[1.8.x] Fixed #23441, #24555 -- Improved the behavior of InclusionNode.

This change:

* Makes the InclusionNode cache-safe by removing render-time side effects
  to its nodelist.
* Ensures the render_context stack is properly scoped and reset by updating
  the render call to use Template.render rather than Nodelist.render.

Backport of 0808ccce38 from master

django/template/base.py

@@ -1262,10 +1262,16 @@ class Library(object):
             class InclusionNode(TagHelperNode):
 
                 def render(self, context):
+                    """
+                    Renders the specified template and context. Caches the
+                    template object in render_context to avoid reparsing and
+                    loading when used in a for loop.
+                    """
                     resolved_args, resolved_kwargs = self.get_resolved_arguments(context)
                     _dict = func(*resolved_args, **resolved_kwargs)
 
-                    if not getattr(self, 'nodelist', False):
+                    t = context.render_context.get(self)
+                    if t is None:
                         if isinstance(file_name, Template):
                             t = file_name
                         elif isinstance(getattr(file_name, 'template', None), Template):
@@ -1274,7 +1280,7 @@ class Library(object):
                             t = context.template.engine.select_template(file_name)
                         else:
                             t = context.template.engine.get_template(file_name)
-                        self.nodelist = t.nodelist
+                        context.render_context[self] = t
                     new_context = context.new(_dict)
                     # Copy across the CSRF token, if present, because
                     # inclusion tags are often used for forms, and we need
@@ -1283,7 +1289,7 @@ class Library(object):
                     csrf_token = context.get('csrf_token', None)
                     if csrf_token is not None:
                         new_context['csrf_token'] = csrf_token
-                    return self.nodelist.render(new_context)
+                    return t.render(new_context)
 
             function_name = (name or
                 getattr(func, '_decorated_function', func).__name__)

tests/template_tests/templates/inclusion_base.html

@@ -0,0 +1 @@
+{% block content %}base{% endblock %}

tests/template_tests/templates/inclusion_extends1.html

@@ -0,0 +1,2 @@
+{% extends 'inclusion_base.html' %}
+{% block content %}one{% endblock %}

tests/template_tests/templates/inclusion_extends2.html

@@ -0,0 +1,2 @@
+{% extends 'inclusion_base.html' %}
+{% block content %}two{% endblock %}

tests/template_tests/templatetags/inclusion.py

@@ -164,3 +164,13 @@ def inclusion_tag_without_context_parameter(arg):
     """Expected inclusion_tag_without_context_parameter __doc__"""
     return {}
 inclusion_tag_without_context_parameter.anything = "Expected inclusion_tag_without_context_parameter __dict__"
+
+
+@register.inclusion_tag('inclusion_extends1.html')
+def inclusion_extends1():
+    return {}
+
+
+@register.inclusion_tag('inclusion_extends2.html')
+def inclusion_extends2():
+    return {}

tests/template_tests/test_custom.py

@@ -2,7 +2,8 @@ from __future__ import unicode_literals
 
 import os
 
-from django.template import Context, Template, TemplateSyntaxError
+from django.template import Context, Engine, Template, TemplateSyntaxError
+from django.template.base import Node
 from django.test import SimpleTestCase, ignore_warnings
 from django.test.utils import extend_sys_path
 from django.utils import six
@@ -274,6 +275,26 @@ class CustomTagTests(SimpleTestCase):
         c.use_l10n = True
         self.assertEqual(t.render(c).strip(), 'True')
 
+    def test_no_render_side_effect(self):
+        """
+        #23441 -- InclusionNode shouldn't modify its nodelist at render time.
+        """
+        engine = Engine(app_dirs=True)
+        template = engine.from_string('{% load inclusion %}{% inclusion_no_params %}')
+        count = template.nodelist.get_nodes_by_type(Node)
+        template.render(Context({}))
+        self.assertEqual(template.nodelist.get_nodes_by_type(Node), count)
+
+    def test_render_context_is_cleared(self):
+        """
+        #24555 -- InclusionNode should push and pop the render_context stack
+        when rendering. Otherwise, leftover values such as blocks from
+        extending can interfere with subsequent rendering.
+        """
+        engine = Engine(app_dirs=True)
+        template = engine.from_string('{% load inclusion %}{% inclusion_extends1 %}{% inclusion_extends2 %}')
+        self.assertEqual(template.render(Context({})).strip(), 'one\ntwo')
+
     def test_assignment_tags(self):
         c = Context({'value': 42})