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

changelog/unreleased/pull-4519

@@ -0,0 +1,16 @@
+Enhancement: Add config option to set SFTP command arguments
+
+The `sftp.args` option can be passed to restic (using `-o`) to specify
+custom arguments to be used by the SSH command executed by the SFTP
+backend.
+
+Before this change, a common scenario where a custom identity file was
+needed for the SSH connection, required the full command to be
+specified:
+`-o sftp.command='ssh user@host:port -i /ssh/my_private_key -s sftp'`
+
+With this new configuration option:
+`-o sftp.args='-i /ssh/my_private_key'`
+
+https://github.com/restic/restic/pull/4519
+https://github.com/restic/restic/issues/4241

doc/030_preparing_a_new_repo.rst

@@ -119,10 +119,10 @@ user's home directory.
 Also, if the SFTP server is enforcing domain-confined users, you can
 specify the user this way: ``user@domain@host``.
 
-.. note:: Please be aware that sftp servers do not expand the tilde character
+.. note:: Please be aware that SFTP servers do not expand the tilde character
           (``~``) normally used as an alias for a user's home directory. If you
           want to specify a path relative to the user's home directory, pass a
-          relative path to the sftp backend.
+          relative path to the SFTP backend.
 
 If you need to specify a port number or IPv6 address, you'll need to use
 URL syntax. E.g., the repository ``/srv/restic-repo`` on ``[::1]`` (localhost)
@@ -174,7 +174,11 @@ Last, if you'd like to use an entirely different program to create the
 SFTP connection, you can specify the command to be run with the option
 ``-o sftp.command="foobar"``.
 
-.. note:: Please be aware that sftp servers close connections when no data is
+The SFTP backend has the following additional option:
+
+ * ``-o sftp.args`` allows setting the arguments passed to the default SSH command (ignored when ``sftp.command`` is set)
+
+.. note:: Please be aware that SFTP servers close connections when no data is
           received by the client. This can happen when restic is processing huge
           amounts of unchanged data. To avoid this issue add the following lines 
           to the client's .ssh/config file:

internal/backend/sftp/config.go

@@ -15,6 +15,7 @@ type Config struct {
 
 	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"},