upload: Fix resizing non-animated images.

5dab6e9d31 began honoring the list of disposals for every frame.
Unfortunately, passing a list of disposals for a non-animated image
raises an exception:
```
  File "zerver/lib/upload.py", line 212, in resize_emoji
    image_data = resize_gif(im, size)
  File "zerver/lib/upload.py", line 165, in resize_gif
    frames[0].save(
  File "[...]/PIL/Image.py", line 2212, in save
    save_handler(self, fp, filename)
  File "[...]/PIL/GifImagePlugin.py", line 605, in _save
    _write_single_frame(im, fp, palette)
  File "[...]/PIL/GifImagePlugin.py", line 506, in _write_single_frame
    _write_local_header(fp, im, (0, 0), flags)
  File "[...]/PIL/GifImagePlugin.py", line 647, in _write_local_header
    disposal = int(im.encoderinfo.get("disposal", 0))
TypeError: int() argument must be a string, a bytes-like object or a
number, not 'list'
```

`check_add_realm_emoji` calls this as:

```
    try:
        is_animated = upload_emoji_image(image_file, emoji_file_name, a
uthor)
        emoji_uploaded_successfully = True
    finally:
        if not emoji_uploaded_successfully:
            realm_emoji.delete()
            return None
        # ...
```

This is equivalent to dropping _all_ exceptions silently.  As such,
Zulip has silently rejected all non-animated images larger than 64x64
since 5dab6e9d31.

Adjust to only pass a single disposal if there are no additional
frames.  Add a test for non-animated images, which requires also
fixing the incidental bug that all GIF images were being recorded as
animated, regardless of if they had more than 1 frame or not.

zerver/lib/upload.py

@@ -169,7 +169,7 @@ def resize_gif(im: GifImageFile, size: int = DEFAULT_EMOJI_SIZE) -> bytes:
         format="GIF",
         append_images=frames[1:],
         duration=duration_info,
-        disposal=disposals,
+        disposal=disposals if len(frames) > 1 else disposals[0],
         loop=loop,
     )
     return out.getvalue()
@@ -211,7 +211,10 @@ def resize_emoji(
             if should_resize:
                 image_data = resize_gif(im, size)
 
-            return image_data, True, still_image_data
+            if im.n_frames > 1:
+                return image_data, True, still_image_data
+            else:
+                return image_data, False, None
         else:
             # Note that this is essentially duplicated in the
             # still_image code path, above.

zerver/tests/test_realm_emoji.py

@@ -271,6 +271,18 @@ class RealmEmojiTest(ZulipTestCase):
             result = self.client_post("/json/realm/emoji/my_emoji", {"f1": fp1, "f2": fp2})
         self.assert_json_error(result, "You must upload exactly one file.")
 
+    def test_emoji_upload_success(self) -> None:
+        self.login("iago")
+        with get_test_image_file("img.gif") as fp:
+            result = self.client_post("/json/realm/emoji/my_emoji", {"file": fp})
+        self.assert_json_success(result)
+
+    def test_emoji_upload_resize_success(self) -> None:
+        self.login("iago")
+        with get_test_image_file("still_large_img.gif") as fp:
+            result = self.client_post("/json/realm/emoji/my_emoji", {"file": fp})
+        self.assert_json_success(result)
+
     def test_emoji_upload_file_size_error(self) -> None:
         self.login("iago")
         with get_test_image_file("img.png") as fp:

zerver/tests/test_upload.py

@@ -1343,6 +1343,14 @@ class EmojiTest(UploadSerializeMixin, ZulipTestCase):
             still_image = Image.open(io.BytesIO(still_img_data))
             self.assertEqual((50, 50), still_image.size)
 
+        # Test a non-animated image which does need to be resized
+        still_large_img_data = read_test_image_file("still_large_img.gif")
+        resized_img_data, is_animated, no_still_data = resize_emoji(still_large_img_data, size=50)
+        im = Image.open(io.BytesIO(resized_img_data))
+        self.assertEqual((50, 50), im.size)
+        self.assertFalse(is_animated)
+        assert no_still_data is None
+
     def tearDown(self) -> None:
         destroy_uploads()
         super().tearDown()