[mfaferek93]

Problem

GET /faults (list_all_faults) fans out to peers through AggregationManager::fan_out_get, which marks the request with X-Medkit-No-Fan-Out. The handler on the receiving side never checked that header, so it fanned out again. The other collection handlers already honour it through merge_peer_items, only this path called fan_out_get directly.

With two gateways that aggregate each other (A lists B, B lists A) a single GET /faults bounces between them. Each hop holds a worker while it waits, so both gateways run out of workers. In the test below A then answers 200 with an empty item list, and every peer call logs Failed to connect to peer.

Fix

A request carrying X-Medkit-No-Fan-Out is answered from the local fault manager only. A request without it fans out as before.

Tests

New test_mutual_peer_faults (two gateways on two domains, each the other's static peer, B runs the faulty lidar):

• test_no_fan_out_request_answers_local_faults_only: A with the header returns only its own faults.
• test_mutual_peers_answer_without_bouncing: GET /faults on A returns LIDAR_RANGE_INVALID exactly once, not partial, faster than one peer timeout.

Without the fix both cases fail: the header request returns B's faults, and the plain request returns an empty list after the bounce. With the fix: 3 of 3 runs pass. test_cross_ecu_fanout, test_peer_aggregation, test_daisy_chain_aggregation, test_aggregator_fault_stream and test_fault_handlers (38) still pass. clang-format-18 and ament_flake8 clean.

Note: #700 and #680 also touch fault_handlers.cpp. This change is one small hunk in list_all_faults, so a rebase either way should be trivial.

[Copilot]

🟡 Changes recommended

OpenAPI and documentation remain inconsistent with the new behavior, and required test conventions are unmet.

2 open findings

• Use GatewayTestCase and declare required integration apps · New
• Update API and design docs for fan-out suppression · New

What changed in this PR

Prevents recursive global fault fan-out between mutually peered gateways.

Changes:

• Suppresses peer fan-out when the request carries X-Medkit-No-Fan-Out.
• Adds a two-gateway regression test.
• Registers the test with two isolated ROS domains.

File	Description
fault_handlers.cpp	Adds global fault fan-out suppression.
test_mutual_peer_faults.test.py	Tests mutual-peer loop prevention.
CMakeLists.txt	Registers the two-domain test.

🧠 Review effort: Balanced

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[bburda]

Three places outside this diff still describe the old behaviour:

• docs/api/rest.rst, the X-Medkit-No-Fan-Out entry, says the global GET /api/v1/faults neither declares nor honours the header.
• src/ros2_medkit_gateway/design/aggregation.rst, the warning in the fan-out section, says list_all_faults never reads the header and two mutual peers recurse.
• The comment in fan_out_aware() in src/ros2_medkit_gateway/src/core/openapi/route_registry.cpp says the same.

Could you update them, and add .fan_out_aware() to the GET /faults registration in rest_server.cpp so the OpenAPI document declares the header?