Fix image saving on backends that expect a rewound file pointer

Fixes #10046

wagtail/images/models.py

@@ -214,6 +214,8 @@ class WagtailImageFieldFile(models.fields.files.ImageFieldFile):
         finally:
             if close:
                 self.close()
+            else:
+                self.seek(0)
 
 
 class WagtailImageField(models.ImageField):

wagtail/test/dummy_external_storage.py

@@ -5,6 +5,7 @@
 #  - Calling .path on the storage or image file raises NotImplementedError
 #  - File.open() after the file has been closed raises an error
 #  - File.size exceptions raise DummyExternalStorageError
+#  - Storage._save() fails loudly if the content file's pointer is not at the start
 
 from django.core.files.base import File
 from django.core.files.storage import FileSystemStorage, Storage
@@ -26,11 +27,16 @@ class DummyExternalStorage(Storage):
         # File object
         return DummyExternalStorageFile(open(self.wrapped.path(name), mode))
 
-    # Wrap all other functions
-
     def _save(self, name, content):
+        file_pos = content.tell()
+        if file_pos != 0:
+            raise ValueError(
+                "Content file pointer should be at 0 - got %d instead" % file_pos
+            )
         return self.wrapped._save(name, content)
 
+    # Wrap all other functions
+
     def delete(self, name):
         self.wrapped.delete(name)
 

wagtail/tests/test_tests.py

@@ -1,9 +1,11 @@
 import json
 
+from django.core.files.uploadedfile import SimpleUploadedFile
 from django.test import TestCase
 
 from wagtail.admin.tests.test_contentstate import content_state_equal
 from wagtail.models import PAGE_MODEL_CLASSES, Page, Site
+from wagtail.test.dummy_external_storage import DummyExternalStorage
 from wagtail.test.testapp.models import (
     BusinessChild,
     BusinessIndex,
@@ -429,3 +431,28 @@ class TestFormDataHelpers(TestCase):
     def test_rich_text_with_alternative_editor(self):
         result = rich_text("<h2>title</h2><p>para</p>", editor="custom")
         self.assertEqual(result, "<h2>title</h2><p>para</p>")
+
+
+class TestDummyExternalStorage(WagtailTestUtils, TestCase):
+    def test_save_with_incorrect_file_object_position(self):
+        """
+        Test that DummyExternalStorage correctly warns about attempts
+        to write files that are not rewound to the start
+        """
+        # This is a 1x1 black png
+        png = (
+            b"\x89PNG\r\n\x1a\n\x00\x00\x00\rIHDR\x00\x00"
+            b"\x00\x01\x00\x00\x00\x01\x08\x06\x00\x00\x00"
+            b"\x1f\x15\xc4\x89\x00\x00\x00\rIDATx\x9cc````"
+            b"\x00\x00\x00\x05\x00\x01\xa5\xf6E@\x00\x00"
+            b"\x00\x00IEND\xaeB`\x82"
+        )
+        simple_png = SimpleUploadedFile(
+            name="test.png", content=png, content_type="image/png"
+        )
+        simple_png.read()
+        with self.assertRaisesMessage(
+            ValueError,
+            "Content file pointer should be at 0 - got 70 instead",
+        ):
+            DummyExternalStorage().save("test.png", simple_png)