systemd: support sending file descriptors to systemd

systemd/sdnotify_linux.go

@@ -0,0 +1,119 @@
+// -*- Mode: Go; indent-tabs-mode: t -*-
+//go:build linux
+
+/*
+ * Copyright (C) 2017-2026 Canonical Ltd
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 3 as
+ * published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+package systemd
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"runtime"
+	"strings"
+
+	"golang.org/x/sys/unix"
+)
+
+// SdNotify sends the given state string notification to systemd.
+//
+// inspired by libsystemd/sd-daemon/sd-daemon.c from the systemd source
+func SdNotify(notifyState string) error {
+	if notifyState == "" {
+		return fmt.Errorf("invalid empty notify state")
+	}
+
+	conn, err := sdNotifyConn()
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	_, err = conn.Write([]byte(notifyState))
+	return err
+}
+
+// SdNotifyWithFds sends the given state string notification and file
+// descriptors associated with passed files to systemd.
+//
+// Caller is responsible for closing the passed files.
+//
+// inspired by libsystemd/sd-daemon/sd-daemon.c from the systemd source
+func SdNotifyWithFds(notifyState string, files ...*os.File) error {
+	if notifyState == "" {
+		return fmt.Errorf("invalid empty notify state")
+	}
+
+	if len(files) == 0 {
+		return fmt.Errorf("at least one file is required")
+	}
+
+	conn, err := sdNotifyConn()
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+
+	rawConn, err := conn.SyscallConn()
+	if err != nil {
+		return err
+	}
+
+	var sendMsgErr error
+	err = rawConn.Control(func(sdNotifyFd uintptr) {
+		fds := make([]int, len(files))
+		for i := range files {
+			fds[i] = int(files[i].Fd())
+		}
+
+		rights := unix.UnixRights(fds...)
+		creds := unix.UnixCredentials(&unix.Ucred{
+			Pid: int32(os.Getpid()),
+			Uid: uint32(os.Getuid()),
+			Gid: uint32(os.Getgid()),
+		})
+		oob := append(creds, rights...)
+		sendMsgErr = unix.Sendmsg(int(sdNotifyFd), []byte(notifyState), oob, nil, 0)
+
+		// Ensure finalizer is not called for passed files.
+		runtime.KeepAlive(files)
+	})
+	if err != nil {
+		return err
+	}
+
+	return sendMsgErr
+}
+
+var osGetenv = os.Getenv
+
+func sdNotifyConn() (*net.UnixConn, error) {
+	notifySocket := osGetenv("NOTIFY_SOCKET")
+	if notifySocket == "" {
+		return nil, fmt.Errorf("cannot find NOTIFY_SOCKET environment variable")
+	}
+	if !strings.HasPrefix(notifySocket, "@") && !strings.HasPrefix(notifySocket, "/") {
+		return nil, fmt.Errorf("cannot use NOTIFY_SOCKET %q", notifySocket)
+	}
+
+	raddr := &net.UnixAddr{
+		Name: notifySocket,
+		Net:  "unixgram",
+	}
+	return net.DialUnix("unixgram", nil, raddr)
+}