From 3088eda994ea0812e4aebf414334304723108671 Mon Sep 17 00:00:00 2001
From: Timo Weingärtner <timo@tiwe.de>
Date: Mon, 17 May 2021 12:37:22 +0200
Subject: factor out alter_fd_flags()

this also saves one syscall each time the flags are already as they should be
---
 ssh-agent-filter.C | 25 +++++++++++++++++--------
 1 file changed, 17 insertions(+), 8 deletions(-)

diff --git a/ssh-agent-filter.C b/ssh-agent-filter.C
index fbeb85f..61255a0 100644
--- a/ssh-agent-filter.C
+++ b/ssh-agent-filter.C
@@ -121,8 +121,19 @@ string base64_encode (string const & s) {
 	return {begin(b64), end(b64)};
 }
 
-void cloexec (int fd) {
-	if (fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC))
+template <int write_cmd, int set_flags, int clear_flags>
+void alter_fd_flags (int const fd) {
+	static_assert(write_cmd == F_SETFD || write_cmd == F_SETFL, "unsupported fcntl() operation");
+	constexpr int read_cmd = (write_cmd == F_SETFD) ? F_GETFD : F_GETFL;
+	static_assert((set_flags & clear_flags) == 0, "overlap in set and clear");
+
+	int const current_flags = fcntl(fd, read_cmd);
+	if (current_flags == -1)
+		throw system_error(errno, system_category(), "fcntl");
+	int const new_flags = (current_flags | set_flags) & ~clear_flags;
+	if (current_flags == new_flags)
+		return;
+	if (fcntl(fd, write_cmd, new_flags))
 		throw system_error(errno, system_category(), "fcntl");
 }
 
@@ -131,7 +142,7 @@ int make_cloexec_socket (int const address_family, int const type, int const pro
 	std::lock_guard lock{fd_fork_mutex};
 	if ((sock = socket(address_family, type | SOCK_CLOEXEC, protocol)) == -1)
 		throw system_error(errno, system_category(), "socket");
-	cloexec(sock);
+	alter_fd_flags<F_SETFD, FD_CLOEXEC, 0>(sock);
 	return sock;
 }
 
@@ -163,8 +174,7 @@ int make_upstream_agent_conn () {
 int make_listen_sock () {
 	int const sock = make_cloexec_socket(AF_UNIX, SOCK_STREAM, 0);
 
-	if (fcntl(sock, F_SETFL, fcntl(sock, F_GETFL) | O_NONBLOCK))
-		throw system_error(errno, system_category(), "fcntl");
+	alter_fd_flags<F_SETFL, O_NONBLOCK, 0>(sock);
 
 	struct sockaddr_un addr{AF_UNIX, {}};
 	
@@ -555,8 +565,7 @@ rfc4251::string handle_request (rfc4251::string const & r) {
 }
 
 void handle_client (int const sock) try {
-	if (fcntl(sock, F_SETFL, fcntl(sock, F_GETFL) & ~O_NONBLOCK))
-		throw system_error(errno, system_category(), "fcntl");
+	alter_fd_flags<F_SETFL, 0, O_NONBLOCK>(sock);
 
 	// we could use only one streambuf and iostream but when
 	// switching from read to write an lseek call is made that
@@ -633,7 +642,7 @@ int main (int const argc, char const * const * const argv) {
 				else
 					break;
 			}
-			cloexec(client_sock);
+			alter_fd_flags<F_SETFD, FD_CLOEXEC, 0>(client_sock);
 		}
 		std::thread t{handle_client, client_sock};
 		t.detach();
-- 
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