fix: CoAP Ping response uses RST instead of ACK per RFC 7252 §4.2

[sandrolain]

Files changed: udp/client/conn.go (+3), udp/client/conn_test.go (+54)

Problem: sendPong() was setting message.Acknowledgement on the response
to a Confirmable empty message (CoAP Ping). RFC 7252 §4.2 requires the receiver
to reply with a matching Reset message, not an Acknowledgement. This caused
interoperability failures with compliant CoAP clients (reported in #603, #486).

Fix: One-line change — message.Acknowledgement → message.Reset in
udp/client/conn.go. A new test TestConnPingResponseIsReset verifies the
correct message type is sent.

Summary by CodeRabbit

• Bug Fixes

  • UDP client now replies to confirmable ping requests with a Reset message while preserving the original message ID.

• Tests

  • Added a test validating that ping responses are Reset messages with the expected code and message ID.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 37fb32de-242c-4af1-b01c-a085d38920d6

📥 Commits

Reviewing files that changed from the base of the PR and between d1e0008 and 35aadb0.

📒 Files selected for processing (2)

• udp/client/conn.go
• udp/client/conn_test.go

🚧 Files skipped from review as they are similar to previous changes (2)

• udp/client/conn.go
• udp/client/conn_test.go

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📝 Walkthrough

Walkthrough

Respond to empty Confirmable CoAP pings with Reset (RST) instead of Acknowledgement (ACK) in the UDP client; add a test that sends a raw Confirmable Empty and asserts the server responds with RST and matching MID.

Changes

Cohort / File(s)	Summary
Core Logic udp/client/conn.go	Changed sendPong to set outgoing type to message.Reset when replying to incoming message.Confirmable (keeps same MID). Non-confirmable handling unchanged.
Tests udp/client/conn_test.go	Added TestConnPingResponseIsReset which sends a raw Confirmable Empty over UDP and verifies the first response is RST with matching message ID; added net import.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Possibly related issues

• "CoAP ping" is acknowleged #603: Fixes the reported behavior where empty Confirmable pings were acknowledged instead of being reset.

Suggested reviewers

• jkralik

Poem

🐰 A ping so quiet, a tiny request,
I hop to reply with a Reset — not the rest.
MID kept intact, the message is terse,
Tested and proven: the protocol's terse verse. 🥕

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title clearly and specifically describes the main change: fixing CoAP Ping responses to use RST instead of ACK per RFC 7252, which is the primary fix in the changeset.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[coderabbitai]

🧹 Nitpick comments (1)

udp/client/conn_test.go (1)

874-874: Minor: Unnecessary type assertion.

net.Conn interface already includes SetReadDeadline, so the type assertion to *net.UDPConn is not required.

♻️ Suggested simplification

-	err = conn.(*net.UDPConn).SetReadDeadline(time.Now().Add(time.Second * 2))
+	err = conn.SetReadDeadline(time.Now().Add(time.Second * 2))

🤖 Prompt for AI Agents

Verify each finding against the current code and only fix it if needed.

In `@udp/client/conn_test.go` at line 874, The line uses an unnecessary
concrete-type assertion on conn before calling SetReadDeadline; replace the
assertion form conn.(*net.UDPConn).SetReadDeadline(...) with the interface call
conn.SetReadDeadline(...) (keeping the same deadline expression
time.Now().Add(time.Second * 2)) so the code uses the net.Conn interface method
directly and removes the needless type cast.

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Nitpick comments:
In `@udp/client/conn_test.go`:
- Line 874: The line uses an unnecessary concrete-type assertion on conn before
calling SetReadDeadline; replace the assertion form
conn.(*net.UDPConn).SetReadDeadline(...) with the interface call
conn.SetReadDeadline(...) (keeping the same deadline expression
time.Now().Add(time.Second * 2)) so the code uses the net.Conn interface method
directly and removes the needless type cast.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e7255feb-b296-4077-aebf-106af4eebdb7

📥 Commits

Reviewing files that changed from the base of the PR and between 2d71bd2 and d1e0008.

📒 Files selected for processing (2)

• udp/client/conn.go
• udp/client/conn_test.go