Fix ValueError: image has wrong mode in thumbnail generation (#1128)

* Initial plan

* Fix ValueError: image has wrong mode in thumbnail generation

Convert images with unsupported PIL modes (P, CMYK, I, F, 1, LA, PA)
to RGB or RGBA before calling thumbnail() in get_thumbnail_bytes().
This prevents the ValueError that occurs when PIL tries to resize
images with these modes using high-quality resamplers.

Co-authored-by: robertatakenaka <505143+robertatakenaka@users.noreply.github.com>

* Improve thumbnail tests: use 300x300 images and assert JPEG output

Use image dimensions larger than thumbnail_size (267x140) to ensure
the resize path is exercised. Assert the result is valid JPEG with
dimensions bounded by thumbnail_size.

Co-authored-by: robertatakenaka <505143+robertatakenaka@users.noreply.github.com>

* Simplify mode normalization: single rule converting to RGB

Remove LA/PA special-casing since _get_bytes() always converts to RGB
before saving. Add comment explaining why the conversion is needed.

Co-authored-by: robertatakenaka <505143+robertatakenaka@users.noreply.github.com>

* Extract _normalize_mode() helper and reuse in both create_thumbnail() and get_thumbnail_bytes()

Co-authored-by: robertatakenaka <505143+robertatakenaka@users.noreply.github.com>

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Apply suggestion from @Copilot

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: robertatakenaka <505143+robertatakenaka@users.noreply.github.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: 3 people

packtools/utils.py

@@ -369,7 +369,7 @@ def create_thumbnail(self, destination_path=None):
                 'Error opening image file "%s": %s' % (self.image_file_path, str(exc))
             )
         else:
-            thumbnail_file = image_file.copy()
+            thumbnail_file = self._normalize_mode(image_file.copy())
             new_filename = os.path.splitext(self.image_file_path)[0] + ".thumbnail.jpg"
             if destination_path is not None and len(destination_path) > 0:
                 new_filename = os.path.join(
@@ -386,6 +386,32 @@ def create_thumbnail(self, destination_path=None):
             finally:
                 image_file.close()
 
+    @staticmethod
+    def _normalize_mode(image):
+        """Normalize the image mode so it is compatible with ``Image.thumbnail()``
+        and with saving as JPEG.
+
+        Some modes such as P, CMYK, I, F, 1, LA, and PA are not accepted by
+        ``Image.thumbnail()`` and may raise ``ValueError``. Additionally, Pillow
+        cannot save JPEGs from images with an alpha channel (e.g. RGBA). This
+        helper converts images in unsupported modes to RGB, and composites RGBA
+        images onto an RGB background, while leaving RGB and L images unchanged,
+        so that both ``create_thumbnail()`` and ``get_thumbnail_bytes()`` share
+        the same normalization logic and can be safely saved as JPEG.
+        """
+        # Handle images with alpha channel explicitly so they can be saved as JPEG.
+        if image.mode == "RGBA":
+            # Composite onto a white background to remove transparency safely.
+            background = Image.new("RGB", image.size, (255, 255, 255))
+            alpha = image.split()[3]
+            background.paste(image, mask=alpha)
+            return background
+
+        # Convert any other unsupported mode to RGB.
+        if image.mode not in ("RGB", "L"):
+            return image.convert("RGB")
+        return image
+
     def _get_bytes(self, format):
         image_file = io.BytesIO()
         try:
@@ -418,6 +444,8 @@ def get_thumbnail_bytes(self):
                 % self.filename
             )
 
+        self._image_object = self._normalize_mode(self._image_object)
+
         self._image_object.thumbnail(self.thumbnail_size)
         return self._get_bytes("JPEG")
 

tests/test_utils.py

@@ -463,6 +463,58 @@ def test_get_thumbnail_bytes_ok(self):
         image_copy.save(image_expected, "JPEG")
         self.assertEqual(result, image_expected.getvalue())
 
+    def test_get_thumbnail_bytes_with_palette_mode(self):
+        mocked_image = Image.new("P", (300, 300))
+        web_image_generator = utils.WebImageGenerator(
+            "image.png", self.extracted_package
+        )
+        web_image_generator._image_object = mocked_image
+
+        result = web_image_generator.get_thumbnail_bytes()
+        result_image = Image.open(io.BytesIO(result))
+        self.assertEqual(result_image.format, "JPEG")
+        self.assertLessEqual(result_image.size[0], web_image_generator.thumbnail_size[0])
+        self.assertLessEqual(result_image.size[1], web_image_generator.thumbnail_size[1])
+
+    def test_get_thumbnail_bytes_with_cmyk_mode(self):
+        mocked_image = Image.new("CMYK", (300, 300))
+        web_image_generator = utils.WebImageGenerator(
+            "image.tiff", self.extracted_package
+        )
+        web_image_generator._image_object = mocked_image
+
+        result = web_image_generator.get_thumbnail_bytes()
+        result_image = Image.open(io.BytesIO(result))
+        self.assertEqual(result_image.format, "JPEG")
+        self.assertLessEqual(result_image.size[0], web_image_generator.thumbnail_size[0])
+        self.assertLessEqual(result_image.size[1], web_image_generator.thumbnail_size[1])
+
+    def test_get_thumbnail_bytes_with_la_mode(self):
+        mocked_image = Image.new("LA", (300, 300))
+        web_image_generator = utils.WebImageGenerator(
+            "image.png", self.extracted_package
+        )
+        web_image_generator._image_object = mocked_image
+
+        result = web_image_generator.get_thumbnail_bytes()
+        result_image = Image.open(io.BytesIO(result))
+        self.assertEqual(result_image.format, "JPEG")
+        self.assertLessEqual(result_image.size[0], web_image_generator.thumbnail_size[0])
+        self.assertLessEqual(result_image.size[1], web_image_generator.thumbnail_size[1])
+
+    def test_get_thumbnail_bytes_with_1_mode(self):
+        mocked_image = Image.new("1", (300, 300))
+        web_image_generator = utils.WebImageGenerator(
+            "image.tiff", self.extracted_package
+        )
+        web_image_generator._image_object = mocked_image
+
+        result = web_image_generator.get_thumbnail_bytes()
+        result_image = Image.open(io.BytesIO(result))
+        self.assertEqual(result_image.format, "JPEG")
+        self.assertLessEqual(result_image.size[0], web_image_generator.thumbnail_size[0])
+        self.assertLessEqual(result_image.size[1], web_image_generator.thumbnail_size[1])
+
 
 class TestXMLWebOptimiser(unittest.TestCase):
     def setUp(self):