[PATCH] [Outreachy] cleanup: use list.h in mru.h and mru.c

[PATCH] [Outreachy] cleanup: use list.h in mru.h and mru.c

[Оля Тележная]

Remove implementation of double-linked list in mru.c and mru.h and use
implementation from list.h.

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>, Jeff King
<peff@peff.net>
---
 builtin/pack-objects.c |  5 +++--
 mru.c                  | 51 +++++++++++++++-----------------------------------
 mru.h                  | 31 +++++++++++++-----------------
 packfile.c             |  6 ++++--
 4 files changed, 35 insertions(+), 58 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index f721137ea..fb4c9be89 100644
--- a/builtin/pack-objects.c
+++ b/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;

diff --git a/mru.c b/mru.c
index 9dedae028..8b6ba3d9b 100644
--- a/mru.c
+++ b/mru.c
@@ -1,50 +1,29 @@
 #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;
-
- while (p) {
- struct mru_entry *to_free = p;
- p = p->next;
+ struct list_head *p1;
+ struct list_head *p2;
+ struct mru *to_free;
+
+ list_for_each_safe(p1, p2, &head->list) {
+ to_free = list_entry(p1, struct mru, list);
  free(to_free);
  }
- mru->head = mru->tail = NULL;
+ INIT_LIST_HEAD(&head->list);
 }
diff --git a/mru.h b/mru.h
index 42e4aeaa1..36a332af0 100644
--- a/mru.h
+++ b/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 */
diff --git a/packfile.c b/packfile.c
index f69a5c8d6..ae3b0b2e9 100644
--- a/packfile.c
+++ b/packfile.c
@@ -876,6 +876,7 @@ void prepare_packed_git(void)
  for (alt = alt_odb_list; alt; alt = alt->next)
  prepare_packed_git_one(alt->path, 0);
  rearrange_packed_git();
+ INIT_LIST_HEAD(&packed_git_mru.list);
  prepare_packed_git_mru();
  prepare_packed_git_run_once = 1;
 }
@@ -1824,13 +1825,14 @@ 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;
-- 
2.14.1.727.g9ddaf86b0

Re: [PATCH] [Outreachy] cleanup: use list.h in mru.h and mru.c

[Christian Couder]

About the title, we don't usually start with "fix:" or "cleanup:" or
"feature:". We usually start with the (main) area of the code that is
changed.

So maybe something like "mru: use double-linked list implementation
from list.h" would be better.

On Wed, Sep 27, 2017 at 12:18 PM, Оля Тележная <olyatelezhnaya@gmail.com> wrote:
> Remove implementation of double-linked list in mru.c and mru.h and use
> implementation from list.h.

It is important in the commit message to get a good idea of the reason
why the patch is a good thing.
So for example "Simplify mru.c, mru.h and related code by reusing the
double-linked list implementation from list.h instead of a custom
one." could be a bit better.

> Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
> Mentored-by: Christian Couder <christian.couder@gmail.com>, Jeff King
> <peff@peff.net>

I think it's better if Peff is on another "Mentored-by: ..." line.

> ---
>  builtin/pack-objects.c |  5 +++--
>  mru.c                  | 51 +++++++++++++++-----------------------------------
>  mru.h                  | 31 +++++++++++++-----------------
>  packfile.c             |  6 ++++--
>  4 files changed, 35 insertions(+), 58 deletions(-)

Here we can see that saying that we simplify things is probably true
as the patch deletes more lines than it adds.

> diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
> index f721137ea..fb4c9be89 100644
> --- a/builtin/pack-objects.c
> +++ b/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;

It looks like there are indentation problems in the patch.

Did you try to send it to you and apply what you received?

When I try to apply it to master I get:

> git am ~/Downloads/original_msg.txt
Applying: cleanup: use list.h in mru.h and mru.c
.git/rebase-apply/patch:18: indent with spaces.
        struct list_head *pos;
.git/rebase-apply/patch:152: indent with spaces.
        void *item;
error: patch failed: builtin/pack-objects.c:995
error: builtin/pack-objects.c: patch does not apply
error: patch failed: mru.c:1
error: mru.c: patch does not apply
error: patch failed: mru.h:8
error: mru.h: patch does not apply
error: patch failed: packfile.c:876
error: packfile.c: patch does not apply
Patch failed at 0001 cleanup: use list.h in mru.h and mru.c
The copy of the patch that failed is found in: .git/rebase-apply/patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

I am stopping here as it's quite difficult to read patches that have
indentation problems.

[PATCH Outreachy] mru: use double-linked list from list.h

[Olga Telezhnaya]

Simplify mru.c, mru.h and related code by reusing the double-linked list implementation from list.h instead of a custom one.

