Fixed RuntimeError: dictionary changed size during iteration, raised while purging stale preview data from session

Added TestPreview.test_preview_on_edit_expiry to exercise PreviewOnEdit.remove_old_preview_data
Added freezegun package in testing_extras

setup.py

@@ -44,6 +44,7 @@ testing_extras = [
     'elasticsearch>=1.0.0,<3.0',
     'Jinja2>=2.8,<3.0',
     'boto3>=1.4,<1.5',
+    'freezegun>=0.3.8',
 
     # For coverage and PEP8 linting
     'coverage>=3.7.0',

wagtail/wagtailadmin/tests/test_pages_views.py

@@ -6,6 +6,7 @@ import os
 
 import django
 import mock
+from freezegun import freeze_time
 from django.conf import settings
 from django.contrib.auth import get_user_model
 from django.contrib.auth.models import Group, Permission
@@ -28,6 +29,7 @@ from wagtail.wagtailcore.models import GroupPagePermission, Page, PageRevision,
 from wagtail.wagtailcore.signals import page_published, page_unpublished
 from wagtail.wagtailsearch.index import SearchField
 from wagtail.wagtailusers.models import UserProfile
+from wagtail.wagtailadmin.views.pages import PreviewOnEdit
 
 
 def submittable_timestamp(timestamp):
@@ -4272,3 +4274,30 @@ class TestPreview(TestCase, WagtailTestUtils):
         self.assertContains(response, "Beach party")
         self.assertContains(response, "<li>Parties</li>")
         self.assertContains(response, "<li>Holidays</li>")
+
+    def test_preview_on_edit_expiry(self):
+        initial_datetime = timezone.now()
+        expiry_datetime = initial_datetime + datetime.timedelta(
+            seconds=PreviewOnEdit.preview_expiration_timeout + 1)
+
+        with freeze_time(initial_datetime) as frozen_datetime:
+            preview_url = reverse('wagtailadmin_pages:preview_on_edit',
+                                  args=(self.event_page.id,))
+            response = self.client.post(preview_url, self.post_data)
+
+            # Check the JSON response
+            self.assertEqual(response.status_code, 200)
+
+            response = self.client.get(preview_url)
+
+            # Check the HTML response
+            self.assertEqual(response.status_code, 200)
+
+            frozen_datetime.move_to(expiry_datetime)
+
+            preview_url = reverse('wagtailadmin_pages:preview_on_edit',
+                                  args=(self.home_page.id,))
+            response = self.client.post(preview_url, self.post_data)
+            self.assertEqual(response.status_code, 200)
+            response = self.client.get(preview_url)
+            self.assertEqual(response.status_code, 200)

wagtail/wagtailadmin/views/pages.py

@@ -562,13 +562,12 @@ class PreviewOnEdit(View):
 
     def remove_old_preview_data(self):
         expiration = time() - self.preview_expiration_timeout
-        for k, v in self.request.session.items():
-            if not k.startswith(self.session_key_prefix):
-                continue
-            post_data, timestamp = v
-            if timestamp < expiration:
-                # Removes the session key gracefully
-                self.request.session.pop(k)
+        expired_keys = [
+            k for k, v in self.request.session.items()
+            if k.startswith(self.session_key_prefix) and v[1] < expiration]
+        # Removes the session key gracefully
+        for k in expired_keys:
+            self.request.session.pop(k)
 
     @property
     def session_key(self):