ScheduledWorkItem::ScheduleClear: remove item from runnable queue

The existing behavior is unexpected: if the work item is already on the
runnable queue, it will still be triggered after a call to ScheduleClear().
This can lead to race conditions.

platforms/common/include/px4_platform_common/px4_work_queue/WorkItem.hpp

@@ -81,6 +81,10 @@ protected:
 
 	virtual ~WorkItem();
 
+	/**
+	 * Remove work item from the runnable queue, if it's there
+	 */
+	void ScheduleClear();
 protected:
 
 	void RunPreamble() { _run_count++; }

platforms/common/px4_work_queue/ScheduledWorkItem.cpp

@@ -66,7 +66,9 @@ ScheduledWorkItem::ScheduleOnInterval(uint32_t interval_us, uint32_t delay_us)
 void
 ScheduledWorkItem::ScheduleClear()
 {
+	// first clear any scheduled hrt call, then remove the item from the runnable queue
 	hrt_cancel(&_call);
+	WorkItem::ScheduleClear();
 }
 
 void

platforms/common/px4_work_queue/WorkItem.cpp

@@ -89,6 +89,14 @@ WorkItem::Deinit()
 	}
 }
 
+void
+WorkItem::ScheduleClear()
+{
+	if (_wq != nullptr) {
+		_wq->Remove(this);
+	}
+}
+
 float
 WorkItem::elapsed_time() const
 {

platforms/common/px4_work_queue/WorkQueue.cpp

@@ -160,6 +160,7 @@ WorkQueue::Run()
 			work_unlock(); // unlock work queue to run (item may requeue itself)
 			work->RunPreamble();
 			work->Run();
+			// Note: after Run() we cannot access work anymore, as it might have been deleted
 			work_lock(); // re-lock
 		}