Subject: Re: [PATCH v1 2/7] mv: add documentation for check_dir_in_index()

From: Victoria Dye <[email-masked]>
Date: Tue, 19 Jul 2022 11:10:56 -0700

Victoria Dye wrote:
> Shaoxuan Yuan wrote:
>> Using check_dir_in_index without checking if the directory is on-disk
>> could get a false positive for partially sparsified directory.
>>
>> Add a note in the documentation to warn potential user.
>>
>> Signed-off-by: Shaoxuan Yuan <[email-masked]>
>> ---
>>  builtin/mv.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/builtin/mv.c b/builtin/mv.c
>> index 4729bb1a1a..c8b9069db8 100644
>> --- a/builtin/mv.c
>> +++ b/builtin/mv.c
>> @@ -132,6 +132,11 @@ static int index_range_of_same_dir(const char *src, int length,
>>   * Return 0 if such directory exist (i.e. with any of its contained files not
>>   * marked with CE_SKIP_WORKTREE, the directory would be present in working tree).
>>   * Return 1 otherwise.
>> + *
>> + * Note: *always* check the directory is not on-disk before this function
>> + * (i.e. using lstat());
>> + * otherwise it may return a false positive for a partially sparsified
>> + * directory.
> 
> To me, a comment like this indicates that either the function is not doing
> what it should be doing, or its name doesn't properly describe the
> function's behavior.
> 
> Per the function description:
> 
> 	Check if an out-of-cone directory should be in the index. Imagine
> 	this case that all the files under a directory are marked with
> 	'CE_SKIP_WORKTREE' bit and thus the directory is sparsified.
> 
> But neither the name of the function ('check_dir_in_index') nor the
> function's behavior (hence the comment you're adding here) match that
> description.
> 
> Since this function is intended to verify that a directory 1) exists in the
> index, and 2) is *entirely* sparse, I have two suggestions:
> 
> * Change the description to specify that the non-existence of the directory
>   on disk is an *assumption*, not an opportunity for a "false positive."
>   It's a subtle distinction, but it clarifies that the function should be
>   used only when the caller already knows the directory is empty. For
>   example:
> 
> 	/*
> 	 * Given the path of a directory that does not exist on-disk, check whether the
> 	 * directory contains any entries in the index with the SKIP_WORKTREE flag 
> 	 * enabled.
> 	 *
> 	 * Return 1 if such index entries exist.
> 	 * Return 0 otherwise.
> 	 */

Whoops, I mixed up the return values (I assumed this returned a boolean
based on the 'check' in the function name). In that case...
> 
> * 'check_dir_in_index' doesn't reflect the "is not on disk" prerequisite, so
>   the function name can be updated to clarify that (e.g.,
>   'empty_dir_has_sparse_contents')

...there are two options. Either you can use a more "boolean-y" name (like
the one I suggest above) and flip the return values (where "1" means "the
empty dir has sparse contents"), or change the name to something that
explicitly *doesn't* imply boolean. I'm personally in favor of the former
(I'm really struggling to come up with a descriptive, non-boolean name for
this function), but I'm fine with either if you can come up with a good
function name.

> 
>>   */
>>  static int check_dir_in_index(const char *name)
>>  {
>