Constify the arguments of ilist.c/h functions

Const qualifiers ensure that we don't do something stupid in the
function implementation.  Additionally they clarify the interface.  As
an example:

    void
    slist_delete(slist_head *head, const slist_node *node)

Here one can instantly tell that node->next is not going to be set to
NULL.  Finally, const qualifiers potentially allow the compiler to do
more optimizations.  This being said, no benchmarking was done for
this patch.

The functions that return non-const pointers like slist_next_node(),
dclist_next_node() etc. are not affected by the patch intentionally.

Author: Aleksander Alekseev
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/CAJ7c6TM2%3D08mNKD9aJg8vEY9hd%2BG4L7%2BNvh30UiNT3kShgRgNg%40mail.gmail.com

src/backend/lib/ilist.c

@@ -28,7 +28,7 @@
  * Caution: this is O(n); consider using slist_delete_current() instead.
  */
 void
-slist_delete(slist_head *head, slist_node *node)
+slist_delete(slist_head *head, const slist_node *node)
 {
 	slist_node *last = &head->head;
 	slist_node *cur;
@@ -57,7 +57,7 @@ slist_delete(slist_head *head, slist_node *node)
  *		Validate that 'node' is a member of 'head'
  */
 void
-dlist_member_check(dlist_head *head, dlist_node *node)
+dlist_member_check(const dlist_head *head, const dlist_node *node)
 {
 	dlist_iter	iter;
 
@@ -73,7 +73,7 @@ dlist_member_check(dlist_head *head, dlist_node *node)
  * Verify integrity of a doubly linked list
  */
 void
-dlist_check(dlist_head *head)
+dlist_check(const dlist_head *head)
 {
 	dlist_node *cur;
 
@@ -110,7 +110,7 @@ dlist_check(dlist_head *head)
  * Verify integrity of a singly linked list
  */
 void
-slist_check(slist_head *head)
+slist_check(const slist_head *head)
 {
 	slist_node *cur;
 

src/include/lib/ilist.h

@@ -286,12 +286,12 @@ typedef struct slist_mutable_iter
 /* Prototypes for functions too big to be inline */
 
 /* Caution: this is O(n); consider using slist_delete_current() instead */
-extern void slist_delete(slist_head *head, slist_node *node);
+extern void slist_delete(slist_head *head, const slist_node *node);
 
 #ifdef ILIST_DEBUG
-extern void dlist_member_check(dlist_head *head, dlist_node *node);
-extern void dlist_check(dlist_head *head);
-extern void slist_check(slist_head *head);
+extern void dlist_member_check(const dlist_head *head, const dlist_node *node);
+extern void dlist_check(const dlist_head *head);
+extern void slist_check(const slist_head *head);
 #else
 /*
  * These seemingly useless casts to void are here to keep the compiler quiet
@@ -322,7 +322,7 @@ dlist_init(dlist_head *head)
  * An empty list has either its first 'next' pointer set to NULL, or to itself.
  */
 static inline bool
-dlist_is_empty(dlist_head *head)
+dlist_is_empty(const dlist_head *head)
 {
 	dlist_check(head);
 
@@ -465,7 +465,7 @@ dlist_move_tail(dlist_head *head, dlist_node *node)
  * Caution: unreliable if 'node' is not in the list.
  */
 static inline bool
-dlist_has_next(dlist_head *head, dlist_node *node)
+dlist_has_next(const dlist_head *head, const dlist_node *node)
 {
 	return node->next != &head->head;
 }
@@ -475,7 +475,7 @@ dlist_has_next(dlist_head *head, dlist_node *node)
  * Caution: unreliable if 'node' is not in the list.
  */
 static inline bool
-dlist_has_prev(dlist_head *head, dlist_node *node)
+dlist_has_prev(const dlist_head *head, const dlist_node *node)
 {
 	return node->prev != &head->head;
 }
@@ -629,7 +629,7 @@ dclist_init(dclist_head *head)
  *		Returns true if the list is empty, otherwise false.
  */
 static inline bool
-dclist_is_empty(dclist_head *head)
+dclist_is_empty(const dclist_head *head)
 {
 	Assert(dlist_is_empty(&head->dlist) == (head->count == 0));
 	return (head->count == 0);
@@ -773,7 +773,7 @@ dclist_move_tail(dclist_head *head, dlist_node *node)
  * Caution: 'node' must be a member of 'head'.
  */
 static inline bool
-dclist_has_next(dclist_head *head, dlist_node *node)
+dclist_has_next(const dclist_head *head, const dlist_node *node)
 {
 	dlist_member_check(&head->dlist, node);
 	Assert(head->count > 0);
@@ -788,7 +788,7 @@ dclist_has_next(dclist_head *head, dlist_node *node)
  * Caution: 'node' must be a member of 'head'.
  */
 static inline bool
-dclist_has_prev(dclist_head *head, dlist_node *node)
+dclist_has_prev(const dclist_head *head, const dlist_node *node)
 {
 	dlist_member_check(&head->dlist, node);
 	Assert(head->count > 0);
@@ -866,7 +866,7 @@ dclist_tail_node(dclist_head *head)
  *		Returns the stored number of entries in 'head'
  */
 static inline uint32
-dclist_count(dclist_head *head)
+dclist_count(const dclist_head *head)
 {
 	Assert(dlist_is_empty(&head->dlist) == (head->count == 0));
 
@@ -929,7 +929,7 @@ slist_init(slist_head *head)
  * Is the list empty?
  */
 static inline bool
-slist_is_empty(slist_head *head)
+slist_is_empty(const slist_head *head)
 {
 	slist_check(head);
 
@@ -977,7 +977,7 @@ slist_pop_head_node(slist_head *head)
  * Check whether 'node' has a following node.
  */
 static inline bool
-slist_has_next(slist_head *head, slist_node *node)
+slist_has_next(const slist_head *head, const slist_node *node)
 {
 	slist_check(head);