[network-diag] introduce AnswerBuilder to manage answer messages

[abtink]

This commit introduces AnswerBuilder class to track and manage Network Diagnostic answer messages. This class is used when the response to a query requires multiple CoAP answer messages. It automatically manages the inclusion of the Query ID and the Answer TLVs(providing message indexing and "more-to-follow" flags) in each allocated answer message, while maintaining all answer messages in a queue. The NetworkDiagnostic::Server is updated to use the AnswerBuilder, simplifying the logic for preparing and sending answers.

The AnswerBuilder class is added in a new header file network_diagnostic_types.hpp to allow for its reuse by other modules in the future.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new AnswerBuilder utility class to handle the segmentation and management of Network Diagnostic answer messages. By centralizing the logic for tracking Query IDs, message indexing, and 'more-to-follow' flags, the change significantly simplifies the NetworkDiagnostic::Server implementation and provides a reusable component for future modules.

Highlights

• Introduction of AnswerBuilder: Added the AnswerBuilder class to encapsulate the logic for managing and segmenting Network Diagnostic answer messages, improving code maintainability.
• Message Queue Enhancement: Added MessageQueue::EnqueueAllFrom to efficiently transfer messages between queues, facilitating the management of segmented diagnostic responses.
• Refactoring NetworkDiagnostic::Server: Updated NetworkDiagnostic::Server to utilize AnswerBuilder, which simplifies the preparation and transmission of diagnostic answers.

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[github-actions]

Merging #12887 into main

name	branch	text	data	bss	total
ot-cli-ftd	8a03257	483080	860	66540	550480
	e307cfa	483144	860	66540	550544
	+/-	+64	+0	+0	+64
ot-ncp-ftd	8a03257	448916	764	61776	511456
	e307cfa	448996	764	61776	511536
	+/-	+80	+0	+0	+80
ot-cli-mtd	8a03257	375184	764	50972	426920
	e307cfa	375184	764	50972	426920
	+/-	+0	+0	+0	+0
ot-ncp-mtd	8a03257	356260	764	46240	403264
	e307cfa	356260	764	46240	403264
	+/-	+0	+0	+0	+0
ot-cli-ftd-br	8a03257	599920	864	136732	737516
	e307cfa	599984	864	136732	737580
	+/-	+64	+0	+0	+64
ot-rcp	8a03257	63344	568	20804	84716
	e307cfa	63344	568	20804	84716
	+/-	+0	+0	+0	+0

Library files

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libopenthread-ftd.a	8a03257	251045	95	40319	291459
	e307cfa	251111	95	40319	291525
	+/-	+66	+0	+0	+66
libopenthread-cli-ftd.a	8a03257	61765	0	8091	69856
	e307cfa	61765	0	8091	69856
	+/-	+0	+0	+0	+0
libopenthread-ncp-ftd.a	8a03257	33583	0	5948	39531
	e307cfa	33583	0	5948	39531
	+/-	+0	+0	+0	+0
libopenthread-mtd.a	8a03257	169389	0	24783	194172
	e307cfa	169649	0	24783	194432
	+/-	+260	+0	+0	+260
libopenthread-cli-mtd.a	8a03257	41462	0	8059	49521
	e307cfa	41462	0	8059	49521
	+/-	+0	+0	+0	+0
libopenthread-ncp-mtd.a	8a03257	25807	0	5948	31755
	e307cfa	25807	0	5948	31755
	+/-	+0	+0	+0	+0
libopenthread-ftd-br.a	8a03257	368891	99	110471	479461
	e307cfa	368957	99	110471	479527
	+/-	+66	+0	+0	+66
libopenthread-cli-ftd-br.a	8a03257	80972	0	8131	89103
	e307cfa	80972	0	8131	89103
	+/-	+0	+0	+0	+0
libopenthread-rcp.a	8a03257	9946	0	5060	15006
	e307cfa	9946	0	5060	15006
	+/-	+0	+0	+0	+0
libopenthread-radio.a	8a03257	19859	0	238	20097
	e307cfa	19859	0	238	20097
	+/-	+0	+0	+0	+0

