sftp: test that Args and Command options cannot be set at the same time

internal/backend/sftp/sshcmd_test.go

@@ -9,43 +9,57 @@ var sshcmdTests = []struct {
 	cfg  Config
 	cmd  string
 	args []string
+	err  string
 }{
 	{
 		Config{User: "user", Host: "host", Path: "dir/subdir"},
 		"ssh",
 		[]string{"host", "-l", "user", "-s", "sftp"},
+		"",
 	},
 	{
 		Config{Host: "host", Path: "dir/subdir"},
 		"ssh",
 		[]string{"host", "-s", "sftp"},
+		"",
 	},
 	{
 		Config{Host: "host", Port: "10022", Path: "/dir/subdir"},
 		"ssh",
 		[]string{"host", "-p", "10022", "-s", "sftp"},
+		"",
 	},
 	{
 		Config{User: "user", Host: "host", Port: "10022", Path: "/dir/subdir"},
 		"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"},
+		"",
+	},
+	{
+		Config{Command: "ssh something", Args: "-i /path/to/id_rsa"},
+		"",
+		nil,
+		"cannot specify both sftp.command and sftp.args options",
 	},
 	{
 		// IPv6 address.
 		Config{User: "user", Host: "::1", Path: "dir"},
 		"ssh",
 		[]string{"::1", "-l", "user", "-s", "sftp"},
+		"",
 	},
 	{
 		// IPv6 address with zone and port.
 		Config{User: "user", Host: "::1%lo0", Port: "22", Path: "dir"},
 		"ssh",
 		[]string{"::1%lo0", "-p", "22", "-l", "user", "-s", "sftp"},
+		"",
 	},
 }
 
@@ -53,8 +67,14 @@ func TestBuildSSHCommand(t *testing.T) {
 	for i, test := range sshcmdTests {
 		t.Run("", func(t *testing.T) {
 			cmd, args, err := buildSSHCommand(test.cfg)
-			if err != nil {
+			if test.err != "" {
-				t.Fatalf("%v in test %d", err, i)
+				if err.Error() != test.err {
+					t.Fatalf("expected error %v got %v", test.err, err.Error())
+				}
+			} else {
+				if err != nil {
+					t.Fatalf("%v in test %d", err, i)
+				}
 			}
 
 			if cmd != test.cmd {