[mle] evaluate neighbor links before upgrade to router (#7765)

OpenThread only etablishes links when the link margin is above a
certain threshold, to help ensure that links in the routing topology
are stable. However, if a device has no existing links to neighboring
routers above threshold, then upgrading to a router is useless and
disruptive.

This commit adds a check to ensure there is at least one neighbor
above the link margin threshold before upgrading to a router.

tests/scripts/thread-cert/Makefile.am

@@ -184,6 +184,7 @@ EXTRA_DIST                                                         = \
     test_reset.py                                                    \
     test_route_table.py                                              \
     test_router_reattach.py                                          \
+    test_router_upgrade.py                                           \
     test_service.py                                                  \
     test_set_mliid.py                                                \
     test_srp_auto_start_mode.py                                      \
@@ -258,6 +259,7 @@ check_SCRIPTS                                                      = \
     test_reset.py                                                    \
     test_route_table.py                                              \
     test_router_reattach.py                                          \
+    test_router_upgrade.py                                           \
     test_service.py                                                  \
     test_srp_auto_start_mode.py                                      \
     test_srp_client_remove_host.py                                   \

tests/scripts/thread-cert/test_router_upgrade.py

@@ -0,0 +1,99 @@
+#!/usr/bin/env python3
+#
+#  Copyright (c) 2022, 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.
+#
+
+import unittest
+
+import config
+import mle
+import network_layer
+import thread_cert
+
+LEADER = 1
+REED = 2
+
+RSSI_LOW = -95
+RSSI_HIGH = -45
+
+ROUTER_UPGRADE_DELAY = 150
+
+# Test Purpose and Description:
+# -----------------------------
+# The purpose of this test case is to show that a REED does not
+# upgrade to router if it does not have a neighbor with link margin
+# above threshold.
+#
+# Test Topology:
+# -------------
+# Leader
+#    |
+# REED
+#
+# DUT Types:
+# ----------
+#  Leader
+#  REED
+
+
+class TestRouterUpgrade(thread_cert.TestCase):
+    USE_MESSAGE_FACTORY = False
+
+    TOPOLOGY = {
+        LEADER: {
+            'name': 'LEADER',
+            'mode': 'rdn',
+            'allowlist': [REED]
+        },
+        REED: {
+            'name': 'REED',
+            'mode': 'rdn',
+            'allowlist': [LEADER]
+        },
+    }
+
+    def test(self):
+        self.nodes[LEADER].start()
+        self.simulator.go(5)
+        self.assertEqual(self.nodes[LEADER].get_state(), 'leader')
+
+        self.nodes[REED].add_allowlist(self.nodes[LEADER].get_addr64(), rssi=RSSI_LOW)
+        self.nodes[REED].enable_allowlist()
+
+        self.nodes[REED].start()
+        self.simulator.go(ROUTER_UPGRADE_DELAY)
+        self.assertEqual(self.nodes[REED].get_state(), 'child')
+
+        self.nodes[REED].add_allowlist(self.nodes[LEADER].get_addr64(), rssi=RSSI_HIGH)
+        self.nodes[REED].enable_allowlist()
+
+        self.simulator.go(ROUTER_UPGRADE_DELAY)
+        self.assertEqual(self.nodes[REED].get_state(), 'router')
+
+
+if __name__ == '__main__':
+    unittest.main()