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored by: Jeff King <peff@peff.net>
---
 builtin/pack-objects.c |  5 +++--
 mru.c                  | 51 +++++++++++++++-----------------------------------
 mru.h                  | 31 +++++++++++++-----------------
 packfile.c             |  6 ++++--
 4 files changed, 35 insertions(+), 58 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index f721137eaf881..ba812349e0aab 100644
--- a/builtin/pack-objects.c
+++ b/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;
 
diff --git a/mru.c b/mru.c
index 9dedae0287ed2..8b6ba3d9b7fad 100644
--- a/mru.c
+++ b/mru.c
@@ -1,50 +1,29 @@
 #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;
-
-	while (p) {
-		struct mru_entry *to_free = p;
-		p = p->next;
+	struct list_head *p1;
+	struct list_head *p2;
+	struct mru *to_free;
+	
+	list_for_each_safe(p1, p2, &head->list) {
+		to_free = list_entry(p1, struct mru, list);
 		free(to_free);
 	}
-	mru->head = mru->tail = NULL;
+	INIT_LIST_HEAD(&head->list);
 }
diff --git a/mru.h b/mru.h
index 42e4aeaa1098a..36a332af0bf88 100644
--- a/mru.h
+++ b/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 */
diff --git a/packfile.c b/packfile.c
index f69a5c8d607af..ae3b0b2e9c09a 100644
--- a/packfile.c
+++ b/packfile.c
@@ -876,6 +876,7 @@ void prepare_packed_git(void)
 	for (alt = alt_odb_list; alt; alt = alt->next)
 		prepare_packed_git_one(alt->path, 0);
 	rearrange_packed_git();
+	INIT_LIST_HEAD(&packed_git_mru.list);
 	prepare_packed_git_mru();
 	prepare_packed_git_run_once = 1;
 }
@@ -1824,13 +1825,14 @@ 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;

--
https://github.com/git/git/pull/409

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Junio C Hamano]

Olga Telezhnaya <olyatelezhnaya@gmail.com> writes:

> Simplify mru.c, mru.h and related code by reusing the double-linked list implementation from list.h instead of a custom one.

An overlong line (I can locally wrap it, so the patch does not have
to be re-sent only to fix this alone).

> Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>

Thanks for making your "From:" name match the "Signed-off-by:" name;
anglicising like you did is probably more friendly to the readers
than writing both in Cyrillic, which is another valid way to make
them match.

> Mentored-by: Christian Couder <christian.couder@gmail.com>
> Mentored by: Jeff King <peff@peff.net>
> ---
>  builtin/pack-objects.c |  5 +++--
>  mru.c                  | 51 +++++++++++++++-----------------------------------
>  mru.h                  | 31 +++++++++++++-----------------
>  packfile.c             |  6 ++++--
>  4 files changed, 35 insertions(+), 58 deletions(-)

As Christian mentioned earlier, nice line reduction ;-)

> diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
> index f721137eaf881..ba812349e0aab 100644
> --- a/builtin/pack-objects.c
%> +++ b/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;

I was a bit surprised to see a change outside mru.[ch] like this
one.  The reason why I was surprised was because I expected mru.[ch]
would offer its own API that encapsulates enumeration like this one
and this patch would just be reimplementing that API using the list
API, instead of rewriting the users of mru API to directly access
the list API.

Alas, there is no such mru API that lets a mru user to iterate over
elements, so the original of the above code were using mru's
implementation detail directly.

We probably want to invent mru_for_each() that hides the fact that
mru is implemented in terms of list_head from the users of mru API
and use it here.

> diff --git a/mru.c b/mru.c
> index 9dedae0287ed2..8b6ba3d9b7fad 100644
> --- a/mru.c
> +++ b/mru.c
> @@ -1,50 +1,29 @@
>  #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;
> -
> -	while (p) {
> -		struct mru_entry *to_free = p;
> -		p = p->next;
> +	struct list_head *p1;
> +	struct list_head *p2;
> +	struct mru *to_free;
> +	
> +	list_for_each_safe(p1, p2, &head->list) {
> +		to_free = list_entry(p1, struct mru, list);
>  		free(to_free);
>  	}
> -	mru->head = mru->tail = NULL;
> +	INIT_LIST_HEAD(&head->list);
>  }
> diff --git a/mru.h b/mru.h
> index 42e4aeaa1098a..36a332af0bf88 100644
> --- a/mru.h
> +++ b/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);

"Zero-initialization is required." is no longer true, it seems, and
the comment above needs to be updated, right?

More importantly, this leaks to the user of the API the fact that
mru is internally implemented in terms of the list API, which is
not necessary (when we want to update the implementation later, we'd
need to update all the users again).  Perhaps you'd want

	INIT_MRU(cache);

which is #define'd in this file, perhaps like so:

	#define INIT_MRU(mru)	INIT_LIST_HEAD(&((mru).list))



> - *   // Iterate in MRU order.
> - *   struct mru_entry *p;
> - *   for (p = cache.head; p; p = p->next) {
> - *	if (matches(p->item))
> - *		break;
> - *   }

Ah, here is a good piece that illustrates what I meant earlier.
This could have been something like:

	// Iterate in MRU order.
	struct mru *item;
	mru_for_each(item, &cache) {
		if (matches(item))
			break;
	}

and then the user would not have to know mru is implemented in terms
of the list API.

