tag: don't warn if target is missing but promised

deref_tag() prints a warning if the object that a tag refers to does not
exist. However, when a partial clone has an annotated tag from its
promisor remote, but not the object that it refers to, printing a
warning on such a tag is incorrect.

This occurs, for example, when the checkout that happens after a partial
clone causes some objects to be fetched - and as part of the fetch, all
local refs are read. The test included in this patch demonstrates this
situation.

Therefore, do not print a warning in this case.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5616-partial-clone.sh

@@ -229,9 +229,13 @@ test_expect_success 'when partial cloning, tolerate server not sending target of
 	git -C "$SERVER" tag -m message -a myblob "$BLOB" &&
 
 	# Craft a packfile including the tag, but not the blob it points to.
-	printf "%s\n%s\n--not\n%s\n" \
+	# Also, omit objects referenced from HEAD in order to force a second
+	# fetch (to fetch missing objects) upon the automatic checkout that
+	# happens after a clone.
+	printf "%s\n%s\n--not\n%s\n%s\n" \
 		$(git -C "$SERVER" rev-parse HEAD) \
 		$(git -C "$SERVER" rev-parse myblob) \
+		$(git -C "$SERVER" rev-parse HEAD^{tree}) \
 		$(git -C "$SERVER" rev-parse myblob^{blob}) |
 		git -C "$SERVER" pack-objects --thin --stdout >incomplete.pack &&
 
@@ -249,7 +253,8 @@ test_expect_success 'when partial cloning, tolerate server not sending target of
 
 	# Exercise to make sure it works.
 	git -c protocol.version=2 clone \
-		--filter=blob:none $HTTPD_URL/one_time_sed/server repo &&
+		--filter=blob:none $HTTPD_URL/one_time_sed/server repo 2> err &&
+	! grep "missing object referenced by" err &&
 
 	# Ensure that the one-time-sed script was used.
 	! test -e "$HTTPD_ROOT_PATH/one-time-sed"

tag.c

@@ -4,6 +4,7 @@
 #include "tree.h"
 #include "blob.h"
 #include "gpg-interface.h"
+#include "packfile.h"
 
 const char *tag_type = "tag";
 
@@ -64,12 +65,18 @@ int gpg_verify_tag(const struct object_id *oid, const char *name_to_report,
 
 struct object *deref_tag(struct object *o, const char *warn, int warnlen)
 {
+	struct object_id *last_oid = NULL;
 	while (o && o->type == OBJ_TAG)
-		if (((struct tag *)o)->tagged)
-			o = parse_object(&((struct tag *)o)->tagged->oid);
-		else
+		if (((struct tag *)o)->tagged) {
+			last_oid = &((struct tag *)o)->tagged->oid;
+			o = parse_object(last_oid);
+		} else {
+			last_oid = NULL;
 			o = NULL;
+		}
 	if (!o && warn) {
+		if (last_oid && is_promisor_object(last_oid))
+			return NULL;
 		if (!warnlen)
 			warnlen = strlen(warn);
 		error("missing object referenced by '%.*s'", warnlen, warn);