Implement username field protection

man 8 useradd says:

    It is usually recommended to only use usernames that begin with a
    lower case letter or an underscore, followed by lower case
    letters, digits, underscores, or dashes. They can end with a
    dollar sign. In regular expression terms: [a-z_][a-z0-9_-]*[$]?

    On Debian, the only constraints are that usernames must neither
    start with a dash ('-') nor plus ('+') nor tilde ('~') nor
    contain a colon (':'), a comma (','), or a whitespace (space: ' ',
    end of line: '\n', tabulation: '\t', etc.).  Note that using a
    slash ('/') may break the default algorithm for the definition
    of the user's home directory.

    Usernames may only be up to 32 characters long.

In this patch we implement most of this.  Subset of the regular
expression suggested above is used to limit input into the username
field.  We've not yet determined how to provide a fixed width widget
so at this time, user can input more than 32 characters, but upon
selectin done, we raise and error and reset the state.

Signed-off-by: Ryan Harper <ryan.harper@canonical.com>

subiquity/ui/interactive.py

@@ -18,6 +18,7 @@
 
 from urwid import (Edit, IntEdit, RadioButton, WidgetWrap)
 import logging
+import re
 
 log = logging.getLogger("subiquity.ui.input")
 
@@ -59,6 +60,26 @@ class PasswordEditor(StringEditor):
         super().__init__(caption, mask=mask)
 
 
+class UsernameEditor(StringEditor):
+    """ Username input prompt with input rules
+    """
+
+    def keypress(self, size, key):
+        ''' restrict what chars we allow for username '''
+
+        userlen = len(self.value)
+        if userlen == 0:
+            username = r'[a-z_]'
+        else:
+            username = r'[a-z0-9_-]'
+
+        # don't allow non username chars
+        if re.match(username, key) is None:
+            return False
+
+        return super().keypress(size, key)
+
+
 class IntegerEditor(WidgetWrap):
     """ IntEdit input class
     """

subiquity/ui/views/identity.py

@@ -21,7 +21,9 @@ Welcome provides user with language selection
 import logging
 from urwid import (Pile, Columns, Text, ListBox)
 from subiquity.ui.buttons import done_btn, cancel_btn
-from subiquity.ui.interactive import StringEditor, PasswordEditor
+from subiquity.ui.interactive import (PasswordEditor,
+                                      StringEditor,
+                                      UsernameEditor)
 from subiquity.ui.utils import Padding, Color
 from subiquity.view import ViewPolicy
 
@@ -34,7 +36,7 @@ class IdentityView(ViewPolicy):
         self.signal = signal
         self.items = []
         self.realname = StringEditor(caption="")
-        self.username = StringEditor(caption="")
+        self.username = UsernameEditor(caption="")
         self.password = PasswordEditor(caption="")
         self.error = Text("", align="center")
         self.confirm_password = PasswordEditor(caption="")
@@ -105,6 +107,12 @@ class IdentityView(ViewPolicy):
             self.password.value = ""
             self.confirm_password.value = ""
             return
+
+        if len(self.username.value) > 32:
+            self.error.set_text("Username too long, must be < 32")
+            self.username.value = ""
+            return
+
         cpassword = self.model.encrypt_password(self.password.value)
         log.debug("*crypted* User input: {} {} {}".format(
             self.username.value, cpassword, cpassword))