fix(mqtt): count the regional roots as the default root topic

src/mqtt/MQTT.cpp

@@ -247,11 +247,6 @@ bool isDefaultServer(const String &host)
     return host.length() == 0 || host == default_mqtt_address;
 }
 
-bool isDefaultRootTopic(const String &root)
-{
-    return root.length() == 0 || root == default_mqtt_root;
-}
-
 // "msh/<region>" is what the default broker's convention produces; any other suffix is the user's own.
 bool isRegionRootTopic(const char *root)
 {
@@ -264,6 +259,12 @@ bool isRegionRootTopic(const char *root)
     return false;
 }
 
+// The regional roots count as default: they are what a region change writes on the default broker.
+bool isDefaultRootTopic(const String &root)
+{
+    return root.length() == 0 || root == default_mqtt_root || isRegionRootTopic(root.c_str());
+}
+
 struct PubSubConfig {
     explicit PubSubConfig(const meshtastic_ModuleConfig_MQTTConfig &config)
     {
@@ -385,7 +386,7 @@ bool MQTT::applyRegionRootTopic(const char *regionName)
     (void)parsedPort;
     if (!isDefaultServer(host))
         return false;
-    if (!isDefaultRootTopic(moduleConfig.mqtt.root) && !isRegionRootTopic(moduleConfig.mqtt.root))
+    if (!isDefaultRootTopic(moduleConfig.mqtt.root))
         return false; // the user picked their own root
     snprintf(moduleConfig.mqtt.root, sizeof(moduleConfig.mqtt.root), "%s/%s", default_mqtt_root, regionName);
     return true;

test/test_mqtt/MQTT.cpp

@@ -1427,6 +1427,20 @@ void test_applyRegionRootTopic_rewritesDefaultBrokerRootsOnly(void)
     TEST_ASSERT_EQUAL_STRING("msh/US", moduleConfig.mqtt.root);
 }
 
+// USERPREFS_EVENT_MODE gates the public broker on isUsingDefaultRootTopic() (Channels::anyMqttEnabled()).
+// A "msh/<region>" root is what a region change writes on the default broker, so it has to keep counting as
+// the default root; otherwise an event build silently loses that guard the first time the region changes.
+void test_regionRootTopic_countsAsTheDefaultRoot(void)
+{
+    strcpy(moduleConfig.mqtt.root, "msh/EU_868");
+    MQTTUnitTest::restart();
+    TEST_ASSERT_TRUE(mqtt->isUsingDefaultRootTopic());
+
+    strcpy(moduleConfig.mqtt.root, "msh/home");
+    MQTTUnitTest::restart();
+    TEST_ASSERT_FALSE(mqtt->isUsingDefaultRootTopic());
+}
+
 // Empty configuration is valid.
 void test_configEmptyIsValid(void)
 {
@@ -1624,6 +1638,7 @@ void setup()
     RUN_TEST(test_customMqttRoot);
     RUN_TEST(test_rootChange_rebuildsTopics);
     RUN_TEST(test_applyRegionRootTopic_rewritesDefaultBrokerRootsOnly);
+    RUN_TEST(test_regionRootTopic_countsAsTheDefaultRoot);
     RUN_TEST(test_configEmptyIsValid);
     RUN_TEST(test_configEnabledEmptyIsValid);
     RUN_TEST(test_configWithDefaultServer);