Hide waiting tasks from n=0.

cylc/flow/task_pool.py

@@ -241,11 +241,17 @@ def add_to_runahead_pool(self, itask, is_new=True):
         return itask
 
     def release_runahead_tasks(self):
-        """Restrict the number of active cycle points.
+        """Release tasks from the runahead pool to the main pool.
+
+        This serves to:
+        - restrict the number of active cycle points
+        - keep partially-satisfied waiting tasks out of the n=0 active pool
 
         Compute max active cycle points or runahead limit, and release tasks to
         the main pool if they are below that point (and <= the stop point, if
-        there is a stop point). Return True if any tasks released, else False.
+        there is a stop point).
+
+        Return True if any tasks were released, else False.
 
         """
         released = False
@@ -339,6 +345,9 @@ def release_runahead_tasks(self):
         for point, itask_id_map in self.runahead_pool.copy().items():
             if point <= latest_allowed_point:
                 for itask in itask_id_map.copy().values():
+                    if itask.is_task_prereqs_not_done():
+                        # Only release if all prerequisites are satisfied.
+                        continue
                     self.release_runahead_task(itask)
                     released = True
         return released