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.

docs/topics/forms/formsets.txt

@@ -32,8 +32,8 @@ would with a regular form::
 
 As you can see it only displayed one empty form. The number of empty forms
 that is displayed is controlled by the ``extra`` parameter. By default,
-``formset_factory`` defines one extra form; the following example will
-display two blank forms::
+:func:`~django.forms.formsets.formset_factory` defines one extra form; the
+following example will display two blank forms::
 
     >>> ArticleFormSet = formset_factory(ArticleForm, extra=2)
 
@@ -84,8 +84,9 @@ list of dictionaries as the initial data.
 Limiting the maximum number of forms
 ------------------------------------
 
-The ``max_num`` parameter to ``formset_factory`` gives you the ability to
-limit the maximum number of empty forms the formset will display::
+The ``max_num`` parameter to :func:`~django.forms.formsets.formset_factory`
+gives you the ability to limit the maximum number of empty forms the formset
+will display::
 
     >>> ArticleFormSet = formset_factory(ArticleForm, extra=2, max_num=1)
     >>> formset = ArticleFormSet()
@@ -101,6 +102,20 @@ so long as the total number of forms does not exceed ``max_num``.
 A ``max_num`` value of ``None`` (the default) puts a high limit on the number
 of forms displayed (1000). In practice this is equivalent to no limit.
 
+If the number of forms in the initial data exceeds ``max_num``, all initial
+data forms will be displayed regardless.  (No extra forms will be displayed.)
+
+By default, ``max_num`` only affects how many forms are displayed and does not
+affect validation.  If ``validate_max=True`` is passed to the
+:func:`~django.forms.formsets.formset_factory`, then ``max_num`` will affect
+validation.  See :ref:`validate_max`.
+
+.. versionchanged:: 1.6
+   The ``validate_max`` parameter was added to
+   :func:`~django.forms.formsets.formset_factory`.  Also, the behavior of
+   ``FormSet`` was brought in line with that of ``ModelFormSet`` so that it
+   displays initial data regardless of ``max_num``.
+
 Formset validation
 ------------------
 
@@ -248,14 +263,59 @@ The formset ``clean`` method is called after all the ``Form.clean`` methods
 have been called. The errors will be found using the ``non_form_errors()``
 method on the formset.
 
+.. _validate_max:
+
+Validating the number of forms in a formset
+-------------------------------------------
+
+If ``validate_max=True`` is passed to
+:func:`~django.forms.formsets.formset_factory`, validation will also check
+that the number of forms in the data set is less than or equal to ``max_num``.
+
+    >>> ArticleFormSet = formset_factory(ArticleForm, max_num=1, validate_max=True)
+    >>> data = {
+    ...     'form-TOTAL_FORMS': u'2',
+    ...     'form-INITIAL_FORMS': u'0',
+    ...     'form-MAX_NUM_FORMS': u'',
+    ...     'form-0-title': u'Test',
+    ...     'form-0-pub_date': u'1904-06-16',
+    ...     'form-1-title': u'Test 2',
+    ...     'form-1-pub_date': u'1912-06-23',
+    ... }
+    >>> formset = ArticleFormSet(data)
+    >>> formset.is_valid()
+    False
+    >>> formset.errors
+    [{}, {}]
+    >>> formset.non_form_errors()
+    [u'Please submit 1 or fewer forms.']
+
+``validate_max=True`` validates against ``max_num`` strictly even if
+``max_num`` was exceeded because the amount of initial data supplied was
+excessive.
+
+Applications which need more customizable validation of the number of forms
+should use custom formset validation.
+
+.. note::
+
+    Regardless of ``validate_max``, if the number of forms in a data set
+    exceeds ``max_num`` by more than 1000, then the form will fail to validate
+    as if ``validate_max`` were set, and additionally only the first 1000
+    forms above ``max_num`` will be validated.  The remainder will be
+    truncated entirely.  This is to protect against memory exhaustion attacks
+    using forged POST requests.
+
+.. versionchanged:: 1.6
+   The ``validate_max`` parameter was added to
+   :func:`~django.forms.formsets.formset_factory`.
+
 Dealing with ordering and deletion of forms
 -------------------------------------------
 
-Common use cases with a formset is dealing with ordering and deletion of the
-form instances. This has been dealt with for you. The ``formset_factory``
-provides two optional parameters ``can_order`` and ``can_delete`` that will do
-the extra work of adding the extra fields and providing simpler ways of
-getting to that data.
+The :func:`~django.forms.formsets.formset_factory` provides two optional
+parameters ``can_order`` and ``can_delete`` to help with ordering of forms in
+formsets and deletion of forms from a formset.
 
 ``can_order``
 ~~~~~~~~~~~~~

docs/topics/forms/modelforms.txt

@@ -597,9 +597,10 @@ with the ``Author`` model. It works just like a regular formset::
 
 .. note::
 
-    :func:`~django.forms.models.modelformset_factory` uses ``formset_factory``
-    to generate formsets. This means that a model formset is just an extension
-    of a basic formset that knows how to interact with a particular model.
+    :func:`~django.forms.models.modelformset_factory` uses
+    :func:`~django.forms.formsets.formset_factory` to generate formsets. This
+    means that a model formset is just an extension of a basic formset that
+    knows how to interact with a particular model.
 
 Changing the queryset
 ---------------------