Subject: Re: pack-objects: Fix segfault when object count is less than thread count

From: Johannes Sixt <[email-masked]>
Date: Mon, 21 Jan 2008 16:12:30 +0100

Sergey Vlasov schrieb:
> When partitioning the work amongst threads, dividing the number of
> objects by the number of threads may return 0 when there are less
> objects than threads; this will cause the subsequent code to segfault
> when accessing list[sub_size-1].  Fix this by ensuring that sub_size
> is not zero if there is at least one object to process.
> 
> Signed-off-by: Sergey Vlasov <[email-masked]>
> ---
>  builtin-pack-objects.c |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/builtin-pack-objects.c b/builtin-pack-objects.c
> index ec10238..cdf8aae 100644
> --- a/builtin-pack-objects.c
> +++ b/builtin-pack-objects.c
> @@ -1665,6 +1665,9 @@ static void ll_find_deltas(struct object_entry **list, unsigned list_size,
>  	for (i = 0; i < delta_search_threads; i++) {
>  		unsigned sub_size = list_size / (delta_search_threads - i);
>  
> +		if (sub_size == 0 && list_size >= 1)
> +			sub_size = 1;
> +
>  		p[i].window = window;
>  		p[i].depth = depth;
>  		p[i].processed = processed;

I think it fits the logic better to include sub_size > 0 in the while loop
that follows, like so:

		/* try to split chunks on "path" boundaries */
		while (0 < sub_size && sub_size < list_size &&
		       list[sub_size]->hash &&
		       list[sub_size]->hash == list[sub_size-1]->hash)
			sub_size++;

because we explicitly want to allow threads to "work" on zero objects
(i.e. do nothing at all), but if a thread does get assigned some work,
then its chunk is extended past the next path boundary. This way you
collapse two special cases - "zero-sized chunk" and "path boundary" - into
one.

-- Hannes