Fixed #20084 -- Provided option to validate formset max_num on server.

This is provided as a new "validate_max" formset_factory option defaulting to
False, since the non-validating behavior of max_num is longstanding, and there
is certainly code relying on it. (In fact, even the Django admin relies on it
for the case where there are more existing inlines than the given max_num). It
may be that at some point we want to deprecate validate_max=False and
eventually remove the option, but this commit takes no steps in that direction.

This also fixes the DoS-prevention absolute_max enforcement so that it causes a
form validation error rather than an IndexError, and ensures that absolute_max
is always 1000 more than max_num, to prevent surprising changes in behavior
with max_num close to absolute_max.

Lastly, this commit fixes the previous inconsistency between a regular formset
and a model formset in the precedence of max_num and initial data. Previously
in a regular formset, if the provided initial data was longer than max_num, it
was truncated; in a model formset, all initial forms would be displayed
regardless of max_num. Now regular formsets are the same as model formsets; all
initial forms are displayed, even if more than max_num. (But if validate_max is
True, submitting these forms will result in a "too many forms" validation
error!) This combination of behaviors was chosen to keep the max_num validation
simple and consistent, and avoid silent data loss due to truncation of initial
data.

Thanks to Preston for discussion of the design choices.

django/forms/formsets.py

@@ -33,6 +33,9 @@ class ManagementForm(Form):
     def __init__(self, *args, **kwargs):
         self.base_fields[TOTAL_FORM_COUNT] = IntegerField(widget=HiddenInput)
         self.base_fields[INITIAL_FORM_COUNT] = IntegerField(widget=HiddenInput)
+        # MAX_NUM_FORM_COUNT is output with the rest of the management form,
+        # but only for the convenience of client-side code. The POST
+        # value of MAX_NUM_FORM_COUNT returned from the client is not checked.
         self.base_fields[MAX_NUM_FORM_COUNT] = IntegerField(required=False, widget=HiddenInput)
         super(ManagementForm, self).__init__(*args, **kwargs)
 
@@ -94,7 +97,11 @@ class BaseFormSet(object):
     def total_form_count(self):
         """Returns the total number of forms in this FormSet."""
         if self.is_bound:
-            return self.management_form.cleaned_data[TOTAL_FORM_COUNT]
+            # return absolute_max if it is lower than the actual total form
+            # count in the data; this is DoS protection to prevent clients
+            # from forcing the server to instantiate arbitrary numbers of
+            # forms
+            return min(self.management_form.cleaned_data[TOTAL_FORM_COUNT], self.absolute_max)
         else:
             initial_forms = self.initial_form_count()
             total_forms = initial_forms + self.extra
@@ -113,14 +120,13 @@ class BaseFormSet(object):
         else:
             # Use the length of the inital data if it's there, 0 otherwise.
             initial_forms = self.initial and len(self.initial) or 0
-            if initial_forms > self.max_num >= 0:
-                initial_forms = self.max_num
         return initial_forms
 
     def _construct_forms(self):
         # instantiate all the forms and put them in self.forms
         self.forms = []
-        for i in xrange(min(self.total_form_count(), self.absolute_max)):
+        # DoS protection is included in total_form_count()
+        for i in xrange(self.total_form_count()):
             self.forms.append(self._construct_form(i))
 
     def _construct_form(self, i, **kwargs):
@@ -168,7 +174,6 @@ class BaseFormSet(object):
         self.add_fields(form, None)
         return form
 
-    # Maybe this should just go away?
     @property
     def cleaned_data(self):
         """
@@ -294,8 +299,11 @@ class BaseFormSet(object):
         for i in range(0, self.total_form_count()):
             form = self.forms[i]
             self._errors.append(form.errors)
-        # Give self.clean() a chance to do cross-form validation.
         try:
+            if (self.validate_max and self.total_form_count() > self.max_num) or \
+                self.management_form.cleaned_data[TOTAL_FORM_COUNT] > self.absolute_max:
+                raise ValidationError(_("Please submit %s or fewer forms." % self.max_num))
+            # Give self.clean() a chance to do cross-form validation.
             self.clean()
         except ValidationError as e:
             self._non_form_errors = self.error_class(e.messages)
@@ -367,16 +375,18 @@ class BaseFormSet(object):
         return mark_safe('\n'.join([six.text_type(self.management_form), forms]))
 
 def formset_factory(form, formset=BaseFormSet, extra=1, can_order=False,
-                    can_delete=False, max_num=None):
+                    can_delete=False, max_num=None, validate_max=False):
     """Return a FormSet for the given form class."""
     if max_num is None:
         max_num = DEFAULT_MAX_NUM
     # hard limit on forms instantiated, to prevent memory-exhaustion attacks
-    # limit defaults to DEFAULT_MAX_NUM, but developer can increase it via max_num
-    absolute_max = max(DEFAULT_MAX_NUM, max_num)
+    # limit is simply max_num + DEFAULT_MAX_NUM (which is 2*DEFAULT_MAX_NUM
+    # if max_num is None in the first place)
+    absolute_max = max_num + DEFAULT_MAX_NUM
     attrs = {'form': form, 'extra': extra,
              'can_order': can_order, 'can_delete': can_delete,
-             'max_num': max_num, 'absolute_max': absolute_max}
+             'max_num': max_num, 'absolute_max': absolute_max,
+             'validate_max' : validate_max}
     return type(form.__name__ + str('FormSet'), (formset,), attrs)
 
 def all_valid(formsets):