Add ParallelSlotSetIdle().

This commit refactors the code for marking a ParallelSlot as idle
to a new static inline function.  This can be used to mark a slot
that was obtained via ParallelSlotGetIdle() but that we don't
intend to actually use for a query as idle again.

This is preparatory work for a follow-up commit that will add a
--dry-run option to vacuumdb.

Reviewed-by: Corey Huinker <corey.huinker@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com
Discussion: https://postgr.es/m/CADkLM%3DckHkX7Of5SrK7g0LokPUwJ%3Dkk8JU1GXGF5pZ1eBVr0%3DQ%40mail.gmail.com

src/fe_utils/parallel_slot.c

@@ -269,8 +269,7 @@ wait_on_slots(ParallelSlotArray *sa)
 			else
 			{
 				/* This connection has become idle */
-				sa->slots[i].inUse = false;
-				ParallelSlotClearHandler(&sa->slots[i]);
+				ParallelSlotSetIdle(&sa->slots[i]);
 				break;
 			}
 		}
@@ -509,8 +508,7 @@ ParallelSlotsWaitCompletion(ParallelSlotArray *sa)
 		if (!consumeQueryResult(&sa->slots[i]))
 			return false;
 		/* Mark connection as idle */
-		sa->slots[i].inUse = false;
-		ParallelSlotClearHandler(&sa->slots[i]);
+		ParallelSlotSetIdle(&sa->slots[i]);
 	}
 
 	return true;

src/include/fe_utils/parallel_slot.h

@@ -58,6 +58,13 @@ ParallelSlotClearHandler(ParallelSlot *slot)
 	slot->handler_context = NULL;
 }
 
+static inline void
+ParallelSlotSetIdle(ParallelSlot *slot)
+{
+	slot->inUse = false;
+	ParallelSlotClearHandler(slot);
+}
+
 extern ParallelSlot *ParallelSlotsGetIdle(ParallelSlotArray *sa,
 										  const char *dbname);