[gemini-code-assist]

Code Review

This pull request refactors the Network Diagnostic answer generation logic by introducing a new AnswerBuilder class to manage message allocation, segmentation, and TLV insertion. It also adds a MessageQueue::EnqueueAllFrom utility to the core message library to facilitate moving messages between queues. The review feedback suggests initializing the mQueryId member in the AnswerBuilder constructor to ensure defined behavior if the Query ID TLV is not found in the request.

[codecov]

Codecov Report

❌ Patch coverage is 97.16981% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.79%. Comparing base (8a03257) to head (e49a966).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
src/core/thread/network_diagnostic_types.cpp	90.90%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12887      +/-   ##
==========================================
- Coverage   74.11%   73.79%   -0.33%     
==========================================
  Files         695      698       +3     
  Lines       95981    96448     +467     
==========================================
+ Hits        71139    71169      +30     
- Misses      24842    25279     +437     

Files with missing lines	Coverage Δ
src/core/common/message.cpp	94.60% <100.00%> (+0.13%)	⬆️
src/core/common/message.hpp	93.42% <ø> (+10.18%)	⬆️
src/core/thread/network_diagnostic.cpp	67.69% <100.00%> (-10.94%)	⬇️
src/core/thread/network_diagnostic.hpp	66.66% <ø> (-15.16%)	⬇️
src/core/thread/network_diagnostic_types.hpp	100.00% <100.00%> (ø)
tests/unit/test_message_queue.cpp	100.00% <100.00%> (ø)
src/core/thread/network_diagnostic_types.cpp	90.90% <90.90%> (ø)

... and 168 files with indirect coverage changes

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[jwhui]

/gemini review

[gemini-code-assist]

Code Review

This pull request refactors the Network Diagnostic answer generation logic by introducing a new AnswerBuilder class, which encapsulates the management of CoAP answer messages and TLVs. Additionally, it adds a MessageQueue::EnqueueAllFrom method to the core message library to allow moving all messages between queues, supported by new unit tests. The review feedback suggests adding a defensive check in the EnqueueAllFrom method to prevent potential circular references if a queue is enqueued into itself.

[AlixBa]

@jwhui could this break ot-br-posix rest API?

{
	"data":	{
		"id":	"90087999-7913-4a2c-b13f-6e0e3f3ce674",
		"type":	"getNetworkDiagnosticTask",
		"attributes":	{
			"destination":	"92844e00385b03db",
			"destinationType":	"extended",
			"types":	["extAddress", "rloc16", "ipv6Addresses", "childTable", "routerNeighbors"],
			"status":	"completed",
			"created":	"2026-04-20T15:03:13+00:00"
		},
		"relationships":	{
			"result":	{
				"data":	{
					"type":	"diagnostics",
					"id":	""
				}
			}
		}
	}

here is what I get after pulling the new openthread:border-router:latest image.
id: "" on a successful response(?)

[abtink]

@jwhui could this break ot-br-posix rest API?

{
 "data": {
 "id": "90087999-7913-4a2c-b13f-6e0e3f3ce674",
 "type": "getNetworkDiagnosticTask",
 "attributes": {
 "destination": "92844e00385b03db",

@AlixBa, it is very unlikely.

• There are existing tests in OT that already validate all diagnostic behavior (and they run on every PR).
• This PR is related to NetDiagAnswer, which is the answer to the NetDiagQuery TMF command (which is different from the NetDiagGet command).
• The REST API typically uses otThreadSendDiagnosticGet (which then uses NetDiagGet and not NetDiagQuery).

[AlixBa]

@abtink Thanks, I'll try to pinpoint then. I don't see any particular commit on ot-br-posix so I guess it's somewhere around here. I might not look at the right place though, haha.