> + *   // 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 */
> diff --git a/packfile.c b/packfile.c
> index f69a5c8d607af..ae3b0b2e9c09a 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -876,6 +876,7 @@ void prepare_packed_git(void)
>  	for (alt = alt_odb_list; alt; alt = alt->next)
>  		prepare_packed_git_one(alt->path, 0);
>  	rearrange_packed_git();
> +	INIT_LIST_HEAD(&packed_git_mru.list);
>  	prepare_packed_git_mru();
>  	prepare_packed_git_run_once = 1;
>  }
> @@ -1824,13 +1825,14 @@ 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;

The same comment as the first hunk applies here, too.

Thanks.

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Thu, Sep 28, 2017 at 08:03:00PM +0900, Junio C Hamano wrote:

> > -	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;
> 
> I was a bit surprised to see a change outside mru.[ch] like this
> one.  The reason why I was surprised was because I expected mru.[ch]
> would offer its own API that encapsulates enumeration like this one
> and this patch would just be reimplementing that API using the list
> API, instead of rewriting the users of mru API to directly access
> the list API.
> 
> Alas, there is no such mru API that lets a mru user to iterate over
> elements, so the original of the above code were using mru's
> implementation detail directly.
> 
> We probably want to invent mru_for_each() that hides the fact that
> mru is implemented in terms of list_head from the users of mru API
> and use it here.

I agree that the caller would be a little shorter with an mru-specific
iterator (e.g., we could probably do the list_entry() part
automatically).

But I also think this patch may be a stepping stone to dropping "struct
mru" entirely, and just pushing a "struct list_head mru" into the
packed_git object itself (or of course any object you like). At which
point we'd just directly use the list iterators anyway.

(One could argue that if that's our end goal, we could go straight
there. But I think this middle state has value, because the individual
steps are easier to verify).

> > @@ -8,18 +10,15 @@
> >   *
> >   *   // Create a list.  Zero-initialization is required.
> >   *   static struct mru cache;
> > - *   mru_append(&cache, item);
> > - *   ...
> > + *   INIT_LIST_HEAD(&cache.list);
> 
> "Zero-initialization is required." is no longer true, it seems, and
> the comment above needs to be updated, right?
> 
> More importantly, this leaks to the user of the API the fact that
> mru is internally implemented in terms of the list API, which is
> not necessary (when we want to update the implementation later, we'd
> need to update all the users again).  Perhaps you'd want
> 
> 	INIT_MRU(cache);
> 
> which is #define'd in this file, perhaps like so:
> 
> 	#define INIT_MRU(mru)	INIT_LIST_HEAD(&((mru).list))

I'd make the same claims here as above (both that I agree your proposed
interface looks nicer, but also that I think we eventually do want to
expose that this is tightly coupled with list.h).

-Peff

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Thu, Sep 28, 2017 at 08:38:39AM +0000, Olga Telezhnaya wrote:

> Simplify mru.c, mru.h and related code by reusing the double-linked
> list implementation from list.h instead of a custom one.

The commit message is a good reason to talk about why we want to do
this. In this case, the answer may be fairly obvious. But I sometimes
find that things that are obvious to me as the patch author are not
quite as obvious to people reading it later (either reviewing, or six
months from now when they are hunting the cause of a bug).

> -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);
>  }

Nice, this hunk is very satisfying. :)

> -void mru_clear(struct mru *mru)
> +void mru_clear(struct mru *head)
>  {
> -	struct mru_entry *p = mru->head;
> -
> -	while (p) {
> -		struct mru_entry *to_free = p;
> -		p = p->next;
> +	struct list_head *p1;
> +	struct list_head *p2;
> +	struct mru *to_free;
> +	
> +	list_for_each_safe(p1, p2, &head->list) {
> +		to_free = list_entry(p1, struct mru, list);
>  		free(to_free);
>  	}
> -	mru->head = mru->tail = NULL;
> +	INIT_LIST_HEAD(&head->list);

Two minor style comments here:

  - Perhaps "tmp" is a better name than "p2" for the second argument of
    a list_for_each_safe, as it makes it less likely to confuse p1 and
    p2 (though admittedly the whole function is short enough that it
    probably doesn't matter much either way).

  - It's a good practice to declare variables in the smallest scope
    possible. So I think the declaration of to_free could go inside the
    loop.

    You could actually get rid of it entirely with:

      free(list_entry(p1, struct mru, list));

    but I certainly don't mind using a variable for better readability.

> @@ -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;
>  };

The decision to get rid of the "mru versus mru_entry" distinction
surprised me a little. In the original, a "struct mru" represented the
whole list. In the list.h implementation, a "struct list_head" serves
that purpose, as a sentinel value. But that sentinel doesn't need to
have an "item", right? I.e., we could have:

  struct mru {
          struct list_head head;
  };

  struct mru_entry {
          void *item;
	  struct list_head list;
  };

As I said in my response to Junio (and as we discussed a little
off-list), I think we can eventually move to having no structs at all
(just list_heads embedded inside the existing packfile objects). At
which point the user of the API would just declare:

  LIST_HEAD(packed_git_mru);

themselves. So I'm actually fine with this direction if we're using it
as the "middle step" that I mentioned there.

>  struct mru {
> -	struct mru_entry *head, *tail;
> +	struct list_head list;
> +        void *item;
>  };

The funny indentation in this diff shows that "void *item" is indented
with spaces, not a tab.

