Netscape GIF extension belongs after the global color table

Tests/test_file_gif.py

@@ -772,9 +772,16 @@ def test_number_of_loops(tmp_path):
     im = Image.new("L", (100, 100), "#000")
     im.save(out, loop=number_of_loops)
     with Image.open(out) as reread:
-
         assert reread.info["loop"] == number_of_loops
 
+    # Check that even if a subsequent GIF frame has the number of loops specified,
+    # only the value from the first frame is used
+    with Image.open("Tests/images/duplicate_number_of_loops.gif") as im:
+        assert im.info["loop"] == 2
+
+        im.seek(1)
+        assert im.info["loop"] == 2
+
 
 def test_background(tmp_path):
     out = str(tmp_path / "temp.gif")

src/PIL/GifImagePlugin.py

@@ -244,15 +244,15 @@ def _seek(self, frame, update_image=True):
                         info["comment"] = comment
                     s = None
                     continue
-                elif s[0] == 255:
+                elif s[0] == 255 and frame == 0:
                     #
                     # application extension
                     #
                     info["extension"] = block, self.fp.tell()
                     if block[:11] == b"NETSCAPE2.0":
                         block = self.data()
                         if len(block) >= 3 and block[0] == 1:
-                            info["loop"] = i16(block, 1)
+                            self.info["loop"] = i16(block, 1)
                 while self.data():
                     pass
 
@@ -399,7 +399,7 @@ def _rgb(color):
 
         if info.get("comment"):
             self.info["comment"] = info["comment"]
-        for k in ["duration", "extension", "loop"]:
+        for k in ["duration", "extension"]:
             if k in info:
                 self.info[k] = info[k]
             elif k in self.info:
@@ -716,18 +716,6 @@ def _write_local_header(fp, im, offset, flags):
             + o8(0)
         )
 
-    if "loop" in im.encoderinfo:
-        number_of_loops = im.encoderinfo["loop"]
-        fp.write(
-            b"!"
-            + o8(255)  # extension intro
-            + o8(11)
-            + b"NETSCAPE2.0"
-            + o8(3)
-            + o8(1)
-            + o16(number_of_loops)  # number of loops
-            + o8(0)
-        )
     include_color_table = im.encoderinfo.get("include_color_table")
     if include_color_table:
         palette_bytes = _get_palette_bytes(im)
@@ -933,6 +921,17 @@ def _get_global_header(im, info):
         # Global Color Table
         _get_header_palette(palette_bytes),
     ]
+    if "loop" in info:
+        header.append(
+            b"!"
+            + o8(255)  # extension intro
+            + o8(11)
+            + b"NETSCAPE2.0"
+            + o8(3)
+            + o8(1)
+            + o16(info["loop"])  # number of loops
+            + o8(0)
+        )
     if info.get("comment"):
         comment_block = b"!" + o8(254)  # extension intro