Avoid O(N^2) behavior in SyncPostCheckpoint().

As in commits 6301c3ada and e9d9ba2a4, avoid doing repetitive
list_delete_first() operations, since that would be expensive when
there are many files waiting to be unlinked.  This is a slightly
larger change than in those cases.  We have to keep the list state
valid for calls to AbsorbSyncRequests(), so it's necessary to invent a
"canceled" field instead of immediately deleting PendingUnlinkEntry
entries.  Also, because we might not be able to process all the
entries, we need a new list primitive list_delete_first_n().

list_delete_first_n() is almost list_copy_tail(), but it modifies the
input List instead of making a new copy.  I found a couple of existing
uses of the latter that could profitably use the new function.  (There
might be more, but the other callers look like they probably shouldn't
overwrite the input List.)

As before, back-patch to v13.

Discussion: https://postgr.es/m/CD2F0E7F-9822-45EC-A411-AE56F14DEA9F@amazon.com

1 files changed, 66 insertions, 0 deletions

diff --git a/src/backend/nodes/list.c b/src/backend/nodes/list.c
index 94fb236dafe..12847e35ea3 100644
--- a/src/backend/nodes/list.c
+++ b/src/backend/nodes/list.c
@@ -907,6 +907,72 @@ list_delete_last(List *list)
 }
 
 /*
+ * Delete the first N cells of the list.
+ *
+ * The List is pfree'd if the request causes all cells to be deleted.
+ */
+List *
+list_delete_first_n(List *list, int n)
+{
+	check_list_invariants(list);
+
+	/* No-op request? */
+	if (n <= 0)
+		return list;
+
+	/* Delete whole list? */
+	if (n >= list_length(list))
+	{
+		list_free(list);
+		return NIL;
+	}
+
+	/*
+	 * Otherwise, we normally just collapse out the removed elements.  But for
+	 * debugging purposes, move the whole list contents someplace else.
+	 *
+	 * (Note that we *must* keep the contents in the same memory context.)
+	 */
+#ifndef DEBUG_LIST_MEMORY_USAGE
+	memmove(&list->elements[0], &list->elements[n],
+			(list->length - n) * sizeof(ListCell));
+	list->length -= n;
+#else
+	{
+		ListCell   *newelems;
+		int			newmaxlen = list->length - n;
+
+		newelems = (ListCell *)
+			MemoryContextAlloc(GetMemoryChunkContext(list),
+							   newmaxlen * sizeof(ListCell));
+		memcpy(newelems, &list->elements[n], newmaxlen * sizeof(ListCell));
+		if (list->elements != list->initial_elements)
+			pfree(list->elements);
+		else
+		{
+			/*
+			 * As in enlarge_list(), clear the initial_elements[] space and/or
+			 * mark it inaccessible.
+			 */
+#ifdef CLOBBER_FREED_MEMORY
+			wipe_mem(list->initial_elements,
+					 list->max_length * sizeof(ListCell));
+#else
+			VALGRIND_MAKE_MEM_NOACCESS(list->initial_elements,
+									   list->max_length * sizeof(ListCell));
+#endif
+		}
+		list->elements = newelems;
+		list->max_length = newmaxlen;
+		list->length = newmaxlen;
+		check_list_invariants(list);
+	}
+#endif
+
+	return list;
+}
+
+/*
  * Generate the union of two lists. This is calculated by copying
  * list1 via list_copy(), then adding to it all the members of list2
  * that aren't already in list1.