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, 14 insertions, 7 deletions

diff --git a/refs/reftable-backend.c b/refs/reftable-backend.c
index 4c3817f4ec..c3d48cc412 100644
--- a/refs/reftable-backend.c
+++ b/refs/reftable-backend.c
@@ -719,15 +719,20 @@ static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
 }
 
 static int reftable_ref_iterator_seek(struct ref_iterator *ref_iterator,
-				      const char *prefix)
+				      const char *refname, unsigned int flags)
 {
 	struct reftable_ref_iterator *iter =
 		(struct reftable_ref_iterator *)ref_iterator;
 
-	free(iter->prefix);
-	iter->prefix = xstrdup_or_null(prefix);
-	iter->prefix_len = prefix ? strlen(prefix) : 0;
-	iter->err = reftable_iterator_seek_ref(&iter->iter, prefix);
+	/* Unset any previously set prefix */
+	FREE_AND_NULL(iter->prefix);
+	iter->prefix_len = 0;
+
+	if (flags & REF_ITERATOR_SEEK_SET_PREFIX) {
+		iter->prefix = xstrdup_or_null(refname);
+		iter->prefix_len = refname ? strlen(refname) : 0;
+	}
+	iter->err = reftable_iterator_seek_ref(&iter->iter, refname);
 
 	return iter->err;
 }
@@ -839,7 +844,8 @@ static struct reftable_ref_iterator *ref_iterator_for_stack(struct reftable_ref_
 	if (ret)
 		goto done;
 
-	ret = reftable_ref_iterator_seek(&iter->base, prefix);
+	ret = reftable_ref_iterator_seek(&iter->base, prefix,
+					 REF_ITERATOR_SEEK_SET_PREFIX);
 	if (ret)
 		goto done;
 
@@ -2042,7 +2048,8 @@ static int reftable_reflog_iterator_advance(struct ref_iterator *ref_iterator)
 }
 
 static int reftable_reflog_iterator_seek(struct ref_iterator *ref_iterator UNUSED,
-					 const char *prefix UNUSED)
+					 const char *refname UNUSED,
+					 unsigned int flags UNUSED)
 {
 	BUG("reftable reflog iterator cannot be seeked");
 	return -1;