Fix indices of colors read from GIMP Palette file

Tests/images/custom_gimp_palette.gpl

@@ -1,12 +1,12 @@
 GIMP Palette
 Name: custompalette
 Columns: 4
-#
-  0   0   0     Index 3
- 65  38  30     Index 4
-103  62  49     Index 6
- 79  73  72     Index 7
-114 101  97     Index 8
-208 127 100     Index 9
-151 144 142     Index 10
-221 207 199     Index 11
+# Original written by David Wetz in https://stackoverflow.com/questions/815836/im-creating-a-program-that-generates-a-palette-from-a-true-color-image-need-hel/815855#815855
+  0   0   0     Index 0
+ 65  38  30     Index 1
+103  62  49     Index 2
+ 79  73  72     Index 3
+114 101  97     Index 4
+208 127 100     Index 5
+151 144 142     Index 6
+221 207 199     Index 7

Tests/test_file_gimppalette.py

@@ -22,6 +22,13 @@ def test_sanity() -> None:
             GimpPaletteFile(fp)
 
 
+def test_large_file_is_truncated(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(GimpPaletteFile, "_max_file_size", 100)
+    with open("Tests/images/custom_gimp_palette.gpl", "rb") as fp:
+        with pytest.warns(UserWarning):
+            GimpPaletteFile(fp)
+
+
 def test_get_palette() -> None:
     # Arrange
     with open("Tests/images/custom_gimp_palette.gpl", "rb") as fp:
@@ -31,5 +38,18 @@ def test_get_palette() -> None:
     palette, mode = palette_file.getpalette()
 
     # Assert
+    expected_palette: list[int] = []
+    for color in (
+        (0, 0, 0),
+        (65, 38, 30),
+        (103, 62, 49),
+        (79, 73, 72),
+        (114, 101, 97),
+        (208, 127, 100),
+        (151, 144, 142),
+        (221, 207, 199),
+    ):
+        expected_palette += color
+    assert palette == bytes(expected_palette)
     assert mode == "RGB"
     assert len(palette) / 3 == 8

src/PIL/GimpPaletteFile.py

@@ -16,6 +16,7 @@
 from __future__ import annotations
 
 import re
+import warnings
 from typing import IO
 
 
@@ -24,24 +25,40 @@ class GimpPaletteFile:
 
     rawmode = "RGB"
 
+    #: override if reading larger palettes is needed
+    max_colors = 256
+    _max_line_size = 100
+    _max_file_size = 2**20  # 1MB
+
     def __init__(self, fp: IO[bytes]) -> None:
         if not fp.readline().startswith(b"GIMP Palette"):
             msg = "not a GIMP palette file"
             raise SyntaxError(msg)
 
+        read = 0
+
         palette: list[int] = []
-        for _ in range(256):
-            s = fp.readline()
+        while len(palette) < 3 * self.max_colors:
+
+            s = fp.readline(self._max_file_size)
             if not s:
                 break
 
+            read += len(s)
+            if read >= self._max_file_size:
+                warnings.warn(
+                    f"Palette file truncated at {self._max_file_size - len(s)} bytes"
+                )
+                break
+
             # skip fields and comment lines
             if re.match(rb"\w+:|#", s):
                 continue
-            if len(s) > 100:
+            if len(s) > self._max_line_size:
                 msg = "bad palette file"
                 raise SyntaxError(msg)
 
+            # 4th column is color name and may contain spaces.
             v = s.split(maxsplit=3)
             if len(v) < 3:
                 msg = "bad palette entry"