Subject: Re: [PATCH 1/4] ref-cache.c: fix prefix matching in ref iteration

From: Junio C Hamano <[email-masked]>
Date: Fri, 06 Oct 2023 14:51:24 -0700

"Victoria Dye via GitGitGadget" <[email-masked]> writes:

> From: Victoria Dye <[email-masked]>
>
> Update 'cache_ref_iterator_advance' to skip over refs that are not matched
> by the given prefix.
>
> Currently, a ref entry is considered "matched" if the entry name is fully
> contained within the prefix:
>
> * prefix: "refs/heads/v1"
> * entry: "refs/heads/v1.0"
>
> OR if the prefix is fully contained in the entry name:
>
> * prefix: "refs/heads/v1.0"
> * entry: "refs/heads/v1"
>
> The first case is always correct, but the second is only correct if the ref
> cache entry is a directory, for example:
>
> * prefix: "refs/heads/example"
> * entry: "refs/heads/"
>
> Modify the logic in 'cache_ref_iterator_advance' to reflect these
> expectations:
>
> 1. If 'overlaps_prefix' returns 'PREFIX_EXCLUDES_DIR', then the prefix and
>    ref cache entry do not overlap at all. Skip this entry.
> 2. If 'overlaps_prefix' returns 'PREFIX_WITHIN_DIR', then the prefix matches
>    inside this entry if it is a directory. Skip if the entry is not a
>    directory, otherwise iterate over it.
> 3. Otherwise, 'overlaps_prefix' returned 'PREFIX_CONTAINS_DIR', indicating
>    that the cache entry (directory or not) is fully contained by or equal to
>    the prefix. Iterate over this entry.
>
> Note that condition 2 relies on the names of directory entries having the
> appropriate trailing slash. The existing function documentation of
> 'create_dir_entry' explicitly calls out the trailing slash requirement, so
> this is a safe assumption to make.

Thanks for explaining it very well and clearly.  

Allowing prefix="refs/heads/v1.0" to yield entry="refs/heads/v1"
(case #2 above that this patch fixes the behaviour for) would cause
ref_iterator_advance() to return a ref outside the hierarhcy,
wouldn't it?  So it appears to me that either one of the two would
be true:

 * the code is structured in such a way that such a condition does
   not actually happen (in which case this patch would be a no-op),
   or

 * there is a bug in the current code that is fixed by this patch,
   whose externally observable behaviour can be verified with a
   test.

It is not quite clear to me which is the case here.  The code with
the patch looks more logical than the original, but I am not sure
how to demonstrate the existing breakage (if any).

> Signed-off-by: Victoria Dye <[email-masked]>
> ---
>  refs/ref-cache.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/refs/ref-cache.c b/refs/ref-cache.c
> index 2294c4564fb..6e3b725245c 100644
> --- a/refs/ref-cache.c
> +++ b/refs/ref-cache.c
> @@ -412,7 +412,8 @@ static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
>  
>  		if (level->prefix_state == PREFIX_WITHIN_DIR) {
>  			entry_prefix_state = overlaps_prefix(entry->name, iter->prefix);
> -			if (entry_prefix_state == PREFIX_EXCLUDES_DIR)
> +			if (entry_prefix_state == PREFIX_EXCLUDES_DIR ||
> +			    (entry_prefix_state == PREFIX_WITHIN_DIR && !(entry->flag & REF_DIR)))
>  				continue;
>  		} else {
>  			entry_prefix_state = level->prefix_state;