thodg/libgit2

diff --git a/src/reflog.c b/src/reflog.c
index c90abb2..7609d9a 100644
--- a/src/reflog.c
+++ b/src/reflog.c
@@ -53,25 +53,15 @@ static int reflog_init(git_reflog **reflog, git_reference *ref)
 	return GIT_SUCCESS;
 }
 
-static int reflog_write(git_repository *repo, const char *ref_name,
-		        const char *oid_old, const char *oid_new,
-		        const git_signature *committer, const char *msg)
+static int reflog_write(const char *log_path, const char *oid_old,
+			const char *oid_new, const git_signature *committer,
+			const char *msg)
 {
 	int error;
-	char log_path[GIT_PATH_MAX];
 	git_buf log = GIT_BUF_INIT;
 	git_filebuf fbuf;
 
-	assert(repo && ref_name && oid_old && oid_new && committer);
-
-	git_path_join_n(log_path, 3, repo->path_repository, GIT_REFLOG_DIR, ref_name);
-
-	if (git_futils_exists(log_path)) {
-		if ((error = git_futils_mkpath2file(log_path)) < GIT_SUCCESS)
-			return git__rethrow(error, "Failed to write reflog. Cannot create reflog directory");
-
-	} else if (git_futils_isfile(log_path))
-		return git__throw(GIT_ERROR, "Failed to write reflog. `%s` is directory", log_path);
+	assert(log_path && oid_old && oid_new && committer);
 
 	git_buf_puts(&log, oid_old);
 	git_buf_putc(&log, ' ');
@@ -222,6 +212,7 @@ int git_reflog_write(git_reference *ref, const git_oid *oid_old,
 	int error;
 	char old[GIT_OID_HEXSZ+1];
 	char new[GIT_OID_HEXSZ+1];
+	char log_path[GIT_PATH_MAX];
 	git_reference *r;
 	const git_oid *oid;
 
@@ -234,12 +225,22 @@ int git_reflog_write(git_reference *ref, const git_oid *oid_old,
 
 	git_oid_to_string(new, GIT_OID_HEXSZ+1, oid);
 
+	git_path_join_n(log_path, 3, ref->owner->path_repository, GIT_REFLOG_DIR, ref->name);
+
+	if (git_futils_exists(log_path)) {
+		if ((error = git_futils_mkpath2file(log_path)) < GIT_SUCCESS)
+			return git__rethrow(error, "Failed to write reflog. Cannot create reflog directory");
+	} else if (git_futils_isfile(log_path)) {
+		return git__throw(GIT_ERROR, "Failed to write reflog. `%s` is directory", log_path);
+	} else if (oid_old == NULL)
+		return git__throw(GIT_ERROR, "Failed to write reflog. Old OID cannot be NULL for existing reference");
+
 	if (oid_old)
 		git_oid_to_string(old, GIT_OID_HEXSZ+1, oid_old);
 	else
 		snprintf(old, GIT_OID_HEXSZ+1, "%0*d", GIT_OID_HEXSZ, 0);
 
-	return reflog_write(ref->owner, ref->name, old, new, committer, msg);
+	return reflog_write(log_path, old, new, committer, msg);
 }
 
 unsigned int git_reflog_entrycount(git_reflog *reflog)
diff --git a/tests/t10-refs.c b/tests/t10-refs.c
index f151b5a..c54ff7c 100644
--- a/tests/t10-refs.c
+++ b/tests/t10-refs.c
@@ -1069,6 +1069,34 @@ BEGIN_TEST(reflog0, "write a reflog for a given reference and ensure it can be r
 	close_temp_repo(repo2);
 END_TEST
 
+BEGIN_TEST(reflog1, "avoid writing an obviously wrong reflog")
+	git_repository *repo;
+	git_reference *ref;
+	git_oid oid;
+	git_signature *committer;
+
+	must_pass(open_temp_repo(&repo, REPOSITORY_FOLDER));
+
+	/* Create a new branch pointing at the HEAD */
+	git_oid_fromstr(&oid, current_master_tip);
+	must_pass(git_reference_create_oid(&ref, repo, new_ref, &oid, 0));
+	must_pass(git_reference_lookup(&ref, repo, new_ref));
+
+	committer = git_signature_now("foo", "foo@bar");
+
+	/* Write the reflog for the new branch */
+	must_pass(git_reflog_write(ref, NULL, committer, NULL));
+
+	/* Try to update the reflog with wrong information:
+	 * It's no new reference, so the ancestor OID cannot
+	 * be NULL. */
+	must_fail(git_reflog_write(ref, NULL, committer, NULL));
+
+	git_signature_free(committer);
+
+	close_temp_repo(repo);
+END_TEST
+
 BEGIN_SUITE(refs)
 	ADD_TEST(readtag0);
 	ADD_TEST(readtag1);
@@ -1114,4 +1142,5 @@ BEGIN_SUITE(refs)
 	ADD_TEST(list1);
 
 	ADD_TEST(reflog0);
+	ADD_TEST(reflog1);
 END_SUITE