thodg/libgit2

diff --git a/src/refs.c b/src/refs.c
index 7aaa1e7..1c2d1a8 100644
--- a/src/refs.c
+++ b/src/refs.c
@@ -83,6 +83,7 @@ static int packed_write(git_repository *repo);
 static int reference_create_symbolic(git_reference **ref_out, git_repository *repo, const char *name, const char *target, int force);
 static int reference_create_oid(git_reference **ref_out, git_repository *repo, const char *name, const git_oid *id, int force);
 static int reference_rename(git_reference *ref, const char *new_name, int force);
+static int reference_available(git_repository *repo, const char *ref, const char *old_ref);
 
 /* name normalization */
 static int check_valid_ref_char(char ch);
@@ -1006,6 +1007,9 @@ static int reference_create_oid(git_reference **ref_out, git_repository *repo, c
 	if(git_reference_lookup(&ref, repo, name) == GIT_SUCCESS && !force)
 		return git__throw(GIT_EEXISTS, "Failed to create reference OID. Reference already exists");
 
+	if ((error = reference_available(repo, name, NULL)) < GIT_SUCCESS)
+		return git__rethrow(error, "Failed to create reference");
+
 	/*
 	 * If they old ref was of the same type, then we can just update
 	 * it (once we've checked that the target is valid). Otherwise we
@@ -1045,6 +1049,47 @@ cleanup:
 	return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to create reference OID");
 }
 
+static int _reference_available_cb(const char *ref, void *data)
+{
+	assert(ref && data);
+
+	git_vector *refs = (git_vector *)data;
+
+	const char *new = (const char *)git_vector_get(refs, 0);
+	const char *old = (const char *)git_vector_get(refs, 1);
+
+	if (!old || strcmp(old, ref)) {
+		int reflen = strlen(ref);
+		int newlen = strlen(new);
+		int cmplen = reflen < newlen ? reflen : newlen;
+		const char *lead = reflen < newlen ? new : ref;
+
+		if (!strncmp(new, ref, cmplen) &&
+		    lead[cmplen] == '/')
+			return GIT_EEXISTS;
+	}
+
+	return GIT_SUCCESS;
+}
+
+static int reference_available(git_repository *repo, const char *ref, const char* old_ref)
+{
+	int error;
+	git_vector refs;
+
+	if (git_vector_init(&refs, 2, NULL) < GIT_SUCCESS)
+		return GIT_ENOMEM;
+
+	git_vector_insert(&refs, (void *)ref);
+	git_vector_insert(&refs, (void *)old_ref);
+
+	error = git_reference_listcb(repo, GIT_REF_LISTALL, _reference_available_cb, (void *)&refs);
+
+	git_vector_free(&refs);
+
+	return error == GIT_SUCCESS ? GIT_SUCCESS : git__throw(GIT_EEXISTS, "Reference name `%s` conflicts with existing reference", ref);
+}
+
 /*
  * Rename a reference
  *
@@ -1082,6 +1127,8 @@ static int reference_rename(git_reference *ref, const char *new_name, int force)
 	    error != GIT_ENOTFOUND)
 		return git__rethrow(error, "Failed to rename reference");
 
+	if ((error = reference_available(ref->owner, new_name, ref->name)) < GIT_SUCCESS)
+		return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to rename reference. Reference already exists");
 
 	old_name = ref->name;
 	ref->name = git__strdup(new_name);
diff --git a/tests/t10-refs.c b/tests/t10-refs.c
index 3e7b079..33d2840 100644
--- a/tests/t10-refs.c
+++ b/tests/t10-refs.c
@@ -654,6 +654,38 @@ BEGIN_TEST(rename5, "can force-rename a reference with the name of an existing r
 	close_temp_repo(repo);
 END_TEST
 
+static const char *ref_one_name = "refs/heads/one/branch";
+static const char *ref_one_name_new = "refs/heads/two/branch";
+static const char *ref_two_name = "refs/heads/two";
+
+BEGIN_TEST(rename6, "can not overwrite name of existing reference")
+	git_reference *ref, *ref_one, *ref_one_new, *ref_two;
+	git_repository *repo;
+	git_oid id;
+
+	must_pass(open_temp_repo(&repo, REPOSITORY_FOLDER));
+
+	must_pass(git_reference_lookup(&ref, repo, ref_master_name));
+	must_be_true(ref->type & GIT_REF_OID);
+
+	git_oid_cpy(&id, git_reference_oid(ref));
+
+	/* Create loose references */
+	must_pass(git_reference_create_oid(&ref_one, repo, ref_one_name, &id));
+	must_pass(git_reference_create_oid(&ref_two, repo, ref_two_name, &id));
+
+	/* Pack everything */
+	must_pass(git_reference_packall(repo));
+
+	/* Attempt to create illegal reference */
+	must_fail(git_reference_create_oid(&ref_one_new, repo, ref_one_name_new, &id));
+
+	/* Illegal reference couldn't be created so this is supposed to fail */
+	must_fail(git_reference_lookup(&ref_one_new, repo, ref_one_name_new));
+
+	close_temp_repo(repo);
+END_TEST
+
 BEGIN_TEST(delete0, "deleting a ref which is both packed and loose should remove both tracks in the filesystem")
 	git_reference *looked_up_ref, *another_looked_up_ref;
 	git_repository *repo;
@@ -950,6 +982,7 @@ BEGIN_SUITE(refs)
 	ADD_TEST(rename3);
 	ADD_TEST(rename4);
 	ADD_TEST(rename5);
+	ADD_TEST(rename6);
 
 	ADD_TEST(delete0);
 	ADD_TEST(list0);