Subject: Re: [PATCH] cache-tree: fix strbuf growth in prime_cache_tree_rec()

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Sun, 05 Feb 2023 22:12:25 +0100

On Sat, Feb 04 2023, René Scharfe wrote:

> Use size_t to store the original length of the strbuf tree_len, as
> that's the correct type.
>
> Don't double the allocated size of the strbuf when adding a subdirectory
> name.  Only extend it to fit that name and a slash.
>
> Signed-off-by: René Scharfe <[email-masked]>
> ---
>  cache-tree.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/cache-tree.c b/cache-tree.c
> index 9af457f47c..35f7617164 100644
> --- a/cache-tree.c
> +++ b/cache-tree.c
> @@ -760,7 +760,7 @@ static void prime_cache_tree_rec(struct repository *r,
>  	struct tree_desc desc;
>  	struct name_entry entry;
>  	int cnt;
> -	int base_path_len = tree_path->len;
> +	size_t base_path_len = tree_path->len;
>
>  	oidcpy(&it->oid, &tree->object.oid);
>
> @@ -785,7 +785,7 @@ static void prime_cache_tree_rec(struct repository *r,
>  			 */
>  			if (r->index->sparse_index) {
>  				strbuf_setlen(tree_path, base_path_len);
> -				strbuf_grow(tree_path, base_path_len + entry.pathlen + 1);
> +				strbuf_grow(tree_path, entry.pathlen + 1);
>  				strbuf_add(tree_path, entry.path, entry.pathlen);
>  				strbuf_addch(tree_path, '/');
>  			}

The size_t conversion is trivially correct.

But what do you mean with "don't double the[...]"? Do you mean that this
manages to evade growing these to 24 etc?

One wonders if (even for this index-related code) we really need such
careful management of growth, and could instead do with:

	strbuf_setlen(tree_path, base_path_len);
	strbuf_add(tree_path, entry.path, entry.pathlen);
	strbuf_addch(tree_path, '/');

Or even just:

	strbuf_addf(tree_path, "%*.s/", (int)entry.pathlen, entry.path);

As e.g. the t1092 test that runs this codepath shows we're looping
through the index entires here and will (in that case) handle names & a
"tree_path" like (these are the entry.path values):

	""
	"before"

Which here are turned into:

	"before/"

But both before & after your change we'll grow the "alloc" to 24, which
isn't surprising, as both the string length of (sans slash) 6 and 7 plus
1 for "\0" is rounde up to 24 with the standard growth pattern.

So I think if we're doing a "while-at-it" fixing of the off-by-one here
we might be better of asking whether it's needed at all, and whether
this case can't just be left to the growth smartness of the
strbuf+ALLOC_GROW() API instead.