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, 15 insertions, 11 deletions

diff --git a/refs/iterator.c b/refs/iterator.c
index 766d96e795..17ef841d8a 100644
--- a/refs/iterator.c
+++ b/refs/iterator.c
@@ -15,10 +15,10 @@ int ref_iterator_advance(struct ref_iterator *ref_iterator)
 	return ref_iterator->vtable->advance(ref_iterator);
 }
 
-int ref_iterator_seek(struct ref_iterator *ref_iterator,
-		      const char *prefix)
+int ref_iterator_seek(struct ref_iterator *ref_iterator, const char *refname,
+		      unsigned int flags)
 {
-	return ref_iterator->vtable->seek(ref_iterator, prefix);
+	return ref_iterator->vtable->seek(ref_iterator, refname, flags);
 }
 
 int ref_iterator_peel(struct ref_iterator *ref_iterator,
@@ -57,7 +57,8 @@ static int empty_ref_iterator_advance(struct ref_iterator *ref_iterator UNUSED)
 }
 
 static int empty_ref_iterator_seek(struct ref_iterator *ref_iterator UNUSED,
-				   const char *prefix UNUSED)
+				   const char *refname UNUSED,
+				   unsigned int flags UNUSED)
 {
 	return 0;
 }
@@ -224,7 +225,7 @@ error:
 }
 
 static int merge_ref_iterator_seek(struct ref_iterator *ref_iterator,
-				   const char *prefix)
+				   const char *refname, unsigned int flags)
 {
 	struct merge_ref_iterator *iter =
 		(struct merge_ref_iterator *)ref_iterator;
@@ -234,11 +235,11 @@ static int merge_ref_iterator_seek(struct ref_iterator *ref_iterator,
 	iter->iter0 = iter->iter0_owned;
 	iter->iter1 = iter->iter1_owned;
 
-	ret = ref_iterator_seek(iter->iter0, prefix);
+	ret = ref_iterator_seek(iter->iter0, refname, flags);
 	if (ret < 0)
 		return ret;
 
-	ret = ref_iterator_seek(iter->iter1, prefix);
+	ret = ref_iterator_seek(iter->iter1, refname, flags);
 	if (ret < 0)
 		return ret;
 
@@ -407,13 +408,16 @@ static int prefix_ref_iterator_advance(struct ref_iterator *ref_iterator)
 }
 
 static int prefix_ref_iterator_seek(struct ref_iterator *ref_iterator,
-				    const char *prefix)
+				    const char *refname, unsigned int flags)
 {
 	struct prefix_ref_iterator *iter =
 		(struct prefix_ref_iterator *)ref_iterator;
-	free(iter->prefix);
-	iter->prefix = xstrdup_or_null(prefix);
-	return ref_iterator_seek(iter->iter0, prefix);
+
+	if (flags & REF_ITERATOR_SEEK_SET_PREFIX) {
+		free(iter->prefix);
+		iter->prefix = xstrdup_or_null(refname);
+	}
+	return ref_iterator_seek(iter->iter0, refname, flags);
 }
 
 static int prefix_ref_iterator_peel(struct ref_iterator *ref_iterator,