Modify logic for picking a default username value when lis_person_sourcedid...

Modify logic for picking a default username value when lis_person_sourcedid parameter is not present.  It's believed that this should only be the case when user is added directly in Canvas UI and when admins switch to "Student View" mode.  Use custom_canvas_user_id (or user_id) with a class-level defined prefix in order to have a shorter field to get past a bug where the Test Student username was longer than 30 characters.

django_auth_lti/backends.py

@@ -15,6 +15,7 @@ oauth_creds = settings.LTI_OAUTH_CREDENTIALS
 
 
 class LTIAuthBackend(ModelBackend):
+
     """
     By default, the ``authenticate`` method creates ``User`` objects for
     usernames that don't already exist in the database.  Subclasses can disable
@@ -24,6 +25,8 @@ class LTIAuthBackend(ModelBackend):
 
     # Create a User object if not already in the database?
     create_unknown_user = True
+    # Username prefix for users without an sis source id
+    unknown_user_prefix = "cuid:"
 
     def authenticate(self, request):
 
@@ -80,16 +83,14 @@ class LTIAuthBackend(ModelBackend):
 
         user = None
 
-        if request.POST.get('lis_person_sourcedid'):
-            username = self.clean_username(request.POST.get('lis_person_sourcedid'))
-        elif request.POST.get('custom_canvas_user_login_id'):
-            username = self.clean_username(request.POST.get('custom_canvas_user_login_id'))
-        else:
-            username = self.clean_username(request.POST.get('user_id'))
+        # Retrieve username from LTI parameter or default to an overridable function return value
+        username = tool_provider.lis_person_sourcedid or self.get_default_username(
+            tool_provider, prefix=self.unknown_user_prefix)
+        username = self.clean_username(username)  # Clean it
 
-        email = request.POST.get('lis_person_contact_email_primary')
-        first_name = request.POST.get('lis_person_name_given')
-        last_name = request.POST.get('lis_person_name_family')
+        email = tool_provider.lis_person_contact_email_primary
+        first_name = tool_provider.lis_person_name_given
+        last_name = tool_provider.lis_person_name_family
 
         logger.info("We have a valid username: %s" % username)
 
@@ -109,7 +110,8 @@ class LTIAuthBackend(ModelBackend):
                 logger.debug('authenticate found an existing user for %s' % username)
 
         else:
-            logger.debug('automatic new user creation is turned OFF! just try to find and existing record')
+            logger.debug(
+                'automatic new user creation is turned OFF! just try to find and existing record')
             try:
                 user = UserModel.objects.get_by_natural_key(username)
             except UserModel.DoesNotExist:
@@ -131,3 +133,12 @@ class LTIAuthBackend(ModelBackend):
 
     def clean_username(self, username):
         return username
+
+    def get_default_username(self, tool_provider, prefix=''):
+        """
+        Return a default username value from tool_provider in case offical
+        LTI param lis_person_sourcedid was not present.
+        """
+        # Default back to user_id lti param
+        uname = tool_provider.get_custom_param('canvas_user_id') or tool_provider.user_id
+        return prefix + uname