refs: selectively set prefix in the seek functions

The ref iterator exposes a `ref_iterator_seek()` function. The name
suggests that this would seek the iterator to a specific reference in
some ways similar to how `fseek()` works for the filesystem.

However, the function actually sets the prefix for refs iteration. So
further iteration would only yield references which match the particular
prefix. This is a bit confusing.

Let's add a 'flags' field to the function, which when set with the
'REF_ITERATOR_SEEK_SET_PREFIX' flag, will set the prefix for the
iteration in-line with the existing behavior. Otherwise, the reference
backends will simply seek to the specified reference and clears any
previously set prefix. This allows users to start iteration from a
specific reference.

In the packed and reftable backend, since references are available in a
sorted list, the changes are simply setting the prefix if needed. The
changes on the files-backend are a little more involved, since the files
backend uses the 'ref-cache' mechanism. We move out the existing logic
within `cache_ref_iterator_seek()` to `cache_ref_iterator_set_prefix()`
which is called when the 'REF_ITERATOR_SEEK_SET_PREFIX' flag is set. We
then parse the provided seek string and set the required levels and
their indexes to ensure that seeking is possible.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 3 deletions

diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 03f5df04d5..40c1c0f93d 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -353,11 +353,12 @@ void base_ref_iterator_init(struct ref_iterator *iter,
 typedef int ref_iterator_advance_fn(struct ref_iterator *ref_iterator);
 
 /*
- * Seek the iterator to the first reference matching the given prefix. Should
- * behave the same as if a new iterator was created with the same prefix.
+ * Seek the iterator to the first matching reference. If the
+ * REF_ITERATOR_SEEK_SET_PREFIX flag is set, it would behave the same as if a
+ * new iterator was created with the provided refname as prefix.
  */
 typedef int ref_iterator_seek_fn(struct ref_iterator *ref_iterator,
-				 const char *prefix);
+				 const char *refname, unsigned int flags);
 
 /*
  * Peels the current ref, returning 0 for success or -1 for failure.