Fix saving MPO with more than one appended image (#8979)

Author: hugovk

Tests/test_file_mpo.py

@@ -296,16 +296,18 @@ def test_save_all() -> None:
             assert_image_similar(im, im_reloaded, 30)
 
     im = Image.new("RGB", (1, 1))
-    im2 = Image.new("RGB", (1, 1), "#f00")
-    im_reloaded = roundtrip(im, save_all=True, append_images=[im2])
-
-    assert_image_equal(im, im_reloaded)
-    assert isinstance(im_reloaded, MpoImagePlugin.MpoImageFile)
-    assert im_reloaded.mpinfo is not None
-    assert im_reloaded.mpinfo[45056] == b"0100"
-
-    im_reloaded.seek(1)
-    assert_image_similar(im2, im_reloaded, 1)
+    for colors in (("#f00",), ("#f00", "#0f0")):
+        append_images = [Image.new("RGB", (1, 1), color) for color in colors]
+        im_reloaded = roundtrip(im, save_all=True, append_images=append_images)
+
+        assert_image_equal(im, im_reloaded)
+        assert isinstance(im_reloaded, MpoImagePlugin.MpoImageFile)
+        assert im_reloaded.mpinfo is not None
+        assert im_reloaded.mpinfo[45056] == b"0100"
+
+        for im_expected in append_images:
+            im_reloaded.seek(im_reloaded.tell() + 1)
+            assert_image_similar(im_reloaded, im_expected, 1)
 
     # Test that a single frame image will not be saved as an MPO
     jpg = roundtrip(im, save_all=True)

src/PIL/MpoImagePlugin.py

@@ -19,7 +19,6 @@
 #
 from __future__ import annotations
 
-import itertools
 import os
 import struct
 from typing import IO, Any, cast
@@ -47,12 +46,18 @@ def _save_all(im: Image.Image, fp: IO[bytes], filename: str | bytes) -> None:
 
     mpf_offset = 28
     offsets: list[int] = []
-    for imSequence in itertools.chain([im], append_images):
-        for im_frame in ImageSequence.Iterator(imSequence):
+    im_sequences = [im, *append_images]
+    total = sum(getattr(seq, "n_frames", 1) for seq in im_sequences)
+    for im_sequence in im_sequences:
+        for im_frame in ImageSequence.Iterator(im_sequence):
             if not offsets:
                 # APP2 marker
+                ifd_length = 66 + 16 * total
                 im_frame.encoderinfo["extra"] = (
-                    b"\xff\xe2" + struct.pack(">H", 6 + 82) + b"MPF\0" + b" " * 82
+                    b"\xff\xe2"
+                    + struct.pack(">H", 6 + ifd_length)
+                    + b"MPF\0"
+                    + b" " * ifd_length
                 )
                 exif = im_frame.encoderinfo.get("exif")
                 if isinstance(exif, Image.Exif):