orb posix: fix multi-threading issues

in detail:
- in the write method: the following are not necessarily atomic operations:
 	_last_update = hrt_absolute_time();
 	_generation++;
- appears_updated() was called with a lock held in some cases, but not
  in ioctl
- use the SmartLock class, so that unlock() is not needed before every
  return call. Makes it less error prone

src/modules/uORB/uORBDevices_posix.cpp

@@ -253,20 +253,20 @@ uORB::DeviceNode::write(device::file_t *filp, const char *buffer, size_t buflen)
 		return -EIO;
 	}
 
-	/* Perform an atomic copy. */
 	lock();
 	memcpy(_data, buffer, _meta->o_size);
-	unlock();
 
 	/* update the timestamp and generation count */
 	_last_update = hrt_absolute_time();
 	_generation++;
 
+	_published = true;
+
+	unlock();
+
 	/* notify any poll waiters */
 	poll_notify(POLLIN);
 
-	_published = true;
-
 	return _meta->o_size;
 }
 
@@ -278,16 +278,22 @@ uORB::DeviceNode::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 
 	switch (cmd) {
 	case ORBIOCLASTUPDATE:
+		lock();
 		*(hrt_abstime *)arg = _last_update;
+		unlock();
 		return PX4_OK;
 
 	case ORBIOCUPDATED:
+		lock();
 		*(bool *)arg = appears_updated(sd);
+		unlock();
 		return PX4_OK;
 
 	case ORBIOCSETINTERVAL:
+		lock();
 		sd->update_interval = arg;
 		sd->last_update = hrt_absolute_time();
+		unlock();
 		return PX4_OK;
 
 	case ORBIOCGADVERTISER:
@@ -409,6 +415,7 @@ uORB::DeviceNode::poll_notify_one(px4_pollfd_struct_t *fds, pollevent_t events)
 bool
 uORB::DeviceNode::appears_updated(SubscriberData *sd)
 {
+
 	/* block if in simulation mode */
 	while (px4_sim_delay_enabled()) {
 		usleep(100);
@@ -420,8 +427,7 @@ uORB::DeviceNode::appears_updated(SubscriberData *sd)
 
 	/* check if this topic has been published yet, if not bail out */
 	if (_data == nullptr) {
-		ret = false;
+		return false;
-		goto out;
 	}
 
 	/*
@@ -467,8 +473,6 @@ uORB::DeviceNode::appears_updated(SubscriberData *sd)
 		break;
 	}
 
-out:
-	/* consider it updated */
 	return ret;
 }
 
@@ -535,14 +539,14 @@ int16_t uORB::DeviceNode::process_add_subscription(int32_t rateInHz)
 		ch->send_message(_meta->o_name, _meta->o_size, _data);
 	}
 
-	return 0;
+	return PX4_OK;
 }
 
 //-----------------------------------------------------------------------------
 //-----------------------------------------------------------------------------
 int16_t uORB::DeviceNode::process_remove_subscription()
 {
-	return 0;
+	return PX4_OK;
 }
 
 //-----------------------------------------------------------------------------
@@ -608,9 +612,6 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 				return ret;
 			}
 
-			/* ensure that only one advertiser runs through this critical section */
-			lock();
-
 			ret = ERROR;
 
 			/* try for topic groups */
@@ -624,11 +625,12 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 				group_tries = *adv->instance;
 
 				if (group_tries >= max_group_tries) {
-					unlock();
 					return -ENOMEM;
 				}
 			}
 
+			SmartLock smart_lock(*this);
+
 			do {
 				/* if path is modifyable change try index */
 				if (adv->instance != nullptr) {
@@ -641,7 +643,6 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 				objname = strdup(meta->o_name);
 
 				if (objname == nullptr) {
-					unlock();
 					return -ENOMEM;
 				}
 
@@ -649,7 +650,6 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 				devpath = strdup(nodepath);
 
 				if (devpath == nullptr) {
-					unlock();
 					free((void *)objname);
 					return -ENOMEM;
 				}
@@ -659,7 +659,6 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 
 				/* if we didn't get a device, that's bad */
 				if (node == nullptr) {
-					unlock();
 					free((void *)objname);
 					free((void *)devpath);
 					return -ENOMEM;
@@ -704,9 +703,6 @@ uORB::DeviceMaster::ioctl(device::file_t *filp, int cmd, unsigned long arg)
 				ret = -ENOMEM;
 			}
 
-			/* the file handle for the driver has been created, unlock */
-			unlock();
-
 			return ret;
 		}