client.go

// SPDX-License-Identifier: BSD-2-Clause
// SPDX-FileCopyrightText: Copyright (c) 2025 Runxi Yu <https://runxiyu.org>

package main

import (
	"io"
	"log"
	"os"
	"syscall"

	"golang.org/x/sys/unix"
)

func main() {
	if len(os.Args) != 2 {
		log.Fatalf("Usage: %s <socket-path>", os.Args[0])
	}

	socketPath := os.Args[1]

	conn, err := unix.Socket(unix.AF_UNIX, unix.SOCK_STREAM, 0)
	if err != nil {
		log.Fatalf("socket: %v", err)
	}
	addr := &unix.SockaddrUnix{Name: socketPath}
	if err := unix.Connect(conn, addr); err != nil {
		log.Fatalf("connect: %v", err)
	}

	message, err := io.ReadAll(os.Stdin)
	if err != nil {
		log.Fatalf("ReadAll: %v", err)
	}

	_, err = unix.Write(conn, message)
	if err != nil {
		log.Fatalf("write: %v", err)
	}

	if err := unix.Shutdown(conn, unix.SHUT_WR); err != nil {
		log.Fatalf("shutdown: %v", err)
	}

	oob := make([]byte, 1024)
	buf := make([]byte, 1)

	n, oobn, _, _, err := unix.Recvmsg(conn, buf, oob, 0)
	if err != nil || n == 0 {
		log.Fatalf("recvmsg: %v", err)
	}

	scms, err := unix.ParseSocketControlMessage(oob[:oobn])
	if err != nil {
		log.Fatalf("ParseSocketControlMessage: %v", err)
	}

	var memfd int = -1
	for _, scm := range scms {
		fds, err := unix.ParseUnixRights(&scm)
		if err != nil {
			log.Fatalf("ParseUnixRights: %v", err)
		}
		if len(fds) > 0 {
			memfd = fds[0]
			break
		}
	}
	if memfd < 0 {
		log.Fatal("invalid memfd")
	}

	var stat unix.Stat_t
	if err := unix.Fstat(memfd, &stat); err != nil {
		log.Fatalf("fstat: %v", err)
	}
	size := int(stat.Size)

	data, err := unix.Mmap(memfd, 0, size, syscall.PROT_READ, syscall.MAP_SHARED)
	if err != nil {
		log.Fatalf("mmap: %v", err)
	}

	_, err = os.Stdout.Write(data)
	if err != nil {
		log.Fatalf("write: %v", err)
	}

	unix.Munmap(data)
	unix.Close(memfd)
	unix.Close(conn)
}