mru: use double-linked list from list.h

Simplify mru.[ch] and related code by reusing the double-linked list
implementation from list.h instead of a custom one.
This commit is an intermediate step. Our final goal is to get rid of
mru.[ch] at all and inline all logic.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored by: Jeff King <peff@peff.net>
Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -995,8 +995,8 @@ static int want_object_in_pack(const unsigned char *sha1,
 			       struct packed_git **found_pack,
 			       off_t *found_offset)
 {
-	struct mru_entry *entry;
 	int want;
+	struct list_head *pos;
 
 	if (!exclude && local && has_loose_object_nonlocal(sha1))
 		return 0;
@@ -1012,7 +1012,8 @@ static int want_object_in_pack(const unsigned char *sha1,
 			return want;
 	}
 
-	for (entry = packed_git_mru.head; entry; entry = entry->next) {
+	list_for_each(pos, &packed_git_mru.list) {
+		struct mru *entry = list_entry(pos, struct mru, list);
 		struct packed_git *p = entry->item;
 		off_t offset;
 

mru.c

@@ -1,50 +1,27 @@
 #include "cache.h"
 #include "mru.h"
 
-void mru_append(struct mru *mru, void *item)
+void mru_append(struct mru *head, void *item)
 {
-	struct mru_entry *cur = xmalloc(sizeof(*cur));
+	struct mru *cur = xmalloc(sizeof(*cur));
 	cur->item = item;
-	cur->prev = mru->tail;
-	cur->next = NULL;
-
-	if (mru->tail)
-		mru->tail->next = cur;
-	else
-		mru->head = cur;
-	mru->tail = cur;
+	list_add_tail(&cur->list, &head->list);
 }
 
-void mru_mark(struct mru *mru, struct mru_entry *entry)
+void mru_mark(struct mru *head, struct mru *entry)
 {
-	/* If we're already at the front of the list, nothing to do */
-	if (mru->head == entry)
-		return;
-
-	/* Otherwise, remove us from our current slot... */
-	if (entry->prev)
-		entry->prev->next = entry->next;
-	if (entry->next)
-		entry->next->prev = entry->prev;
-	else
-		mru->tail = entry->prev;
-
-	/* And insert us at the beginning. */
-	entry->prev = NULL;
-	entry->next = mru->head;
-	if (mru->head)
-		mru->head->prev = entry;
-	mru->head = entry;
+	/* To mark means to put at the front of the list. */
+	list_del(&entry->list);
+	list_add(&entry->list, &head->list);
 }
 
-void mru_clear(struct mru *mru)
+void mru_clear(struct mru *head)
 {
-	struct mru_entry *p = mru->head;
+	struct list_head *pos;
+	struct list_head *tmp;
 
-	while (p) {
-		struct mru_entry *to_free = p;
-		p = p->next;
-		free(to_free);
+	list_for_each_safe(pos, tmp, &head->list) {
+		free(list_entry(pos, struct mru, list));
 	}
-	mru->head = mru->tail = NULL;
+	INIT_LIST_HEAD(&head->list);
 }

mru.h

@@ -1,6 +1,8 @@
 #ifndef MRU_H
 #define MRU_H
 
+#include "list.h"
+
 /**
  * A simple most-recently-used cache, backed by a doubly-linked list.
  *
@@ -8,18 +10,15 @@
  *
  *   // Create a list.  Zero-initialization is required.
  *   static struct mru cache;
- *   mru_append(&cache, item);
- *   ...
+ *   INIT_LIST_HEAD(&cache.list);
  *
- *   // Iterate in MRU order.
- *   struct mru_entry *p;
- *   for (p = cache.head; p; p = p->next) {
- *	if (matches(p->item))
- *		break;
- *   }
+ *   // Add new item to the end of the list.
+ *   void *item;
+ *   ...
+ *   mru_append(&cache, item);
  *
  *   // Mark an item as used, moving it to the front of the list.
- *   mru_mark(&cache, p);
+ *   mru_mark(&cache, item);
  *
  *   // Reset the list to empty, cleaning up all resources.
  *   mru_clear(&cache);
@@ -29,17 +28,13 @@
  * you will begin traversing the whole list again.
  */
 
-struct mru_entry {
-	void *item;
-	struct mru_entry *prev, *next;
-};
-
 struct mru {
-	struct mru_entry *head, *tail;
+	struct list_head list;
+	void *item;
 };
 
-void mru_append(struct mru *mru, void *item);
-void mru_mark(struct mru *mru, struct mru_entry *entry);
-void mru_clear(struct mru *mru);
+void mru_append(struct mru *head, void *item);
+void mru_mark(struct mru *head, struct mru *entry);
+void mru_clear(struct mru *head);
 
 #endif /* MRU_H */

packfile.c

@@ -40,7 +40,7 @@ static unsigned int pack_max_fds;
 static size_t peak_pack_mapped;
 static size_t pack_mapped;
 struct packed_git *packed_git;
-struct mru packed_git_mru;
+struct mru packed_git_mru = {{&packed_git_mru.list, &packed_git_mru.list}};
 
 #define SZ_FMT PRIuMAX
 static inline uintmax_t sz_fmt(size_t s) { return s; }
@@ -1824,13 +1824,14 @@ static int fill_pack_entry(const unsigned char *sha1,
  */
 int find_pack_entry(const unsigned char *sha1, struct pack_entry *e)
 {
-	struct mru_entry *p;
+	struct list_head *pos;
 
 	prepare_packed_git();
 	if (!packed_git)
 		return 0;
 
-	for (p = packed_git_mru.head; p; p = p->next) {
+	list_for_each(pos, &packed_git_mru.list) {
+		struct mru *p = list_entry(pos, struct mru, list);
 		if (fill_pack_entry(sha1, e, p->item)) {
 			mru_mark(&packed_git_mru, p);
 			return 1;