sftp: support listening on passed FDs

cmd/serve/sftp/server.go

@@ -21,6 +21,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	sdActivation "github.com/coreos/go-systemd/v22/activation"
 	"github.com/rclone/rclone/cmd/serve/proxy"
 	"github.com/rclone/rclone/cmd/serve/proxy/proxyflags"
 	"github.com/rclone/rclone/fs"
@@ -266,10 +267,27 @@ func (s *server) serve() (err error) {
 
 	// Once a ServerConfig has been configured, connections can be
 	// accepted.
-	s.listener, err = net.Listen("tcp", s.opt.ListenAddr)
+	var listener net.Listener
+
+	// In case we run in a socket-activated environment, listen on (the first)
+	// passed FD.
+	sdListeners, err := sdActivation.Listeners()
 	if err != nil {
-		return fmt.Errorf("failed to listen for connection: %w", err)
+		return fmt.Errorf("unable to acquire listeners: %w", err)
 	}
+
+	if len(sdListeners) > 0 {
+		if len(sdListeners) > 1 {
+			fs.LogPrintf(fs.LogLevelWarning, nil, "more than one listener passed, ignoring all but the first.\n")
+		}
+		listener = sdListeners[0]
+	} else {
+		listener, err = net.Listen("tcp", s.opt.ListenAddr)
+		if err != nil {
+			return fmt.Errorf("failed to listen for connection: %w", err)
+		}
+	}
+	s.listener = listener
 	fs.Logf(nil, "SFTP server listening on %v\n", s.listener.Addr())
 
 	go s.acceptConnections()

cmd/serve/sftp/sftp.go

@@ -115,6 +115,17 @@ directory.
 By default the server binds to localhost:2022 - if you want it to be
 reachable externally then supply ` + "`--addr :2022`" + ` for example.
 
+This also supports being run with socket activation, in which case it will
+listen on the first passed FD.
+It can be configured with .socket and .service unit files as described in
+https://www.freedesktop.org/software/systemd/man/latest/systemd.socket.html
+
+Socket activation can be tested ad-hoc with the ` + "`systemd-socket-activate`" + `command:
+
+	systemd-socket-activate -l 2222 -- rclone serve sftp :local:vfs/
+
+This will socket-activate rclone on the first connection to port 2222 over TCP.
+
 Note that the default of ` + "`--vfs-cache-mode off`" + ` is fine for the rclone
 sftp backend, but it may not be with other SFTP clients.