>TCPConnections: Close both connections after completion or error

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Merge Risk: ⚪ Minimal · up to a6805

The forwarding change has regression coverage for request/response half-closes and no actionable current-head risk remains.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 16.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 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 and concisely describes the main change: preserving SSH forwarding half-close semantics.
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

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch ragged-ladybug

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