shelldriver: allow login with empty password

Make 'password' default to None, so when is not set, no password is
entered, but the user can set "" (empty string) to support boards which
prompt for a password, but accept an enter.

Fixes #441

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
[r.czerwinski@pengutronix.de: fix commit message, use better None check]
Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>

Author: rohieb

CHANGES.rst

@@ -29,6 +29,7 @@ Bug fixes in 23.1
   test run.
 - ManagedFile NFS detection heuristic now does symlink resolution on the
   local host.
+- The password for the ShellDriver can now be an empty string.
 
 Breaking changes in 23.1
 ~~~~~~~~~~~~~~~~~~~~~~~~~

doc/configuration.rst

@@ -1558,7 +1558,8 @@ Arguments:
   - prompt (regex): shell prompt to match after logging in
   - login_prompt (regex): match for the login prompt
   - username (str): username to use during login
-  - password (str): optional, password to use during login
+  - password (str): optional, password to use during login.
+    Can be an empty string.
   - keyfile (str): optional, keyfile to upload after login, making the
     `SSHDriver`_ usable
   - login_timeout (int, default=60): timeout for login prompt detection in

labgrid/driver/shelldriver.py

@@ -48,7 +48,7 @@ class ShellDriver(CommandMixin, Driver, CommandProtocol, FileTransferProtocol):
     prompt = attr.ib(validator=attr.validators.instance_of(str))
     login_prompt = attr.ib(validator=attr.validators.instance_of(str))
     username = attr.ib(validator=attr.validators.instance_of(str))
-    password = attr.ib(default="", validator=attr.validators.instance_of(str))
+    password = attr.ib(default=None, validator=attr.validators.optional(attr.validators.instance_of(str)))
     keyfile = attr.ib(default="", validator=attr.validators.instance_of(str))
     login_timeout = attr.ib(default=60, validator=attr.validators.instance_of(int))
     console_ready = attr.ib(default="", validator=attr.validators.instance_of(str))
@@ -152,7 +152,7 @@ def _await_login(self):
                 did_login = True
 
             elif index == 2:
-                if self.password:
+                if self.password is not None:
                     self.console.sendline(self.password)
                 else:
                     raise Exception("Password entry needed but no password set")