[PATCH 04/10] fast-export: avoid dying when filtering by paths and old tags exist

[Elijah Newren <newren@gmail.com>]

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 2d8ad4691921,
"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 4ee1b225b99f, "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 <newren@gmail.com>
---
 builtin/fast-export.c  |  9 ++++++---
 t/t9350-fast-export.sh | 20 ++++++++++++++++++++
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index 1a299c2a21..89de9d6400 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -774,9 +774,12 @@ 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, sha1_to_hex(null_sha1));
+					free(buf);
+					return;
+				}
 				p = p->parents->item;
 			}
 			tagged_mark = get_object_mark(&p->object);
diff --git a/t/t9350-fast-export.sh b/t/t9350-fast-export.sh
index 6a392e87bc..5bf21b4908 100755
--- a/t/t9350-fast-export.sh
+++ b/t/t9350-fast-export.sh
@@ -325,6 +325,26 @@ 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 &&
+
+		touch ignored &&
+		git add ignored &&
+		test_commit initial &&
+
+		git tag -a -m "Some old tag" v0.0.0.0.0.0.1 &&
+
+		echo foo >bar &&
+		git add bar &&
+		test_commit add-bar &&
+
+		git fast-export --tag-of-filtered-object=rewrite --all -- bar >output &&
+		grep -A 1 refs/tags/v0.0.0.0.0.0.1 output | grep -E ^from.0{40}
+	)
+'
+
 cat > limit-by-paths/expected << EOF
 blob
 mark :1
-- 
2.19.1.866.g82735bcbde