tag: don't allow tags to non-existent objects

These indicate an inconsistency in the repository which we've created,
so don't allow them.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>

src/tag.c

@@ -209,6 +209,9 @@ static int tag_create(
 		return error;
 	}
 
+	if (!git_odb_exists(repo->db, target))
+		return GIT_ENOTFOUND;
+
 	/* Try to find out what the type is */
 	if (target_type == GIT_OBJ_ANY) {
 		size_t _unused;

tests/t08-tag.c

@@ -119,7 +119,7 @@ BEGIN_TEST(write0, "write a tag to the repository and read it again")
 
 END_TEST
 
-BEGIN_TEST(write1, "write a tag to the repository which points to an unknown oid and read it again")
+BEGIN_TEST(write1, "write a tag to the repository which points to an unknown oid should fail")
 	git_repository *repo;
 	git_tag *tag;
 	git_oid target_id, tag_id;
@@ -135,7 +135,7 @@ BEGIN_TEST(write1, "write a tag to the repository which points to an unknown oid
 	tagger = git_signature_new(TAGGER_NAME, TAGGER_EMAIL, 123456789, 60);
 	must_be_true(tagger != NULL);
 
-	must_pass(git_tag_create(
+	must_fail(git_tag_create(
 		&tag_id, /* out id */
 		repo,
 		"the-zombie-tag",
@@ -146,17 +146,6 @@ BEGIN_TEST(write1, "write a tag to the repository which points to an unknown oid
 
 	git_signature_free((git_signature *)tagger);
 
-	must_pass(git_tag_lookup(&tag, repo, &tag_id));
-	
-	/* The non existent target can not be looked up */
-	must_fail(git_tag_target(&zombie, tag));
-
-	must_pass(git_reference_lookup(&ref_tag, repo, "refs/tags/the-zombie-tag"));
-	
-	must_pass(git_reference_delete(ref_tag));
-	must_pass(remove_loose_object(REPOSITORY_FOLDER, (git_object *)tag));
-
-	git_tag_close(tag);
 	git_repository_free(repo);
 
 END_TEST