Throw error if images.Filter.run cannot create output (#8508)

If Filter.run is requested to create an unknown output format,
it should not fail silently without writing any output, but instead
throw an exception.

Fixes #8503

CHANGELOG.txt

@@ -12,6 +12,7 @@ Changelog
  * Use `InlinePanel`'s label when available for field comparison label (Sandil Ranasinghe)
  * Drop support for Safari 13 by removing left/right positioning in favour of CSS logical properties (Thibaud Colas)
  * Fix: Typo in `ResumeWorkflowActionFormatter` message (Stefan Hammer)
+ * Fix: Throw a meaningful error when saving an image to an unrecognised image format (Christian Franke)
 
 
 3.0 (xx.xx.xxxx) - IN DEVELOPMENT

CONTRIBUTORS.rst

@@ -596,6 +596,7 @@ Contributors
 * Simon Krull
 * Przemysław Buczkowski
 * Josh Woodcock
+* Christian Franke
 
 Translators
 ===========

docs/releases/4.0.md

@@ -22,6 +22,7 @@ depth: 1
 
  * Fix issue where `ModelAdmin` index listings with export list enabled would show buttons with an incorrect layout (Josh Woodcock)
  * Fix typo in `ResumeWorkflowActionFormatter` message (Stefan Hammer)
+ * Throw a meaningful error when saving an image to an unrecognised image format (Christian Franke)
 
 
 ## Upgrade considerations

wagtail/images/exceptions.py

@@ -1,2 +1,10 @@
 class InvalidFilterSpecError(ValueError):
     pass
+
+
+class UnknownOutputImageFormatError(ValueError):
+    """
+    Exception thrown when an unknown output format is requested for an image conversion
+    """
+
+    pass

wagtail/images/models.py

@@ -25,7 +25,10 @@ from willow.image import Image as WillowImage
 from wagtail import hooks
 from wagtail.admin.models import get_object_usage
 from wagtail.coreutils import string_to_ascii
-from wagtail.images.exceptions import InvalidFilterSpecError
+from wagtail.images.exceptions import (
+    InvalidFilterSpecError,
+    UnknownOutputImageFormatError,
+)
 from wagtail.images.image_operations import (
     FilterOperation,
     ImageTransform,
@@ -677,6 +680,9 @@ class Filter:
                     quality = getattr(settings, "WAGTAILIMAGES_WEBP_QUALITY", 85)
 
                 return willow.save_as_webp(output, quality=quality)
+            raise UnknownOutputImageFormatError(
+                f"Unknown output image format '{output_format}'"
+            )
 
     def get_cache_key(self, image):
         vary_parts = []

wagtail/images/tests/test_image_operations.py

@@ -5,12 +5,16 @@ from django.test import TestCase, override_settings
 
 from wagtail import hooks
 from wagtail.images import image_operations
-from wagtail.images.exceptions import InvalidFilterSpecError
+from wagtail.images.exceptions import (
+    InvalidFilterSpecError,
+    UnknownOutputImageFormatError,
+)
 from wagtail.images.image_operations import TransformOperation
 from wagtail.images.models import Filter, Image
 from wagtail.images.tests.utils import (
     get_test_image_file,
     get_test_image_file_jpeg,
+    get_test_image_file_tiff,
     get_test_image_file_webp,
 )
 
@@ -599,6 +603,18 @@ class TestFilter(TestCase):
         self.assertEqual(run_mock.call_count, 2)
 
 
+class TestUnknownOutputImageFormat(TestCase):
+    @hooks.register_temporarily(
+        "register_image_operations", register_image_operations_hook
+    )
+    def test_run_raises_error(self):
+        fil = Filter(spec="operation1|operation2")
+        image = Image.objects.create(
+            title="Test image", file=get_test_image_file_tiff()
+        )
+        self.assertRaises(UnknownOutputImageFormatError, fil.run, image, BytesIO())
+
+
 class TestFormatFilter(TestCase):
     def test_jpeg(self):
         fil = Filter(spec="width-400|format-jpeg")

wagtail/images/tests/utils.py

@@ -27,3 +27,10 @@ def get_test_image_file_webp(filename="test.webp", colour="white", size=(640, 48
     image = PIL.Image.new("RGB", size, colour)
     image.save(f, "WEBP")
     return ImageFile(f, name=filename)
+
+
+def get_test_image_file_tiff(filename="test.tiff", colour="white", size=(640, 480)):
+    f = BytesIO()
+    image = PIL.Image.new("RGB", size, colour)
+    image.save(f, "TIFF")
+    return ImageFile(f, name=filename)