support round-tripping JPEG comments

Tests/test_file_jpeg.py

@@ -87,6 +87,18 @@ class TestFileJpeg:
 
             assert im.info["comment"] == b"File written by Adobe Photoshop\xa8 4.0\x00"
 
+    def test_com_write(self):
+        dummy_text = "this is a test comment"
+        with Image.open(TEST_FILE) as im:
+            with BytesIO() as buf:
+                im.save(buf, format="JPEG")
+                with Image.open(buf) as im2:
+                    assert im.app['COM'] == im2.app['COM']
+            with BytesIO() as buf:
+                im.save(buf, format="JPEG", comment=dummy_text)
+                with Image.open(buf) as im2:
+                    assert im2.app['COM'].decode() == dummy_text
+
     def test_cmyk(self):
         # Test CMYK handling.  Thanks to Tim and Charlie for test data,
         # Michael for getting me to look one more time.

src/PIL/JpegImagePlugin.py

@@ -44,6 +44,7 @@ import warnings
 from . import Image, ImageFile, TiffImagePlugin
 from ._binary import i16be as i16
 from ._binary import i32be as i32
+from ._binary import o16be as o16
 from ._binary import o8
 from ._deprecate import deprecate
 from .JpegPresets import presets
@@ -713,6 +714,15 @@ def _save(im, fp, filename):
 
     extra = info.get("extra", b"")
 
+    comment = info.get("comment")
+    if comment is None and isinstance(im, JpegImageFile):
+        comment = im.app.get('COM')
+    if comment:
+        if isinstance(comment, str):
+            comment = comment.encode()
+        size = o16(2 + len(comment))
+        extra += b'\xFF\xFE%s%s' % (size, comment)
+
     icc_profile = info.get("icc_profile")
     if icc_profile:
         ICC_OVERHEAD_LEN = 14