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

From: Patrick Steinhardt <[email-masked]>
Date: Mon, 9 Oct 2023 12:04:30 +0200

On Fri, Oct 06, 2023 at 02:51:24PM -0700, Junio C Hamano wrote:
> "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).

Agreed, I also had a bit of a hard time to figure out whether this is an
actual bug fix, a performance improvement or merely a refactoring.

Patrick

> > 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;