refdb: don't update when there's no need

If the caller wants to update a ref to point to the same target as it
currently has, we should return early and avoid writing to the reflog.

src/refdb_fs.c

@@ -1021,6 +1021,8 @@ static int refdb_fs_backend__write(
 	refdb_fs_backend *backend = (refdb_fs_backend *)_backend;
 	git_filebuf file = GIT_FILEBUF_INIT;
 	int error = 0, cmp = 0;
+	const char *new_target = NULL;
+	const git_oid *new_id = NULL;
 
 	assert(backend);
 
@@ -1041,6 +1043,21 @@ static int refdb_fs_backend__write(
 		goto on_error;
 	}
 
+	if (ref->type == GIT_REF_SYMBOLIC)
+		new_target = ref->target.symbolic;
+	else
+		new_id = &ref->target.oid;
+
+	error = cmp_old_ref(&cmp, _backend, ref->name, new_id, new_target);
+	if (error < 0 && error != GIT_ENOTFOUND)
+		goto on_error;
+
+	/* Don't update if we have the same value */
+	if (!error && !cmp) {
+		error = 0;
+		goto on_error; /* not really error */
+	}
+
 	if (should_write_reflog(backend->repo, ref->name)) {
 		if ((error = reflog_append(backend, ref, NULL, NULL, who, message)) < 0)
 			goto on_error;

tests/refs/reflog/reflog.c

@@ -237,3 +237,27 @@ void test_refs_reflog_reflog__append_to_HEAD_when_changing_current_branch(void)
 
 	cl_assert_equal_i(nlogs_after, nlogs + 1);
 }
+
+void test_refs_reflog_reflog__do_not_append_when_no_update(void)
+{
+	size_t nlogs, nlogs_after;
+	git_reference *ref, *ref2;
+	git_reflog *log;
+
+	cl_git_pass(git_reflog_read(&log, g_repo, "HEAD"));
+	nlogs = git_reflog_entrycount(log);
+	git_reflog_free(log);
+
+	cl_git_pass(git_reference_lookup(&ref, g_repo, "refs/heads/master"));
+	cl_git_pass(git_reference_create(&ref2, g_repo, "refs/heads/master",
+					 git_reference_target(ref), 1, NULL, NULL));
+
+	git_reference_free(ref);
+	git_reference_free(ref2);
+
+	cl_git_pass(git_reflog_read(&log, g_repo, "HEAD"));
+	nlogs_after = git_reflog_entrycount(log);
+	git_reflog_free(log);
+
+	cl_assert_equal_i(nlogs_after, nlogs);
+}

tests/repo/head.c

@@ -341,3 +341,30 @@ void test_repo_head__orphan_branch_does_not_count(void)
 
 	git_signature_free(sig);
 }
+
+void test_repo_head__set_to_current_target(void)
+{
+	git_signature *sig;
+	const char *msg;
+	git_reflog *log;
+	size_t nentries, nentries_after;
+
+	cl_git_pass(git_reflog_read(&log, repo, GIT_HEAD_FILE));
+	nentries = git_reflog_entrycount(log);
+	git_reflog_free(log);
+
+	cl_git_pass(git_signature_now(&sig, "me", "foo@example.com"));
+
+	msg = "message 1";
+	cl_git_pass(git_repository_set_head(repo, "refs/heads/haacked", sig, msg));
+	cl_git_pass(git_repository_set_head(repo, "refs/heads/haacked", sig, msg));
+
+	cl_git_pass(git_reflog_read(&log, repo, GIT_HEAD_FILE));
+	nentries_after = git_reflog_entrycount(log);
+	git_reflog_free(log);
+
+	cl_assert_equal_i(nentries + 1, nentries_after);
+
+	git_signature_free(sig);
+
+}