> [...]

I pointed out a few minor bits, but overall this is looking very strong.
Great work!

-Peff

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Junio C Hamano]

Jeff King <peff@peff.net> writes:

> But I also think this patch may be a stepping stone to dropping "struct
> mru" entirely, and just pushing a "struct list_head mru" into the
> packed_git object itself (or of course any object you like). At which
> point we'd just directly use the list iterators anyway.

The endgame state implied by your statement would mean that we won't
have mru_mark() and instead have these open-coded in terms of the
two calls into the list API.  There only are two callers of it in
the current system, so it is not the end of the world, so I guess I
can agree that this is a good preparation step toward the longer
term goal, which says "mru API is over-engineered and what it offers
over the plain vanilla list API is almost never used except for a
few callsite; let's remove it and use the bare list API instead".

> I'd make the same claims here as above (both that I agree your proposed
> interface looks nicer, but also that I think we eventually do want to
> expose that this is tightly coupled with list.h).

I agree.  I just do not yet know if I fully embrace the idea that we
just should use bare list API, getting rid of the mru API.

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Fri, Sep 29, 2017 at 06:56:28AM +0900, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > But I also think this patch may be a stepping stone to dropping "struct
> > mru" entirely, and just pushing a "struct list_head mru" into the
> > packed_git object itself (or of course any object you like). At which
> > point we'd just directly use the list iterators anyway.
> 
> The endgame state implied by your statement would mean that we won't
> have mru_mark() and instead have these open-coded in terms of the
> two calls into the list API.  There only are two callers of it in
> the current system, so it is not the end of the world, so I guess I
> can agree that this is a good preparation step toward the longer
> term goal, which says "mru API is over-engineered and what it offers
> over the plain vanilla list API is almost never used except for a
> few callsite; let's remove it and use the bare list API instead".

Thanks, that last sentence is a good summary of my thinking (I think
what I find most silly about it is that it allocates a whole extra
struct per item, which is where most of the complication comes from).

I had envisioned leaving mru_mark() as a wrapper for "move to the front"
that could operate on any list. But seeing how Olga's patch takes it
down to two trivial lines, I'd also be fine with an endgame that just
eliminates it.

> > I'd make the same claims here as above (both that I agree your proposed
> > interface looks nicer, but also that I think we eventually do want to
> > expose that this is tightly coupled with list.h).
> 
> I agree.  I just do not yet know if I fully embrace the idea that we
> just should use bare list API, getting rid of the mru API.

Fair enough.

-Peff

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Thu, Sep 28, 2017 at 08:38:39AM +0000, Olga Telezhnaya wrote:

> diff --git a/packfile.c b/packfile.c
> index f69a5c8d607af..ae3b0b2e9c09a 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -876,6 +876,7 @@ void prepare_packed_git(void)
>  	for (alt = alt_odb_list; alt; alt = alt->next)
>  		prepare_packed_git_one(alt->path, 0);
>  	rearrange_packed_git();
> +	INIT_LIST_HEAD(&packed_git_mru.list);
>  	prepare_packed_git_mru();
>  	prepare_packed_git_run_once = 1;
>  }

I was thinking on this hunk a bit more, and I think it's not quite
right.

The prepare_packed_git_mru() function will clear the mru list and then
re-add each item from the packed_git list. But by calling
INIT_LIST_HEAD() here, we're effectively clearing the packed_git_mru
list, and we end up leaking whatever was on the list before.

So for the first call to prepare_packed_git, we really need this
INIT_LIST_HEAD() call. But for subsequent calls (which come from
reprepare_packed_git()), we must not call it.

There are a few ways to work around it that I can think of:

  1. Check whether packed_git_mru.list.head is NULL, and only initialize
     in that case.

  2. Use a static initializer for packed_git_mru.list, so that we don't
     have do the first-time initializing here.

  3. Teach reprepare_packed_git() to do the mru_clear() call, so that we
     know the list is empty when we get here.

One final and more invasive option is to stop regenerating the
packed_git_mru list from scratch during each prepare_packed_git(). I did
it that way so that we start with the same order that
rearrange_packed_git() will give us, but I'm not sure how much value
that has in practice (it probably had a lot more when we didn't have the
mru, and the time-sorted pack order helped find recent objects more
quickly).

The alternative would be to just teach install_packed_git() to add each
newly-added pack to the mru list, and then never clear the list (and we
wouldn't need an mru_clear() at all, then).

-Peff

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Christian Couder]

On Fri, Sep 29, 2017 at 12:42 AM, Jeff King <peff@peff.net> wrote:
> On Thu, Sep 28, 2017 at 08:38:39AM +0000, Olga Telezhnaya wrote:
>
>> diff --git a/packfile.c b/packfile.c
>> index f69a5c8d607af..ae3b0b2e9c09a 100644
>> --- a/packfile.c
>> +++ b/packfile.c
>> @@ -876,6 +876,7 @@ void prepare_packed_git(void)
>>       for (alt = alt_odb_list; alt; alt = alt->next)
>>               prepare_packed_git_one(alt->path, 0);
>>       rearrange_packed_git();
>> +     INIT_LIST_HEAD(&packed_git_mru.list);
>>       prepare_packed_git_mru();
>>       prepare_packed_git_run_once = 1;
>>  }
>
> I was thinking on this hunk a bit more, and I think it's not quite
> right.
>
> The prepare_packed_git_mru() function will clear the mru list and then
> re-add each item from the packed_git list. But by calling
> INIT_LIST_HEAD() here, we're effectively clearing the packed_git_mru
> list, and we end up leaking whatever was on the list before.

