Serving images to external sites uses renditions

Previously, an image served to an external site was rendered on the fly.
This may have performance issues with large websites.

This commit changes the image serve view to use renditions so images are
only rendered when the url is first accessed. When the user hits the
url, they are now given a permanent redirect to the renditions image.

wagtail/tests/settings.py

@@ -7,6 +7,7 @@ from django.conf import global_settings
 WAGTAIL_ROOT = os.path.dirname(__file__)
 STATIC_ROOT = os.path.join(WAGTAIL_ROOT, 'test-static')
 MEDIA_ROOT = os.path.join(WAGTAIL_ROOT, 'test-media')
+MEDIA_URL = '/media/'
 
 
 DATABASES = {

wagtail/wagtailimages/tests/tests.py

@@ -140,13 +140,7 @@ class TestFrontendServeView(TestCase):
         response = self.client.get(reverse('wagtailimages_serve', args=(signature, self.image.id, 'fill-800x600')))
 
         # Check response
-        self.assertEqual(response.status_code, 200)
-        self.assertEqual(response['Content-Type'], 'image/jpeg')
-
-        # Make sure the cache headers are set to expire after at least one month
-        self.assertIn('Cache-Control', response)
-        self.assertEqual(response['Cache-Control'].split('=')[0], 'max-age')
-        self.assertTrue(int(response['Cache-Control'].split('=')[1]) > datetime.timedelta(days=30).seconds)
+        self.assertRedirects(response, self.image.renditions.first().url, status_code=301, target_status_code=404)
 
     def test_get_invalid_signature(self):
         """

wagtail/wagtailimages/views/frontend.py

@@ -1,13 +1,11 @@
-from django.shortcuts import get_object_or_404
+from django.shortcuts import get_object_or_404, redirect
 from django.http import HttpResponse
 from django.core.exceptions import PermissionDenied
-from django.views.decorators.cache import cache_control
 
 from wagtail.wagtailimages.models import get_image_model, Filter
 from wagtail.wagtailimages.utils import verify_signature
 
 
-@cache_control(max_age=60*60*24*60) # Cache for 60 days
 def serve(request, signature, image_id, filter_spec):
     image = get_object_or_404(get_image_model(), id=image_id)
 
@@ -15,6 +13,7 @@ def serve(request, signature, image_id, filter_spec):
         raise PermissionDenied
 
     try:
-        return Filter(spec=filter_spec).process_image(image.file.file, HttpResponse(content_type='image/jpeg'), focal_point=image.get_focal_point())
+        rendition = image.get_rendition(filter_spec)
+        return redirect(rendition.url, permanent=True)
     except Filter.InvalidFilterSpecError:
         return HttpResponse("Invalid filter spec: " + filter_spec, content_type='text/plain', status=400)