emoji: Add support for animated GIF images.

This commit adds 'resize_gif()' function which extracts each frame,
resize it and coalesces them again to form the resized GIF while
preserving the duration of the GIF. I read some stackoverflow
answers all of which were referring to BiggleZX's script
(https://gist.github.com/BigglesZX/4016539) for working with animated
GIF. I modified the script to fit to our usecase and did some manual
testing but the function was failing for some specific GIFs and was not
preserving the duration of animation. So I went ahead and read about
GIF format itself as well as PIL's `GifImagePlugin` code and came up
with this simple function which gets the worked done in a much cleaner
way. I tested this function on a number of GIF images from giphy.com
and it resized all of them correctly.

Fixes: #9945.

zerver/lib/upload.py

@@ -27,6 +27,7 @@ import base64
 import os
 import re
 from PIL import Image, ImageOps, ExifTags
+from PIL.GifImagePlugin import GifImageFile
 import io
 import random
 import logging
@@ -117,26 +118,39 @@ def resize_avatar(image_data: bytes, size: int=DEFAULT_AVATAR_SIZE) -> bytes:
     return out.getvalue()
 
 
+def resize_gif(im: GifImageFile, size: int=DEFAULT_EMOJI_SIZE) -> bytes:
+    frames = []
+    duration_info = []
+    # If 'loop' info is not set then loop for infinite number of times.
+    loop = im.info.get("loop", 0)
+    for frame_num in range(0, im.n_frames):
+        im.seek(frame_num)
+        new_frame = Image.new("RGBA", im.size)
+        new_frame.paste(im, (0, 0), im.convert("RGBA"))
+        new_frame = ImageOps.fit(new_frame, (size, size), Image.ANTIALIAS)
+        frames.append(new_frame)
+        duration_info.append(im.info['duration'])
+    out = io.BytesIO()
+    frames[0].save(out, save_all=True, optimize=True,
+                   format="GIF", append_images=frames[1:],
+                   duration=duration_info,
+                   loop=loop)
+    return out.getvalue()
+
 def resize_emoji(image_data: bytes, size: int=DEFAULT_EMOJI_SIZE) -> bytes:
     try:
         im = Image.open(io.BytesIO(image_data))
         image_format = im.format
-        im = exif_rotate(im)
-        if image_format == 'GIF' and im.is_animated:
-            if im.size[0] != im.size[1]:
-                raise JsonableError(
-                    _("Animated emoji must have the same width and height."))
-            elif im.size[0] > size:
-                raise JsonableError(
-                    _("Animated emoji can't be larger than 64px in width or height."))
-            else:
-                return image_data
-        im = ImageOps.fit(im, (size, size), Image.ANTIALIAS)
+        if image_format == "GIF":
+            return resize_gif(im, size)
+        else:
+            im = exif_rotate(im)
+            im = ImageOps.fit(im, (size, size), Image.ANTIALIAS)
+            out = io.BytesIO()
+            im.save(out, format=image_format)
+            return out.getvalue()
     except IOError:
         raise BadImageError("Could not decode image; did you upload an image file?")
-    out = io.BytesIO()
-    im.save(out, format=image_format)
-    return out.getvalue()
 
 
 ### Common