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/debug.c b/refs/debug.c
index 485e3079d7..da300efaf3 100644
--- a/refs/debug.c
+++ b/refs/debug.c
@@ -170,12 +170,13 @@ static int debug_ref_iterator_advance(struct ref_iterator *ref_iterator)
 }
 
 static int debug_ref_iterator_seek(struct ref_iterator *ref_iterator,
-				   const char *prefix)
+				   const char *refname, unsigned int flags)
 {
 	struct debug_ref_iterator *diter =
 		(struct debug_ref_iterator *)ref_iterator;
-	int res = diter->iter->vtable->seek(diter->iter, prefix);
-	trace_printf_key(&trace_refs, "iterator_seek: %s: %d\n", prefix ? prefix : "", res);
+	int res = diter->iter->vtable->seek(diter->iter, refname, flags);
+	trace_printf_key(&trace_refs, "iterator_seek: %s flags: %d: %d\n",
+			 refname ? refname : "", flags, res);
 	return res;
 }