Don't reopen files that are from external hosting

Fixes #1397

wagtail/wagtailimages/models.py

@@ -118,8 +118,18 @@ class AbstractImage(models.Model, TagSearchable):
         # Open file if it is closed
         close_file = False
         try:
+            image_file = self.file
+
             if self.file.closed:
-                self.file.open('rb')
+                # Reopen the file
+                if self.is_stored_locally():
+                    self.file.open('rb')
+                else:
+                    # Some external storage backends don't allow reopening
+                    # the file. Get a fresh file instance. #1397
+                    storage = self._meta.get_field('file').storage
+                    image_file = storage.open(self.file.name, 'rb')
+
                 close_file = True
         except IOError as e:
             # re-throw this as a SourceImageIOError so that calling code can distinguish
@@ -127,13 +137,13 @@ class AbstractImage(models.Model, TagSearchable):
             raise SourceImageIOError(text_type(e))
 
         # Seek to beginning
-        self.file.seek(0)
+        image_file.seek(0)
 
         try:
-            yield WillowImage.open(self.file)
+            yield WillowImage.open(image_file)
         finally:
             if close_file:
-                self.file.close()
+                image_file.close()
 
     def get_rect(self):
         return Rect(0, 0, self.width, self.height)

wagtail/wagtailimages/tests/test_admin_views.py

@@ -1,7 +1,6 @@
 from __future__ import unicode_literals
 
 import json
-import unittest
 
 from django.test import TestCase, override_settings
 from django.utils.http import urlquote
@@ -156,7 +155,6 @@ class TestImageEditView(TestCase, WagtailTestUtils):
         self.assertEqual(response.status_code, 200)
         self.assertTemplateUsed(response, 'wagtailimages/images/edit.html')
 
-    @unittest.expectedFailure
     @override_settings(DEFAULT_FILE_STORAGE='wagtail.tests.dummy_external_storage.DummyExternalStorage')
     def test_simple_with_external_storage(self):
         # The view calls get_file_size on the image that closes the file if
@@ -352,7 +350,6 @@ class TestImageChooserUploadView(TestCase, WagtailTestUtils):
         # The form should have an error
         self.assertFormError(response, 'uploadform', 'file', "This field is required.")
 
-    @unittest.expectedFailure
     @override_settings(DEFAULT_FILE_STORAGE='wagtail.tests.dummy_external_storage.DummyExternalStorage')
     def test_upload_with_external_storage(self):
         response = self.client.post(reverse('wagtailimages:chooser_upload'), {