backend/sftp: Add sftp.args option

Allow setting custom arguments for the `sftp` backend, by using the
`sftp.args` option. This is similar to the approach already implemented
in the `rclone` backend, to support new arguments without requiring
future code changes for each different SSH argument.

Closes #4241

internal/backend/sftp/config.go

@@ -13,8 +13,9 @@ import (
 type Config struct {
 	User, Host, Port, Path string
 
-	Layout  string `option:"layout" help:"use this backend directory layout (default: auto-detect)"`
+	Layout  string `option:"layout"  help:"use this backend directory layout (default: auto-detect)"`
 	Command string `option:"command" help:"specify command to create sftp connection"`
+	Args    string `option:"args"    help:"specify arguments for ssh"`
 
 	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 5)"`
 }

internal/backend/sftp/sftp.go

@@ -213,6 +213,9 @@ func buildSSHCommand(cfg Config) (cmd string, args []string, err error) {
 		if err != nil {
 			return "", nil, err
 		}
+		if cfg.Args != "" {
+			return "", nil, errors.New("cannot specify both sftp.command and sftp.args options")
+		}
 
 		return args[0], args[1:], nil
 	}
@@ -226,11 +229,19 @@ func buildSSHCommand(cfg Config) (cmd string, args []string, err error) {
 		args = append(args, "-p", port)
 	}
 	if cfg.User != "" {
-		args = append(args, "-l")
-		args = append(args, cfg.User)
+		args = append(args, "-l", cfg.User)
 	}
-	args = append(args, "-s")
-	args = append(args, "sftp")
+
+	if cfg.Args != "" {
+		a, err := backend.SplitShellStrings(cfg.Args)
+		if err != nil {
+			return "", nil, err
+		}
+
+		args = append(args, a...)
+	}
+
+	args = append(args, "-s", "sftp")
 	return cmd, args, nil
 }
 

internal/backend/sftp/sshcmd_test.go

@@ -30,6 +30,11 @@ var sshcmdTests = []struct {
 		"ssh",
 		[]string{"host", "-p", "10022", "-l", "user", "-s", "sftp"},
 	},
+	{
+		Config{User: "user", Host: "host", Port: "10022", Path: "/dir/subdir", Args: "-i /path/to/id_rsa"},
+		"ssh",
+		[]string{"host", "-p", "10022", "-l", "user", "-i", "/path/to/id_rsa", "-s", "sftp"},
+	},
 	{
 		// IPv6 address.
 		Config{User: "user", Host: "::1", Path: "dir"},