score: Fix _Thread_Priority_change()

The POSIX sporadic server may temporarily remove the real priority of a
thread.  Check that the priority node is active before the change is
propagated.

Update #5168.

cpukit/include/rtems/score/thread.h

@@ -96,9 +96,22 @@ extern "C" {
  */
 
 #if defined(RTEMS_DEBUG)
+/**
+ * @brief This define enables the thread resource count support.
+ */
 #define RTEMS_SCORE_THREAD_ENABLE_RESOURCE_COUNT
 #endif
 
+#if defined(RTEMS_POSIX_API)
+/**
+ * @brief This define enables support for an inactive real thread priority.
+ *
+ * For example, the POSIX sporadic server may temporarily remove the real
+ * priority of a thread while it is in low priority mode.
+ */
+#define RTEMS_SCORE_THREAD_REAL_PRIORITY_MAY_BE_INACTIVE
+#endif
+
 /**
  *  @brief Type of the numeric argument of a thread entry function with at
  *  least one numeric argument.

cpukit/include/rtems/score/threadimpl.h

@@ -800,6 +800,14 @@ static inline void _Thread_Priority_change(
 )
 {
   _Priority_Node_set_priority( priority_node, new_priority );
+
+#if defined(RTEMS_SCORE_THREAD_REAL_PRIORITY_MAY_BE_INACTIVE)
+  if ( !_Priority_Node_is_active( priority_node ) ) {
+    /* The priority change is picked up once the node is added */
+    return;
+  }
+#endif
+
   _Thread_Priority_changed(
     the_thread,
     priority_node,