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/files-backend.c b/refs/files-backend.c
index bf6f89b1d1..8b282f2a60 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -929,11 +929,11 @@ static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
 }
 
 static int files_ref_iterator_seek(struct ref_iterator *ref_iterator,
-				   const char *prefix)
+				   const char *refname, unsigned int flags)
 {
 	struct files_ref_iterator *iter =
 		(struct files_ref_iterator *)ref_iterator;
-	return ref_iterator_seek(iter->iter0, prefix);
+	return ref_iterator_seek(iter->iter0, refname, flags);
 }
 
 static int files_ref_iterator_peel(struct ref_iterator *ref_iterator,
@@ -2316,7 +2316,8 @@ static int files_reflog_iterator_advance(struct ref_iterator *ref_iterator)
 }
 
 static int files_reflog_iterator_seek(struct ref_iterator *ref_iterator UNUSED,
-				      const char *prefix UNUSED)
+				      const char *refname UNUSED,
+				      unsigned int flags UNUSED)
 {
 	BUG("ref_iterator_seek() called for reflog_iterator");
 }