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, 11 insertions, 6 deletions

diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 7fd73a0e6d..5fa4ae6655 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1004,19 +1004,23 @@ static int packed_ref_iterator_advance(struct ref_iterator *ref_iterator)
 }
 
 static int packed_ref_iterator_seek(struct ref_iterator *ref_iterator,
-				    const char *prefix)
+				    const char *refname, unsigned int flags)
 {
 	struct packed_ref_iterator *iter =
 		(struct packed_ref_iterator *)ref_iterator;
 	const char *start;
 
-	if (prefix && *prefix)
-		start = find_reference_location(iter->snapshot, prefix, 0);
+	if (refname && *refname)
+		start = find_reference_location(iter->snapshot, refname, 0);
 	else
 		start = iter->snapshot->start;
 
-	free(iter->prefix);
-	iter->prefix = xstrdup_or_null(prefix);
+	/* Unset any previously set prefix */
+	FREE_AND_NULL(iter->prefix);
+
+	if (flags & REF_ITERATOR_SEEK_SET_PREFIX)
+		iter->prefix = xstrdup_or_null(refname);
+
 	iter->pos = start;
 	iter->eof = iter->snapshot->eof;
 
@@ -1194,7 +1198,8 @@ static struct ref_iterator *packed_ref_iterator_begin(
 	iter->repo = ref_store->repo;
 	iter->flags = flags;
 
-	if (packed_ref_iterator_seek(&iter->base, prefix) < 0) {
+	if (packed_ref_iterator_seek(&iter->base, prefix,
+				     REF_ITERATOR_SEEK_SET_PREFIX) < 0) {
 		ref_iterator_free(&iter->base);
 		return NULL;
 	}