ref-filter: implement '--points-at' option

In 'tag -l' we have '--points-at' option which lets users
list only tags of a given object. Implement this option in
'ref-filter.{c,h}' so that other commands can benefit from this.

This is duplicated from tag.c, we will eventually remove that
when we port tag.c to use ref-filter APIs.

Based-on-patch-by: Jeff King <peff@peff.net>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -56,6 +56,10 @@ static int match_pattern(const char **patterns, const char *ref)
 	return 0;
 }
 
+/*
+ * This is currently duplicated in ref-filter.c, and will eventually be
+ * removed as we port tag.c to use the ref-filter APIs.
+ */
 static const unsigned char *match_points_at(const char *refname,
 					    const unsigned char *sha1)
 {

ref-filter.c

@@ -842,6 +842,38 @@ static int match_name_as_path(const char **pattern, const char *refname)
 	return 0;
 }
 
+/*
+ * Given a ref (sha1, refname), check if the ref belongs to the array
+ * of sha1s. If the given ref is a tag, check if the given tag points
+ * at one of the sha1s in the given sha1 array.
+ * the given sha1_array.
+ * NEEDSWORK:
+ * 1. Only a single level of inderection is obtained, we might want to
+ * change this to account for multiple levels (e.g. annotated tags
+ * pointing to annotated tags pointing to a commit.)
+ * 2. As the refs are cached we might know what refname peels to without
+ * the need to parse the object via parse_object(). peel_ref() might be a
+ * more efficient alternative to obtain the pointee.
+ */
+static const unsigned char *match_points_at(struct sha1_array *points_at,
+					    const unsigned char *sha1,
+					    const char *refname)
+{
+	const unsigned char *tagged_sha1 = NULL;
+	struct object *obj;
+
+	if (sha1_array_lookup(points_at, sha1) >= 0)
+		return sha1;
+	obj = parse_object(sha1);
+	if (!obj)
+		die(_("malformed object at '%s'"), refname);
+	if (obj->type == OBJ_TAG)
+		tagged_sha1 = ((struct tag *)obj)->tagged->sha1;
+	if (tagged_sha1 && sha1_array_lookup(points_at, tagged_sha1) >= 0)
+		return tagged_sha1;
+	return NULL;
+}
+
 /* Allocate space for a new ref_array_item and copy the objectname and flag to it */
 static struct ref_array_item *new_ref_array_item(const char *refname,
 						 const unsigned char *objectname,
@@ -875,6 +907,9 @@ static int ref_filter_handler(const char *refname, const struct object_id *oid,
 	if (*filter->name_patterns && !match_name_as_path(filter->name_patterns, refname))
 		return 0;
 
+	if (filter->points_at.nr && !match_points_at(&filter->points_at, oid->hash, refname))
+		return 0;
+
 	/*
 	 * We do not open the object yet; sort may only need refname
 	 * to do its job and the resulting list may yet to be pruned

ref-filter.h

@@ -42,6 +42,7 @@ struct ref_array {
 
 struct ref_filter {
 	const char **name_patterns;
+	struct sha1_array points_at;
 };
 
 struct ref_filter_cbdata {