threads / patch / 31503

patchclear_child_for_cleanup must correctly manage children_to_clean

Subject: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean

## tl;dr

6 messages between Sep 11, 2012 and Sep 11, 2012. Diffs are folded; open one to read it.

replies: 5people: 4as markdown or json

David Gould· Sep 11, 2012, 14:32 UTC · lore
Iterate through children_to_clean using 'next' fields but with an
extra level of indirection. This allows us to update the chain when
we remove a child and saves us managing several variables around
the loop mechanism.
---
 run-command.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)
Show changes to run-command.c +6 −6
diff --git a/run-command.c b/run-command.c
index f9922b9..c42d63b 100644
--- a/run-command.c
+++ b/run-command.c
@@ -53,13 +53,13 @@ static void mark_child_for_cleanup(pid_t pid)
 
 static void clear_child_for_cleanup(pid_t pid)
 {
-	struct child_to_clean **last, *p;
+	struct child_to_clean **pp;
 
-	last = &children_to_clean;
-	for (p = children_to_clean; p; p = p->next) {
-		if (p->pid == pid) {
-			*last = p->next;
-			free(p);
+	for (pp = &children_to_clean; *pp; pp = &(*pp)->next) {
+		if ((*pp)->pid == pid) {
+			struct child_to_clean *clean_me = *pp;
+			*pp = clean_me->next;
+			free(clean_me);
 			return;
 		}
 	}

-- 
1.7.9.5
Jeff King· Sep 11, 2012, 15:20 UTC · re: David Gould · lore

Re: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean

On Tue, Sep 11, 2012 at 03:32:47PM +0100, David Gould wrote:
> Subject: Re: [PATCH] clear_child_for_cleanup must correctly manage
>   children_to_clean

Thanks for the patch. Overall it looks good, but let me nit-pick your commit message a little (not because it is that horrible, but because you are so close to perfect that I want to fix the minor things and then encourage you to submit more patches :) ).

Your subject is a bit vague, and it is not clear if it is not correct now, and this is a bugfix, or if it is a feature enhancement. I would have said something like:

  Subject: fix broken list iteration in clear_child_for_cleanup

which is _also_ vague about what exactly the breakage is, but is clear that this is a bugfix. So then you can go on to describe the actual problem:

  We iterate through the list of children to cleanup, but do not keep
  our "last" pointer up to date. As a result, we may end up cutting off
  part of the list instead of removing a single element.
And then describe your fix:
> Iterate through children_to_clean using 'next' fields but with an
> extra level of indirection. This allows us to update the chain when
> we remove a child and saves us managing several variables around
> the loop mechanism.
which I think is good.
Show 12 quoted lines
> -	last = &children_to_clean;
> -	for (p = children_to_clean; p; p = p->next) {
> -		if (p->pid == pid) {
> -			*last = p->next;
> -			free(p);
> +	for (pp = &children_to_clean; *pp; pp = &(*pp)->next) {
> +		if ((*pp)->pid == pid) {
> +			struct child_to_clean *clean_me = *pp;
> +			*pp = clean_me->next;
> +			free(clean_me);
>  			return;
>  		}

I think using the indirect pointer is a nice compromise; it makes it clear from just the for loop that this is not an ordinary for-each traversal. You could hoist the extra pointer out of the conditional and save one set of parentheses in the "if" statement, but I don't think it is a big deal either way.

Acked-by: Jeff King <peff@peff.net>
Thanks for the bug report and the patch.
-Peff
Junio C Hamano· Sep 11, 2012, 17:32 UTC · re: Jeff King · lore

Re: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean

Jeff King <peff@peff.net> writes:
Show 7 quoted lines
> Thanks for the patch. Overall it looks good, but let me nit-pick your
> commit message a little (not because it is that horrible, but because
> you are so close to perfect that I want to fix the minor things and then
> encourage you to submit more patches :) ).
> ...
>
> Acked-by: Jeff King <peff@peff.net>
Thanks, both.  I'd have to ask for a Sign-off, though.
Erik Faye-Lund· Sep 11, 2012, 15:30 UTC · re: David Gould · lore

Re: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean

On Tue, Sep 11, 2012 at 4:32 PM, David Gould <david@optimisefitness.com> wrote:
Show 35 quoted lines
> Iterate through children_to_clean using 'next' fields but with an
> extra level of indirection. This allows us to update the chain when
> we remove a child and saves us managing several variables around
> the loop mechanism.
> ---
>  run-command.c |   12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/run-command.c b/run-command.c
> index f9922b9..c42d63b 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -53,13 +53,13 @@ static void mark_child_for_cleanup(pid_t pid)
>
>  static void clear_child_for_cleanup(pid_t pid)
>  {
> -       struct child_to_clean **last, *p;
> +       struct child_to_clean **pp;
>
> -       last = &children_to_clean;
> -       for (p = children_to_clean; p; p = p->next) {
> -               if (p->pid == pid) {
> -                       *last = p->next;
> -                       free(p);
> +       for (pp = &children_to_clean; *pp; pp = &(*pp)->next) {
> +               if ((*pp)->pid == pid) {
> +                       struct child_to_clean *clean_me = *pp;
> +                       *pp = clean_me->next;
> +                       free(clean_me);
>                         return;
>                 }
>         }
> --
> 1.7.9.5
>
Looks good to me. Thanks :)
David Gould· Sep 11, 2012, 20:22 UTC · re: David Gould · lore

[PATCHv2] fix broken list iteration in clear_child_for_cleanup

We iterate through the list of children to cleanup, but do not keep our "last" pointer up to date. As a result, we may end up cutting off part of the list instead of removing a single element.

Iterate through children_to_clean using 'next' fields but with an extra level of indirection. This allows us to update the chain when we remove a child and saves us managing several variables around the loop mechanism.

Signed-off-by: David Gould <david@optimisefitness.com>
Acked-by: Jeff King <peff@peff.net>
---
PATCHv2 updates PATCH only in the commit message: Peff suggested
both a helpful subject and a more-informative introduction.
 run-command.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)
Show changes to run-command.c +6 −6
diff --git a/run-command.c b/run-command.c
index f9922b9..c42d63b 100644
--- a/run-command.c
+++ b/run-command.c
@@ -53,13 +53,13 @@ static void mark_child_for_cleanup(pid_t pid)
 
 static void clear_child_for_cleanup(pid_t pid)
 {
-	struct child_to_clean **last, *p;
+	struct child_to_clean **pp;
 
-	last = &children_to_clean;
-	for (p = children_to_clean; p; p = p->next) {
-		if (p->pid == pid) {
-			*last = p->next;
-			free(p);
+	for (pp = &children_to_clean; *pp; pp = &(*pp)->next) {
+		if ((*pp)->pid == pid) {
+			struct child_to_clean *clean_me = *pp;
+			*pp = clean_me->next;
+			free(clean_me);
 			return;
 		}
 	}

-- 
1.7.9.5

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