The current code is:

static int prepare_packed_git_run_once = 0;
void prepare_packed_git(void)
{
    struct alternate_object_database *alt;

    if (prepare_packed_git_run_once)
        return;
    prepare_packed_git_one(get_object_directory(), 1);
    prepare_alt_odb();
    for (alt = alt_odb_list; alt; alt = alt->next)
        prepare_packed_git_one(alt->path, 0);
    rearrange_packed_git();
    prepare_packed_git_mru();
    prepare_packed_git_run_once = 1;
}

As we use the "prepare_packed_git_run_once" static, this function will
only be called only once when packed_git_mru has not yet been
initialized, so there will be no leak.

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Fri, Sep 29, 2017 at 09:18:11AM +0200, Christian Couder wrote:

> On Fri, Sep 29, 2017 at 12:42 AM, Jeff King <peff@peff.net> wrote:
> > On Thu, Sep 28, 2017 at 08:38:39AM +0000, Olga Telezhnaya wrote:
> >
> >> diff --git a/packfile.c b/packfile.c
> >> index f69a5c8d607af..ae3b0b2e9c09a 100644
> >> --- a/packfile.c
> >> +++ b/packfile.c
> >> @@ -876,6 +876,7 @@ void prepare_packed_git(void)
> >>       for (alt = alt_odb_list; alt; alt = alt->next)
> >>               prepare_packed_git_one(alt->path, 0);
> >>       rearrange_packed_git();
> >> +     INIT_LIST_HEAD(&packed_git_mru.list);
> >>       prepare_packed_git_mru();
> >>       prepare_packed_git_run_once = 1;
> >>  }
> >
> > I was thinking on this hunk a bit more, and I think it's not quite
> > right.
> >
> > The prepare_packed_git_mru() function will clear the mru list and then
> > re-add each item from the packed_git list. But by calling
> > INIT_LIST_HEAD() here, we're effectively clearing the packed_git_mru
> > list, and we end up leaking whatever was on the list before.
> 
> The current code is:
> 
> static int prepare_packed_git_run_once = 0;
> void prepare_packed_git(void)
> {
>     struct alternate_object_database *alt;
> 
>     if (prepare_packed_git_run_once)
>         return;
>     prepare_packed_git_one(get_object_directory(), 1);
>     prepare_alt_odb();
>     for (alt = alt_odb_list; alt; alt = alt->next)
>         prepare_packed_git_one(alt->path, 0);
>     rearrange_packed_git();
>     prepare_packed_git_mru();
>     prepare_packed_git_run_once = 1;
> }
> 
> As we use the "prepare_packed_git_run_once" static, this function will
> only be called only once when packed_git_mru has not yet been
> initialized, so there will be no leak.

Check reprepare_packed_git(). It unsets the run_once flag, and then
calls prepare_packed_git() again.

-Peff

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Christian Couder]

On Fri, Sep 29, 2017 at 9:23 AM, Jeff King <peff@peff.net> wrote:
> On Fri, Sep 29, 2017 at 09:18:11AM +0200, Christian Couder wrote:
>
>> As we use the "prepare_packed_git_run_once" static, this function will
>> only be called only once when packed_git_mru has not yet been
>> initialized, so there will be no leak.
>
> Check reprepare_packed_git(). It unsets the run_once flag, and then
> calls prepare_packed_git() again.

Ah ok.

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Оля Тележная]

Hi everyone,
Many thanks to all of you, I am interested in every opinion. Sorry
that I wasn't in the discussion, unfortunately I got sick, that's why
I skipped all the process.
I want to reply to the main moments and also ask some questions.

>> Simplify mru.c, mru.h and related code by reusing the double-linked list implementation from list.h instead of a custom one.
> An overlong line (I can locally wrap it, so the patch does not have
> to be re-sent only to fix this alone).
I've read only about 50 characters max in commit head (and
highlighting repeats it), but there's nothing about max length of line
in commit message. Sorry, next time I will make it shorter.

About many different opinions how to improve the code: I agree with
the idea that my commit is a middle step to get rid of MRU at all. If
we really need to add initializer/mru_for_each/smth_else - it's
absolutely not a problem, but as it was said, not sure that we need
it.
It really looks that using list implementation from list.h directly
won't be worse.

> I had envisioned leaving mru_mark() as a wrapper for "move to the front"
> that could operate on any list. But seeing how Olga's patch takes it
> down to two trivial lines, I'd also be fine with an endgame that just
> eliminates it.
Let's add needed function to list.h directly? I also wanted to add
list_for_each_entry function to list.h as it's in Linux kernel.
https://www.kernel.org/doc/htmldocs/kernel-api/API-list-for-each-entry.html
It will simplify the code even more, guess that not only in MRU
related code. Maybe we need to do that in separate patch.

About minor issues ( "tmp" vs "p2", variable scope, space indentation)
- fully agree, I will fix it.

So finally I think that I need to fix that minor issues and that's
all. I have plans to rewrite (with --amend) my current commit (I think
so because I will add no new features, so it's better to have single
commit for all changes).
As I understand, Submitgit will send an update in a new thread. And I
need to say there [PATCH v2].
Please correct me if I am wrong in any of the moments mentioned earlier.

By the way, other contributors write smth like "[PATCH v6 0/3]". What
does mean "0/3"? It's about editing separate commits in a single
patch, am I right?

Thank you one more time!
Olga

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Оля Тележная]

> About minor issues ( "tmp" vs "p2", variable scope, space indentation)
> - fully agree, I will fix it.
> ...
> So finally I think that I need to fix that minor issues and that's
> all.

I forgot about leak. I also need to add checking in mru_clear. That's
not beautiful solution but it works reliably.
Question remains the same - do you see something more to fix in this patch?

Thank you!
Olga

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Fri, Sep 29, 2017 at 07:08:28PM +0300, Оля Тележная wrote:

> Many thanks to all of you, I am interested in every opinion. Sorry
> that I wasn't in the discussion, unfortunately I got sick, that's why
> I skipped all the process.

No problem. It's often reasonable to let review comments come in and
think about them as a whole before responding or re-posting anyway.

> > An overlong line (I can locally wrap it, so the patch does not have
> > to be re-sent only to fix this alone).
> I've read only about 50 characters max in commit head (and
> highlighting repeats it), but there's nothing about max length of line
> in commit message. Sorry, next time I will make it shorter.

Usually we shoot for making things look good in an 80-column terminal,
including both code and commit messages. For commit message bodies, we
tend to make them a little shorter, since "git log" will indent them. 72
characters is reasonable there. And we tend to make subject lines a
little shorter than that, even since they often appear with "Subject:"
and "[PATCH]" prefixed. I usually go for about 60 characters, but will
go over if it helps make the subject more clear.

> > I had envisioned leaving mru_mark() as a wrapper for "move to the front"
> > that could operate on any list. But seeing how Olga's patch takes it
> > down to two trivial lines, I'd also be fine with an endgame that just
> > eliminates it.
> Let's add needed function to list.h directly?

Yes, I think we could just call this "list_move_to_front()" or
something. The fact that it's operating on a list called
"packed_git_mru" is probably sufficient to make it clear that the
purpose is managing recentness.

> I also wanted to add
> list_for_each_entry function to list.h as it's in Linux kernel.
> https://www.kernel.org/doc/htmldocs/kernel-api/API-list-for-each-entry.html
> It will simplify the code even more, guess that not only in MRU
> related code. Maybe we need to do that in separate patch.

It would be nice to have list_for_each_entry(), but unfortunately it's
not portable. It relies on having a typeof operator, and we build on
platforms that lack it. It was omitted in 94e99012fc (http-walker:
reduce O(n) ops with doubly-linked list, 2016-07-11) for that reason.

> About minor issues ( "tmp" vs "p2", variable scope, space indentation)
> - fully agree, I will fix it.

Thanks.

> So finally I think that I need to fix that minor issues and that's
> all. I have plans to rewrite (with --amend) my current commit (I think
> so because I will add no new features, so it's better to have single
> commit for all changes).
> As I understand, Submitgit will send an update in a new thread. And I
> need to say there [PATCH v2].
> Please correct me if I am wrong in any of the moments mentioned earlier.

Correct. Until a patch is merged to Junio's "next" branch (at which
point it is set in stone), we generally prefer to rewrite it with
"--amend" (or git-rebase) to fix anything that comes up during the
review.

> By the way, other contributors write smth like "[PATCH v6 0/3]". What
> does mean "0/3"? It's about editing separate commits in a single
> patch, am I right?

Right, it means multiple commits in a logical series that are meant to
be applied together. Your patch is small enough that it makes as a
single patch. If we wanted to do the second step of dropping mru.[ch]
entirely now, then you'd probably have at least a 2-patch series.

(I'm OK with not doing that second step for now, though if we are not
going to polish up the mru_for_each() interface, it may make sense to
make the final step sooner rather than later).

-Peff

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Fri, Sep 29, 2017 at 11:38:27PM +0300, Оля Тележная wrote:

> > About minor issues ( "tmp" vs "p2", variable scope, space indentation)
> > - fully agree, I will fix it.
> > ...
> > So finally I think that I need to fix that minor issues and that's
> > all.
> 
> I forgot about leak. I also need to add checking in mru_clear. That's
> not beautiful solution but it works reliably.

I'm not sure what you mean about checking in mru_clear(). It may make
sense to just send your v2 patch and I can see there what you do.

> Question remains the same - do you see something more to fix in this patch?

I think that's it. Think about writing a bit in the commit message about
the "why" of this. Like I said earlier, the rationale for this commit is
perhaps obvious, but it never hurts to be explicit (and it may be good
practice, since part of the point of this task is getting you familiar
with the patch submission process).

