Subject: Re: [PATCH] bisect: save heap memory. allocate only the required amount

From: Jeff King <[email-masked]>
Date: Mon, 25 Aug 2014 09:35:50 -0400

On Sun, Aug 24, 2014 at 07:47:24PM +0530, Arjun Sreedharan wrote:

> diff --git a/bisect.c b/bisect.c
> index d6e851d..c96aab0 100644
> --- a/bisect.c
> +++ b/bisect.c
> @@ -215,10 +215,13 @@ static struct commit_list *best_bisection_sorted(struct commit_list *list, int n
>  	}
>  	qsort(array, cnt, sizeof(*array), compare_commit_dist);
>  	for (p = list, i = 0; i < cnt; i++) {
> -		struct name_decoration *r = xmalloc(sizeof(*r) + 100);
> +		char name[100];
> +		sprintf(name, "dist=%d", array[i].distance);
> +		int name_len = strlen(name);
> +		struct name_decoration *r = xmalloc(sizeof(*r) + name_len);

This allocation should be name_len + 1 for the NUL-terminator, no?

It looks like add_name_decoration in log-tree already handles half of
what you are adding here. Can we just make that available globally (it
is manipulating the already-global "struct decoration name_decoration")?

I also notice that we do not set r->type at all, meaning the decoration
lookup code in log-tree will access uninitialized memory (worse, it will
use it as a pointer offset into the color list; I got a segfault when I
tried to run "git rev-list --bisect-all v1.8.0..v1.9.0").

I think we need this:

diff --git a/bisect.c b/bisect.c
index d6e851d..e2a7682 100644
--- a/bisect.c
+++ b/bisect.c
@@ -219,6 +219,7 @@ static struct commit_list *best_bisection_sorted(struct commit_list *list, int n
 		struct object *obj = &(array[i].commit->object);
 
 		sprintf(r->name, "dist=%d", array[i].distance);
+		r->type = 0;
 		r->next = add_decoration(&name_decoration, obj, r);
 		p->item = array[i].commit;
 		p = p->next;

at a minimum.

It looks like this was a regression caused by eb3005e (commit.h: add
'type' to struct name_decoration, 2010-06-19). Which makes me wonder if
anybody actually _uses_ --bisect-all (which AFAICT is the only way to
trigger the problem), but since it's public, I guess we should keep it.

I think the sane thing here is to stop advertising name_decoration as a
global, and make all callers use add_name_decoration. That makes it
easier for callers like this one, and would have caught the regression
caused be eb3005e (the compiler would have noticed that we were not
passing a type parameter to the function).

-Peff