Create play_tune plugin

play_tune: Use tune_id as parameter.

play_tune: Run clang-format

play_tune: Fix buggy conditional

The check `!x > 0` evaluates as `(!x) > 0`, but this was probably intended to be
`!(x > 0)`.

However:

The tune id is unsigned, and tune id 0 means custom tune, which we don't
support, so we can simplify this to just issue an error if tune id is zero.

Check out tune_definition.desc in the PX4 Firmware repo.

Fix mavlink#3.

mavros/CMakeLists.txt

@@ -139,6 +139,7 @@ add_library(mavros_plugins
   src/plugins/vfr_hud.cpp
   src/plugins/waypoint.cpp
   src/plugins/wind_estimation.cpp
+  src/plugins/play_tune.cpp
 )
 add_dependencies(mavros_plugins
   mavros

mavros/mavros_plugins.xml

@@ -77,4 +77,7 @@
 	<class name="wind_estimation" type="mavros::std_plugins::WindEstimationPlugin" base_class_type="mavros::plugin::PluginBase">
 		<description>Publish wind estimates</description>
 	</class>
+	<class name="play_tune" type="mavros::std_plugins::PlayTunePlugin" base_class_type="mavros::plugin::PluginBase">
+		<description>Play a tune</description>
+	</class>
 </library>

mavros/src/plugins/play_tune.cpp

@@ -0,0 +1,55 @@
+#include <mavros/mavros_plugin.h>
+#include <std_msgs/UInt8.h>
+
+namespace mavros
+{
+namespace std_plugins
+{
+
+class PlayTunePlugin : public plugin::PluginBase
+{
+  public:
+    PlayTunePlugin() : PluginBase(), nh("~") {}
+
+    void initialize(UAS& uas_)
+    {
+        PluginBase::initialize(uas_);
+        sub = nh.subscribe("play_tune", 1, &PlayTunePlugin::callback, this);
+    }
+
+    Subscriptions get_subscriptions() { return {/* Rx disabled */}; }
+
+  private:
+    ros::NodeHandle nh;
+    ros::Subscriber sub;
+
+    void callback(const std_msgs::UInt8::ConstPtr& tune_id)
+    {
+        ROS_INFO("Got play tune request!");
+
+        if (tune_id->data == 0)
+        {
+            ROS_ERROR("Tune ID 0 (custom tune) not supported, ignoring.");
+            return;
+        }
+
+        mavlink::common::msg::PLAY_TUNE msg{};
+
+        msg.target_system = m_uas->get_tgt_system();
+        msg.target_component = m_uas->get_tgt_component();
+
+        /* Tune definitions:
+            https://github.com/PX4/Firmware/blob/
+            bfab544a64e2dc3b86905771017989502ec40dd2/src/lib/tunes/
+            tune_definition.desc#L102
+        */
+        msg.tune[0] = tune_id->data;
+        UAS_FCU(m_uas)->send_message_ignore_drop(msg);
+    }
+};
+}  // namespace std_plugins
+}  // namespace mavros
+
+#include <pluginlib/class_list_macros.h>
+PLUGINLIB_EXPORT_CLASS(mavros::std_plugins::PlayTunePlugin,
+                       mavros::plugin::PluginBase)