-Peff

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Junio C Hamano]

Jeff King <peff@peff.net> writes:

> Yes, I think we could just call this "list_move_to_front()" or
> something. The fact that it's operating on a list called
> "packed_git_mru" is probably sufficient to make it clear that the
> purpose is managing recentness.

I earlier said I wasn't sure, but I fully agree with your envisioned
endgame state, my understanding of which is that we will not have an
API that is only to be used to manage a MRU list (hence there will
be no mru.[ch] in that future), as there is very small MRU-specific
operation to be offered anyway.  Namely, mru_mark() that is to be
used to "mark the fact that the item was used---do whatever necessary
to maintain the MRU list".

Instead, everybody understands how the list API works, and the fact
that one instance of list_head is called MRU is sufficient to see
that use of "move this to the front of the list" function means we
are marking the item as most-recently-used.

Thanks.

[PATCH v2 Outreachy] mru: use double-linked list from list.h

[Olga Telezhnaya]

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.

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored by: Jeff King <peff@peff.net>
---
 builtin/pack-objects.c |  5 +++--
 mru.c                  | 49 +++++++++++++------------------------------------
 mru.h                  | 31 +++++++++++++------------------
 packfile.c             |  7 ++++---
 4 files changed, 33 insertions(+), 59 deletions(-)

diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index f721137eaf881..ba812349e0aab 100644
--- a/builtin/pack-objects.c
+++ b/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;
 
diff --git a/mru.c b/mru.c
index 9dedae0287ed2..8f3f34c5ba91d 100644
--- a/mru.c
+++ b/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);
 }
diff --git a/mru.h b/mru.h
index 42e4aeaa1098a..80a589eb4c0eb 100644
--- a/mru.h
+++ b/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 */
diff --git a/packfile.c b/packfile.c
index f69a5c8d607af..502d915991bee 100644
--- a/packfile.c
+++ b/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;

--
https://github.com/git/git/pull/409

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Оля Тележная]

I added "v2" after "PATCH", but it does not appeared. Actually it was
written automatically and it was "PATCH Outreachy v2". I rearranged it
in the middle of the phrase.

>> I forgot about leak. I also need to add checking in mru_clear. That's
>> not beautiful solution but it works reliably.
>
> I'm not sure what you mean about checking in mru_clear(). It may make
> sense to just send your v2 patch and I can see there what you do.

I realized that I said something strange before. I solved the problem
with leak by deleting INIT in prepare_packed_git and adding init as
you suggested here:
https://github.com/telezhnaya/git/commit/7f8995835949f83e54efdfd88445feb6d54cb3e9#commitcomment-24576103

By the way, I asked to edit FAQ for Linux kernel newbies about linked
list that confused me a week ago, and that funny picture was deleted.
https://kernelnewbies.org/FAQ/LinkedLists
Maybe it will help to someone else :)

Re: [PATCH v2 Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Sat, Sep 30, 2017 at 05:51:01PM +0000, Olga Telezhnaya wrote:

> 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.

Thanks, this version looks correct to me.

I do think there are a few ugly bits in the result (like that
initializer for packed_git_mru :) ), so I'd prefer not to merge this
down until we do that final step.

So the big question is: who wants to do it?

I think you've done a good job here, and this would count for your
Outreachy application's contribution. But if you'd like to do that next
step, you are welcome to.

We could also consider it a #leftoverbits that perhaps some other
Outreachy candidate would pick up[1].

In the meantime, Junio, I think we'd want to queue this with the intent
to graduate it to "pu" or possibly "next", but not "master". Then if
somebody (Olga or another applicant) produces the endgame patch, we can
queue it on top and move it further. And if nobody does, I can pick it
after the application period is over.

-Peff

[1] For those who find this mail through the archive, there's more
    discussion in this thread:

      https://public-inbox.org/git/CAL21BmnvJSaN+Tnw7Hdc5P5biAnM5dfWR7gX5FrAG1r_D8th=A@mail.gmail.com/

Re: [PATCH Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Sat, Sep 30, 2017 at 09:09:11PM +0300, Оля Тележная wrote:

> I added "v2" after "PATCH", but it does not appeared. Actually it was
> written automatically and it was "PATCH Outreachy v2". I rearranged it
> in the middle of the phrase.

That looks fine.

> > I'm not sure what you mean about checking in mru_clear(). It may make
> > sense to just send your v2 patch and I can see there what you do.
> 
> I realized that I said something strange before. I solved the problem
> with leak by deleting INIT in prepare_packed_git and adding init as
> you suggested here:
> https://github.com/telezhnaya/git/commit/7f8995835949f83e54efdfd88445feb6d54cb3e9#commitcomment-24576103

OK, that makes sense.

> By the way, I asked to edit FAQ for Linux kernel newbies about linked
> list that confused me a week ago, and that funny picture was deleted.
> https://kernelnewbies.org/FAQ/LinkedLists
> Maybe it will help to someone else :)

Thank you. :)

Remember (and this applies to other Outreachy folks, too) to submit your
application in the Outreachy system:

   https://outreachy.gnome.org

-Peff

Re: [PATCH v2 Outreachy] mru: use double-linked list from list.h

[Оля Тележная]

>> 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.
>
> Thanks, this version looks correct to me.

