[nexus] add test for OT_MLE_LONG_ROUTES feature (#12956)

This commit adds a new Nexus test to verify the functionality of the
MLE long routes experimental feature, which allows path costs to
exceed the standard limit of 15.

The new test `TestLongRoutes` in `test_long_routes.cpp` forms a
topology consisting of a leader and a chain of 25 routers. It then
validates that the path cost from the last router in the chain to the
leader is correctly reported as 25 using `GetPathCostToLeader()`.

Supporting changes include:
- Updating `build.sh` to support a `long_routes` build target that
  enables `OT_MLE_LONG_ROUTES`.
- Adding the `long_routes` test to `CMakeLists.txt` with the
  appropriate labels.
- Introducing a new GitHub workflow job `nexus-long-routes-tests` in
  `nexus.yml` to automate the execution of this test.

.github/workflows/nexus.yml

@@ -107,6 +107,35 @@ jobs:
       run: |
         cd build/nexus && ctest -L trel --output-on-failure
 
+  nexus-long-routes-tests:
+    name: nexus-long-routes-tests
+    runs-on: ubuntu-24.04
+    steps:
+    - name: Harden Runner
+      uses: step-security/harden-runner@5ef0c079ce82195b2a36a210272d6b661572d83e # v2.14.2
+      with:
+        egress-policy: audit # TODO: change to 'egress-policy: block' after couple of runs
+
+    - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+      with:
+        submodules: recursive
+
+    - name: Bootstrap
+      env:
+        PR_BODY: "${{ github.event.pull_request.body }}"
+      run: |
+        sudo apt-get update
+        sudo apt-get --no-install-recommends install -y ninja-build lcov
+
+    - name: Build Nexus
+      run: |
+        mkdir -p build/nexus
+        top_builddir=build/nexus ./tests/nexus/build.sh long_routes
+
+    - name: Run LONG_ROUTES Tests
+      run: |
+        cd build/nexus && ctest -L long_routes --output-on-failure
+
   nexus-grpc-tests:
     name: nexus-grpc-tests
     runs-on: ubuntu-24.04

tests/nexus/CMakeLists.txt

@@ -423,6 +423,9 @@ ot_nexus_test(zero_len_external_route "core;nexus")
 # Trel
 ot_nexus_test(trel "trel;nexus")
 
+# Long-routes
+ot_nexus_test(long_routes "long_routes;nexus")
+
 # Grpc
 if(OT_NEXUS_GRPC)
     ot_nexus_test(grpc "core;nexus")

tests/nexus/build.sh

@@ -44,6 +44,8 @@ else
     top_builddir=.
 fi
 
+long_routes=OFF
+
 case $1 in
     trel)
         fifteenfour=OFF
@@ -53,6 +55,11 @@ case $1 in
         fifteenfour=ON
         wasm=ON
         ;;
+    long_routes)
+        fifteenfour=ON
+        wasm=OFF
+        long_routes=ON
+        ;;
     *)
         fifteenfour=ON
         wasm=OFF
@@ -73,6 +80,7 @@ CMAKE_ARGS=(
     -DOT_APP_NCP=OFF
     -DOT_APP_RCP=OFF
     -DOT_15_4="${fifteenfour}"
+    -DOT_MLE_LONG_ROUTES="${long_routes}"
     -DOT_PROJECT_CONFIG="${top_srcdir}/tests/nexus/openthread-core-nexus-config.h"
 )
 

tests/nexus/test_long_routes.cpp

@@ -0,0 +1,95 @@
+/*
+ *  Copyright (c) 2026, The OpenThread Authors.
+ *  All rights reserved.
+ *
+ *  Redistribution and use in source and binary forms, with or without
+ *  modification, are permitted provided that the following conditions are met:
+ *  1. Redistributions of source code must retain the above copyright
+ *     notice, this list of conditions and the following disclaimer.
+ *  2. Redistributions in binary form must reproduce the above copyright
+ *     notice, this list of conditions and the following disclaimer in the
+ *     documentation and/or other materials provided with the distribution.
+ *  3. Neither the name of the copyright holder nor the
+ *     names of its contributors may be used to endorse or promote products
+ *     derived from this software without specific prior written permission.
+ *
+ *  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *  CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *  SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *  INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *  POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "platform/nexus_core.hpp"
+#include "platform/nexus_node.hpp"
+
+namespace ot {
+namespace Nexus {
+
+#if OPENTHREAD_CONFIG_MLE_LONG_ROUTES_ENABLE
+
+void TestLongRoutes(void)
+{
+    static constexpr uint16_t kNumRouters = 25;
+
+    Core  nexus;
+    Node &leader = nexus.CreateNode();
+    Node *routers[kNumRouters];
+
+    Log("---------------------------------------------------------------------------------------");
+    Log("TestLongRoutes");
+
+    // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    Log("Form topology - long chain of routers");
+
+    leader.Form();
+    nexus.AdvanceTime(100 * 1000);
+    VerifyOrQuit(leader.Get<Mle::Mle>().IsLeader());
+
+    routers[0] = &nexus.CreateNode();
+    AllowLinkBetween(leader, *routers[0]);
+    routers[0]->Join(leader, Node::kAsFtd);
+
+    for (uint16_t i = 1; i < kNumRouters; i++)
+    {
+        Log("Router %u", i);
+
+        routers[i] = &nexus.CreateNode();
+        AllowLinkBetween(*routers[i], *routers[i - 1]);
+        routers[i]->Join(*routers[i - 1], Node::kAsFtd);
+        nexus.AdvanceTime(20 * 1000);
+
+        VerifyOrQuit(routers[i - 1]->Get<Mle::Mle>().IsRouter());
+    }
+
+    // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+    Log("Check the path cost from last router to leader");
+
+    VerifyOrQuit(routers[kNumRouters - 1]->Get<RouterTable>().GetPathCostToLeader() == kNumRouters);
+}
+
+#endif // OPENTHREAD_CONFIG_MLE_LONG_ROUTES_ENABLE
+
+} // namespace Nexus
+} // namespace ot
+
+int main(void)
+{
+#if OPENTHREAD_CONFIG_MLE_LONG_ROUTES_ENABLE
+    ot::Nexus::TestLongRoutes();
+    printf("All tests passed\n");
+#else
+    printf("OPENTHREAD_CONFIG_MLE_LONG_ROUTES_ENABLE is not enabled, test is skipped\n");
+#endif
+    return 0;
+}