# [PATCH] clear_child_for_cleanup must correctly manage children_to_clean 6 messages from 2012-09-11 to 2012-09-11. Participants: David Gould, Jeff King, Erik Faye-Lund, Junio C Hamano. Thread: https://gitlist.dev/t/31503 ## David Gould, 2012-09-11 14:32 Subject: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean Message-ID: <1347373967-29248-1-git-send-email-david@optimisefitness.com> URL: https://gitlist.dev/e/1347373967-29248-1-git-send-email-david%40optimisefitness.com In-Reply-To: <62cd8d4a1853cb6fe8fda9f534cc269c8b2e0f6c> ``` 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 ``` ## Jeff King, 2012-09-11 15:20 Subject: Re: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean Message-ID: <20120911152041.GA11994@sigill.intra.peff.net> URL: https://gitlist.dev/e/20120911152041.GA11994%40sigill.intra.peff.net In-Reply-To: <1347373967-29248-1-git-send-email-david@optimisefitness.com> ``` 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 Thanks for the bug report and the patch. -Peff ``` ## Erik Faye-Lund, 2012-09-11 15:30 Subject: Re: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean Message-ID: URL: https://gitlist.dev/e/CABPQNSYEiw2hw-S5x5YYJWrWYR6hNWBqudvHR%3DfxoHf0380jsA%40mail.gmail.com In-Reply-To: <1347373967-29248-1-git-send-email-david@optimisefitness.com> ``` On Tue, Sep 11, 2012 at 4:32 PM, David Gould wrote: > 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 :) ``` ## Junio C Hamano, 2012-09-11 17:32 Subject: Re: [PATCH] clear_child_for_cleanup must correctly manage children_to_clean Message-ID: <7vpq5sh3dw.fsf@alter.siamese.dyndns.org> URL: https://gitlist.dev/e/7vpq5sh3dw.fsf%40alter.siamese.dyndns.org In-Reply-To: <20120911152041.GA11994@sigill.intra.peff.net> ``` Jeff King writes: > 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 Thanks, both. I'd have to ask for a Sign-off, though. ``` ## David Gould, 2012-09-11 20:22 Subject: [PATCHv2] fix broken list iteration in clear_child_for_cleanup Message-ID: <1347394965-29906-1-git-send-email-david@optimisefitness.com> URL: https://gitlist.dev/e/1347394965-29906-1-git-send-email-david%40optimisefitness.com In-Reply-To: <1347373967-29248-1-git-send-email-david@optimisefitness.com> ``` 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 Acked-by: Jeff King --- 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 ``` ## Junio C Hamano, 2012-09-11 20:32 Subject: Re: [PATCHv2] fix broken list iteration in clear_child_for_cleanup Message-ID: <7vzk4wfggd.fsf@alter.siamese.dyndns.org> URL: https://gitlist.dev/e/7vzk4wfggd.fsf%40alter.siamese.dyndns.org In-Reply-To: <1347394965-29906-1-git-send-email-david@optimisefitness.com> ``` Thanks. ```