Great! What is better - to complete my application now (and say only
about this patch) or to complete it in last days (and say about
everything that I've done. Maybe there would be something new).

> I do think there are a few ugly bits in the result (like that
> initializer for packed_git_mru :) ), so I'd prefer not to merge this
> down until we do that final step.
>
> So the big question is: who wants to do it?
>
> I think you've done a good job here, and this would count for your
> Outreachy application's contribution. But if you'd like to do that next
> step, you are welcome to.

I was thinking about starting my small research about main task of the
internship. I could postpone it and end with this task, there's no
problem for me.
Or, if someone from the newbies wants to have a small simple task -
it's a great opportunity for him/her. By the way, I am ready to help
if he/she will have questions or difficulties.
So please make a decision on your own, I will be happy in any scenario.

Re: [PATCH v2 Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Mon, Oct 02, 2017 at 12:37:53PM +0300, Оля Тележная wrote:

> >> 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.
> >
> > Thanks, this version looks correct to me.
> 
> Great! What is better - to complete my application now (and say only
> about this patch) or to complete it in last days (and say about
> everything that I've done. Maybe there would be something new).

I think it's good to get it started now. You can always make changes to
it later. And in particular, you should fill out the eligibility section
s we can make sure that part is good before going further.

> > I think you've done a good job here, and this would count for your
> > Outreachy application's contribution. But if you'd like to do that next
> > step, you are welcome to.
> 
> I was thinking about starting my small research about main task of the
> internship. I could postpone it and end with this task, there's no
> problem for me.

I do think you should spend some time thinking about and researching the
main task. I'd hope to see in the applications that the candidate has
a sense of the problem space and a realistic plan for approaching the
work.

> Or, if someone from the newbies wants to have a small simple task -
> it's a great opportunity for him/her. By the way, I am ready to help
> if he/she will have questions or difficulties.

Wonderful. :) I do hope to get Outreachy interns involve in normal list
activities like discussion and review, not just producing their own
patches.

> So please make a decision on your own, I will be happy in any scenario.

Why don't we give it a week or so to see if anybody else picks it up.

-Peff

Re: [PATCH v2 Outreachy] mru: use double-linked list from list.h

[Junio C Hamano]

Jeff King <peff@peff.net> writes:

> I do think there are a few ugly bits in the result (like that
> initializer for packed_git_mru :) ), so I'd prefer not to merge this
> down until we do that final step.
>
> So the big question is: who wants to do it?
>
> I think you've done a good job here, and this would count for your
> Outreachy application's contribution. But if you'd like to do that next
> step, you are welcome to.
>
> We could also consider it a #leftoverbits that perhaps some other
> Outreachy candidate would pick up[1].
>
> In the meantime, Junio, I think we'd want to queue this with the intent
> to graduate it to "pu" or possibly "next", but not "master". Then if
> somebody (Olga or another applicant) produces the endgame patch, we can
> queue it on top and move it further. And if nobody does, I can pick it
> after the application period is over.

So... do we still want to keep this in 'next', or does somebody
wants to do honors?  

I'd really prefer *not* to see you or me ending up finishing it up,
but at the same time, I really hate seeing a halfway or 3/4 done
topic hanging around in 'pu'.

> [1] For those who find this mail through the archive, there's more
>     discussion in this thread:
>
>       https://public-inbox.org/git/CAL21BmnvJSaN+Tnw7Hdc5P5biAnM5dfWR7gX5FrAG1r_D8th=A@mail.gmail.com/

Re: [PATCH v2 Outreachy] mru: use double-linked list from list.h

[Jeff King]

On Wed, Nov 08, 2017 at 10:44:46AM +0900, Junio C Hamano wrote:

> > We could also consider it a #leftoverbits that perhaps some other
> > Outreachy candidate would pick up[1].
> >
> > In the meantime, Junio, I think we'd want to queue this with the intent
> > to graduate it to "pu" or possibly "next", but not "master". Then if
> > somebody (Olga or another applicant) produces the endgame patch, we can
> > queue it on top and move it further. And if nobody does, I can pick it
> > after the application period is over.
> 
> So... do we still want to keep this in 'next', or does somebody
> wants to do honors?  
> 
> I'd really prefer *not* to see you or me ending up finishing it up,
> but at the same time, I really hate seeing a halfway or 3/4 done
> topic hanging around in 'pu'.

We hung back on it to leave it as low-hanging fruit for other Outreachy
applicants. Perhaps Olga would like to pick it up now that the
application period is over.

Alternatively, it might be a nice task for the Bloomberg hackathon this
weekend. I'll add it to the list of topics there, too.

-Peff

Re: [PATCH v2 Outreachy] mru: use double-linked list from list.h

[Оля Тележная]

> We hung back on it to leave it as low-hanging fruit for other Outreachy
> applicants. Perhaps Olga would like to pick it up now that the
> application period is over.

It's absolutely not a problem for me, I can do that as one more
warm-up exercise in the beginning of the internship.

Thanks!