feat(systemd): Prepare module for passing FDs to service

Signed-off-by: Steffen Vogel <post@steffenvogel.de>

pkg/os/systemd/files_linux.go

@@ -0,0 +1,72 @@
+// SPDX-FileCopyrightText: 2015 CoreOS, Inc.
+// SPDX-FileCopyrightText: 2023-2025 Steffen Vogel <post@steffenvogel.de>
+// SPDX-License-Identifier: Apache-2.0
+
+package systemd
+
+import (
+	"os"
+	"strconv"
+	"strings"
+	"syscall"
+)
+
+const (
+	// listenFdsStart corresponds to `SD_LISTEN_FDS_START`.
+	listenFdsStart = 3
+)
+
+// Files returns a slice containing a `os.File` object for each
+// file descriptor passed to this process via systemd fd-passing protocol.
+//
+// The order of the file descriptors is preserved in the returned slice.
+// `unsetEnv` is typically set to `true` in order to avoid clashes in
+// fd usage and to avoid leaking environment flags to child processes.
+func Files(unsetEnv bool) []*os.File {
+	if unsetEnv {
+		defer os.Unsetenv("LISTEN_PID")
+		defer os.Unsetenv("LISTEN_FDS")
+		defer os.Unsetenv("LISTEN_FDNAMES")
+	}
+
+	pid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
+	if err != nil || pid != os.Getpid() {
+		return nil
+	}
+
+	nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
+	if err != nil || nfds == 0 {
+		return nil
+	}
+
+	names := strings.Split(os.Getenv("LISTEN_FDNAMES"), ":")
+
+	files := make([]*os.File, 0, nfds)
+
+	for fd := listenFdsStart; fd < listenFdsStart+nfds; fd++ {
+		syscall.CloseOnExec(fd)
+
+		name := "LISTEN_FD_" + strconv.Itoa(fd)
+		if offset := fd - listenFdsStart; offset < len(names) && len(names[offset]) > 0 {
+			name = names[offset]
+		}
+
+		files = append(files, os.NewFile(uintptr(fd), name))
+	}
+
+	return files
+}
+
+func NumFiles() int {
+	lpid, err := strconv.Atoi(os.Getenv("LISTEN_PID"))
+	if err != nil || lpid != os.Getpid() {
+		return 0
+	}
+
+	nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
+	if err != nil {
+		return 0
+	}
+
+	return nfds
+}