mavlink & system: add events interface

- sending protocol
- uorb event message & template methods for argument packing
- libevents submodule to send common events and handle json files
- cmake maintains a list of all (PX4) source files for the current build
  (PX4 modules + libs), which is used to extract event metadata and
  generate a json file

platforms/common/events.cpp

@@ -0,0 +1,70 @@
+/****************************************************************************
+ *
+ *   Copyright (c) 2020 PX4 Development Team. 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 PX4 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 OWNER 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 <pthread.h>
+
+#include <drivers/drv_hrt.h>
+#include <px4_platform_common/posix.h>
+#include <px4_platform_common/events.h>
+#include <uORB/uORB.h>
+
+static orb_advert_t orb_event_pub = nullptr;
+static pthread_mutex_t publish_event_mutex = PTHREAD_MUTEX_INITIALIZER;
+static uint16_t event_sequence{events::initial_event_sequence};
+
+namespace events
+{
+
+void send(EventType &event)
+{
+	event.timestamp = hrt_absolute_time();
+
+	// We need some synchronization here because:
+	// - modifying orb_event_pub
+	// - the update of event_sequence needs to be atomic
+	// - we need to ensure ordering of the sequence numbers: the sequence we set here
+	//   has to be the one published next.
+	pthread_mutex_lock(&publish_event_mutex);
+	event.sequence = ++event_sequence; // Set the sequence here so we're able to detect uORB queue overflows
+
+	if (orb_event_pub != nullptr) {
+		orb_publish(ORB_ID(event), orb_event_pub, &event);
+
+	} else {
+		orb_event_pub = orb_advertise_queue(ORB_ID(event), &event, event_s::ORB_QUEUE_LENGTH);
+	}
+
+	pthread_mutex_unlock(&publish_event_mutex);
+}
+
+} /* namespace events */