orb: add a separate uORB::Manager::initialize() method

This fixes a race condition: uORB::Manager::get_instance() is used in a
multi-thread context, but the singleton initialization was not thread-safe.
Further, this avoids having to check for nullptr every time the singleton
is accessed.

uORB::Manager::initialize() is called when uorb is started. No one else
accesses the singleton before that point, because it is only used in the
orb_* methods, and in muorb. Both require uorb to be started already when
they are used.

src/modules/uORB/uORBMain.cpp

@@ -33,6 +33,7 @@
 
 #include <string.h>
 #include "uORBDevices.hpp"
+#include "uORBManager.hpp"
 #include "uORB.h"
 #include "uORBCommon.hpp"
 
@@ -70,6 +71,11 @@ uorb_main(int argc, char *argv[])
 			return 0;
 		}
 
+		if (!uORB::Manager::initialize()) {
+			PX4_ERR("uorb manager alloc failed");
+			return -ENOMEM;
+		}
+
 		/* create the driver */
 		g_dev = new uORB::DeviceMaster(uORB::PUBSUB);
 

src/modules/uORB/uORBManager.cpp

@@ -49,13 +49,13 @@ uORB::Manager *uORB::Manager::_Instance = nullptr;
 
 //-----------------------------------------------------------------------------
 //-----------------------------------------------------------------------------
-uORB::Manager *uORB::Manager::get_instance()
+bool uORB::Manager::initialize()
 {
 	if (_Instance == nullptr) {
 		_Instance = new uORB::Manager();
 	}
 
-	return _Instance;
+	return _Instance != nullptr;
 }
 
 //-----------------------------------------------------------------------------

src/modules/uORB/uORBManager.hpp

@@ -62,11 +62,21 @@ class uORB::Manager  : public uORBCommunicator::IChannelRxHandler
 public:
 	// public interfaces for this class.
 
+	/**
+	 * Initialize the singleton. Call this before everything else.
+	 * @return true on success
+	 */
+	static bool initialize();
+
 	/**
 	 * Method to get the singleton instance for the uORB::Manager.
+	 * Make sure initialize() is called first.
 	 * @return uORB::Manager*
 	 */
-	static uORB::Manager *get_instance();
+	static uORB::Manager *get_instance()
+	{
+		return _Instance;
+	}
 
 	// ==== uORB interface methods ====
 	/**