Rationalize use of list_concat + list_copy combinations.

In the wake of commit 1cff1b95a, the result of list_concat no longer
shares the ListCells of the second input.  Therefore, we can replace
"list_concat(x, list_copy(y))" with just "list_concat(x, y)".

To improve call sites that were list_copy'ing the first argument,
or both arguments, invent "list_concat_copy()" which produces a new
list sharing no ListCells with either input.  (This is a bit faster
than "list_concat(list_copy(x), y)" because it makes the result list
the right size to start with.)

In call sites that were not list_copy'ing the second argument, the new
semantics mean that we are usually leaking the second List's storage,
since typically there is no remaining pointer to it.  We considered
inventing another list_copy variant that would list_free the second
input, but concluded that for most call sites it isn't worth worrying
about, given the relative compactness of the new List representation.
(Note that in cases where such leakage would happen, the old code
already leaked the second List's header; so we're only discussing
the size of the leak not whether there is one.  I did adjust two or
three places that had been troubling to free that header so that
they manually free the whole second List.)

Patch by me; thanks to David Rowley for review.

Discussion: https://postgr.es/m/11587.1550975080@sss.pgh.pa.us

1 files changed, 44 insertions, 6 deletions

diff --git a/src/backend/nodes/list.c b/src/backend/nodes/list.c
index 9163464de24..6bf13ae0d4a 100644
--- a/src/backend/nodes/list.c
+++ b/src/backend/nodes/list.c
@@ -501,12 +501,15 @@ lcons_oid(Oid datum, List *list)
 }
 
 /*
- * Concatenate list2 to the end of list1, and return list1. list1 is
- * destructively changed, list2 is not. (However, in the case of pointer
- * lists, list1 and list2 will point to the same structures.) Callers
- * should be sure to use the return value as the new pointer to the
- * concatenated list: the 'list1' input pointer may or may not be the
- * same as the returned pointer.
+ * Concatenate list2 to the end of list1, and return list1.
+ *
+ * This is equivalent to lappend'ing each element of list2, in order, to list1.
+ * list1 is destructively changed, list2 is not.  (However, in the case of
+ * pointer lists, list1 and list2 will point to the same structures.)
+ *
+ * Callers should be sure to use the return value as the new pointer to the
+ * concatenated list: the 'list1' input pointer may or may not be the same
+ * as the returned pointer.
  */
 List *
 list_concat(List *list1, const List *list2)
@@ -535,6 +538,41 @@ list_concat(List *list1, const List *list2)
 }
 
 /*
+ * Form a new list by concatenating the elements of list1 and list2.
+ *
+ * Neither input list is modified.  (However, if they are pointer lists,
+ * the output list will point to the same structures.)
+ *
+ * This is equivalent to, but more efficient than,
+ * list_concat(list_copy(list1), list2).
+ * Note that some pre-v13 code might list_copy list2 as well, but that's
+ * pointless now.
+ */
+List *
+list_concat_copy(const List *list1, const List *list2)
+{
+	List	   *result;
+	int			new_len;
+
+	if (list1 == NIL)
+		return list_copy(list2);
+	if (list2 == NIL)
+		return list_copy(list1);
+
+	Assert(list1->type == list2->type);
+
+	new_len = list1->length + list2->length;
+	result = new_list(list1->type, new_len);
+	memcpy(result->elements, list1->elements,
+		   list1->length * sizeof(ListCell));
+	memcpy(result->elements + list1->length, list2->elements,
+		   list2->length * sizeof(ListCell));
+
+	check_list_invariants(result);
+	return result;
+}
+
+/*
  * Truncate 'list' to contain no more than 'new_size' elements. This
  * modifies the list in-place! Despite this, callers should use the
  * pointer returned by this function to refer to the newly truncated