fast-export: avoid dying when filtering by paths and old tags exist

If --tag-of-filtered-object=rewrite is specified along with a set of
paths to limit what is exported, then any tags pointing to old commits
that do not contain any of those specified paths cause problems.  Since
the old tagged commit is not exported, fast-export attempts to rewrite
such tags to an ancestor commit which was exported.  If no such commit
exists, then fast-export currently die()s.  Five years after the tag
rewriting logic was added to fast-export (see commit 2d8ad46,
"fast-export: Add a --tag-of-filtered-object  option for newly dangling
tags", 2009-06-25), fast-import gained the ability to delete refs (see
commit 4ee1b22, "fast-import: add support to delete refs",
2014-04-20), so now we do have a valid option to rewrite the tag to.
Delete these tags instead of dying.

Signed-off-by: Elijah Newren <[email protected]>
Acked-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

builtin/fast-export.c

@@ -775,9 +775,12 @@ static void handle_tag(const char *name, struct tag *tag)
 					break;
 				if (!(p->object.flags & TREESAME))
 					break;
-				if (!p->parents)
-					die("can't find replacement commit for tag %s",
-					     oid_to_hex(&tag->object.oid));
+				if (!p->parents) {
+					printf("reset %s\nfrom %s\n\n",
+					       name, oid_to_hex(&null_oid));
+					free(buf);
+					return;
+				}
 				p = p->parents->item;
 			}
 			tagged_mark = get_object_mark(&p->object);

t/t9350-fast-export.sh

@@ -325,6 +325,22 @@ test_expect_success 'rewriting tag of filtered out object' '
 )
 '
 
+test_expect_success 'rewrite tag predating pathspecs to nothing' '
+	test_create_repo rewrite_tag_predating_pathspecs &&
+	(
+		cd rewrite_tag_predating_pathspecs &&
+
+		test_commit initial &&
+
+		git tag -a -m "Some old tag" v0.0.0.0.0.0.1 &&
+
+		test_commit bar &&
+
+		git fast-export --tag-of-filtered-object=rewrite --all -- bar.t >output &&
+		grep from.$ZERO_OID output
+	)
+'
+
 cat > limit-by-paths/expected << EOF
 blob
 mark :1