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

[Derrick Stolee <derrickstolee@github.com>]

On 2/5/2023 4:12 PM, Ævar Arnfjörð Bjarmason wrote:
> 
> 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 <l.s.r@web.de>
>> ---
>>  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.

I agree, and thanks for finding and fixing this issue.

Upon reading strbuf_grow(), I would expect it to work the same
as ALLOC_GROW(), but its documentation clearly states a very
different result.

> 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, '/');

This would be my preferred way to go here.

> Or even just:
> 
> 	strbuf_addf(tree_path, "%*.s/", (int)entry.pathlen, entry.path);

Please do not add "addf" functions that can be run in tight loops.
It's faster to do strbuf_add() followed by strbuf_addch().

Thanks,
-Stolee