[PATCH] clear_child_for_cleanup must correctly manage children_to_clean

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

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.

-	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 <redacted>

Thanks for the bug report and the patch.

-Peff

On Tue, Sep 11, 2012 at 4:32 PM, David Gould [off-list ref] wrote:

Looks good to me. Thanks :)

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 <redacted>
Acked-by: Jeff King <redacted>
---
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(-)

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