Support getting listeners from systemd

Add a new module for getting listener sockets via systemd's file descriptor
passing (see sd_listen_fds(3) for more details), and make the main daemon use
it when "systemd" is given an address.

internal/systemd/systemd.go

@@ -0,0 +1,70 @@
+// Package systemd implements utility functions to interact with systemd.
+package systemd
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"strconv"
+	"syscall"
+)
+
+var (
+	// Error to return when $LISTEN_PID does not refer to us.
+	PIDMismatch = errors.New("$LISTEN_PID != our PID")
+
+	// First FD for listeners.
+	// It's 3 by definition, but using a variable simplifies testing.
+	firstFD = 3
+)
+
+// Listeners creates a slice net.Listener from the file descriptors passed
+// by systemd, via the LISTEN_FDS environment variable.
+// See sd_listen_fds(3) for more details.
+func Listeners() ([]net.Listener, error) {
+	pidStr := os.Getenv("LISTEN_PID")
+	nfdsStr := os.Getenv("LISTEN_FDS")
+
+	// Nothing to do if the variables are not set.
+	if pidStr == "" || nfdsStr == "" {
+		return nil, nil
+	}
+
+	pid, err := strconv.Atoi(pidStr)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"error converting $LISTEN_PID=%q: %v", pidStr, err)
+	} else if pid != os.Getpid() {
+		return nil, PIDMismatch
+	}
+
+	nfds, err := strconv.Atoi(os.Getenv("LISTEN_FDS"))
+	if err != nil {
+		return nil, fmt.Errorf(
+			"error reading $LISTEN_FDS=%q: %v", nfdsStr, err)
+	}
+
+	listeners := []net.Listener{}
+
+	for fd := firstFD; fd < firstFD+nfds; fd++ {
+		// We don't want childs to inherit these file descriptors.
+		syscall.CloseOnExec(fd)
+
+		name := fmt.Sprintf("[systemd-fd-%d]", fd)
+		lis, err := net.FileListener(os.NewFile(uintptr(fd), name))
+		if err != nil {
+			return nil, fmt.Errorf(
+				"Error making listener out of fd %d: %v", fd, err)
+		}
+
+		listeners = append(listeners, lis)
+	}
+
+	// Remove them from the environment, to prevent accidental reuse (by
+	// us or children processes).
+	os.Unsetenv("LISTEN_PID")
+	os.Unsetenv("LISTEN_FDS")
+
+	return listeners, nil
+}