Validate on max length of string field type

This blueprint aims to add a max_length validation to the
`nova.objects.fields.String` class.

Change-Id: I1ed8f0804996af86c7eadc83914d81df1ab4a88d
Blueprint: string-field-max-length

specs/juno/string-field-max-length.rst

@@ -0,0 +1,174 @@
+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+=======================================
+Allow StringField to enforce max length
+=======================================
+
+https://blueprints.launchpad.net/nova/+spec/string-field-max-length
+
+This blueprint aims to add a max length constraint to the
+`nova.objects.fields.StringField` class.
+
+Problem description
+===================
+
+Currently, the nova object framework revolves around the use of field type
+classes that describe the schema of an object. Each object model is simply
+a collection of fields, each of which have a particular type, such as
+IntegerField or StringField.
+
+In much the same way that a SQL database schema describes the constraints
+that a given column in a table must adhere to -- e.g. the length of characters
+possible in a CHAR field, or a valid DATETIME string -- the nova objects
+should be self-validating.
+
+Proposed change
+===============
+
+This specification proposes to change the `coerce` method of the
+`String` class to validate on the number of characters in the
+field's string value.
+
+The `StringField` concrete field class shall have a new `max_length` kwarg
+added to its constructor that will control the validation. The default
+value will be None, and no `StringField` objects defined in the schemas of
+any of the nova object models shall be changed in this spec.
+
+Alternatives
+------------
+
+None (keep things the way they are now)
+
+Data model impact
+-----------------
+
+None (the existing models themselves won't be changed in this specification
+at all)
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Roughly, the code the `String` field type class would change from this:
+
+.. code:: python
+
+    class String(FieldType):
+        @staticmethod
+        def coerce(obj, attr, value):
+            # FIXME(danms): We should really try to avoid the need to do this
+            if isinstance(value, (six.string_types, int, long, float,
+                                  datetime.datetime)):
+                return unicode(value)
+            else:
+                raise ValueError(_('A string is required here, not %s') %
+                                 value.__class__.__name__)
+
+to this:
+
+.. code:: python
+
+    class String(FieldType):
+
+        def __init__(self, max_length=None):
+            """
+            :param max_length: Optional constraint on the number of Unicode
+                               characters the string value can be.
+            """
+            self._max_length = max_length
+
+        @staticmethod
+        def coerce(self, obj, attr, value):
+            # FIXME(danms): We should really try to avoid the need to do this
+            if isinstance(value, (six.string_types, int, long, float,
+                                  datetime.datetime)):
+                result = unicode(value)
+            else:
+                raise ValueError(_('A string is required here, not %s') %
+                                 value.__class__.__name__)
+            if self._max_length is not None:
+                if len(value) > self._max_length):
+                    msg = _("String %(result)s is longer than maximum allowed "
+                            "length of %(max_length)d.")
+                    msg = msg % dict(result=result,
+                                     max_length=self._max_length)
+                    raise ValueError(msg)
+            return result
+
+The StringField class would then need to be modified to allow passing the
+max_length parameter along to its type class.
+
+Work Items
+----------
+
+N/A
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  jaypipes
+
+Dependencies
+============
+
+None
+
+Testing
+=======
+
+Would need new unit tests. No need for any integration test changes.
+
+Documentation Impact
+====================
+
+None
+
+References
+==========
+
+The server-instance-tagging work will likely be the first work to use
+this functionality, as the tag string has a max length associated with
+it and we need to be very careful about changing existing model fields' string
+length validation code, so a new field like the tag field is an ideal place to
+begin with this implementation.
+
+http://git.openstack.org/cgit/openstack/nova-specs/tree/specs/juno/tag-instances.rst