From c2fe53c5fa2133019f07a95fa397acd2b6465485 Mon Sep 17 00:00:00 2001
From: Benjamin Gilbert <bgilbert@backtick.net>
Date: Mon, 4 Dec 2023 07:31:24 -0600
Subject: [PATCH] Fail if chroma subsampling selected when writing RGB JPEG

The user presumably doesn't intend to subsample the green and blue
channels.
---
 Tests/test_file_jpeg.py              | 12 +++++++++---
 docs/handbook/image-file-formats.rst |  3 +++
 src/libImaging/JpegEncode.c          | 10 ++++++++++
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/Tests/test_file_jpeg.py b/Tests/test_file_jpeg.py
index 748c07119ba..75851e4773e 100644
--- a/Tests/test_file_jpeg.py
+++ b/Tests/test_file_jpeg.py
@@ -447,9 +447,15 @@ def getsampling(im):
             im = self.roundtrip(hopper(), subsampling=subsampling)
             assert getsampling(im) == (2, 2, 1, 1, 1, 1)
 
-        # RGB colorspace, no subsampling by default
-        im = self.roundtrip(hopper(), keep_rgb=True)
-        assert getsampling(im) == (1, 1, 1, 1, 1, 1)
+        # RGB colorspace
+        for subsampling in (-1, 0, "4:4:4"):
+            # "4:4:4" doesn't really make sense for RGB, but the conversion
+            # to an integer happens at a higher level
+            im = self.roundtrip(hopper(), keep_rgb=True, subsampling=subsampling)
+            assert getsampling(im) == (1, 1, 1, 1, 1, 1)
+        for subsampling in (1, "4:2:2", 2, "4:2:0", 3):
+            with pytest.raises(OSError):
+                self.roundtrip(hopper(), keep_rgb=True, subsampling=subsampling)
 
         with pytest.raises(TypeError):
             self.roundtrip(hopper(), subsampling="1:1:1")
diff --git a/docs/handbook/image-file-formats.rst b/docs/handbook/image-file-formats.rst
index 20310dedd35..2324c997d58 100644
--- a/docs/handbook/image-file-formats.rst
+++ b/docs/handbook/image-file-formats.rst
@@ -491,6 +491,9 @@ The :py:meth:`~PIL.Image.Image.save` method supports the following options:
     If this option is present and true, those images will be stored as RGB
     instead.
 
+    When this option is enabled, attempting to chroma-subsample RGB images
+    with the ``subsampling`` option will result in an error.
+
     .. versionadded:: 10.2.0
 
 **subsampling**
diff --git a/src/libImaging/JpegEncode.c b/src/libImaging/JpegEncode.c
index 2946cc530a1..fc9966235c3 100644
--- a/src/libImaging/JpegEncode.c
+++ b/src/libImaging/JpegEncode.c
@@ -144,6 +144,16 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
 #ifdef JCS_EXTENSIONS
                     case JCS_EXT_RGBX:
 #endif
+                        switch (context->subsampling) {
+                            case -1:  /* Default */
+                            case 0:   /* No subsampling */
+                                break;
+                            default:
+                                /* Would subsample the GB channels, which
+                                   doesn't make sense */
+                                state->errcode = IMAGING_CODEC_CONFIG;
+                                return -1;
+                        }
                         jpeg_set_colorspace(&context->cinfo, JCS_RGB);
                         break;
                     default: