ROB: Attempt to deal with non-rectangular FlateDecode streams (#3245)

Closes #3241.

Author: stefan6419846

pypdf/filters.py

@@ -179,8 +179,10 @@ def decode(
     @staticmethod
     def _decode_png_prediction(data: bytes, columns: int, rowlength: int) -> bytes:
         # PNG prediction can vary from row to row
-        if len(data) % rowlength != 0:
-            raise PdfReadError("Image data is not rectangular")
+        if (remainder := len(data) % rowlength) != 0:
+            logger_warning("Image data is not rectangular. Adding padding.", __name__)
+            data += b"\x00" * (rowlength - remainder)
+            assert len(data) % rowlength == 0
         output = []
         prev_rowdata = (0,) * rowlength
         bpp = (rowlength - 1) // columns  # recomputed locally to not change params

tests/test_filters.py

@@ -374,9 +374,8 @@ def test_iss1787():
     obj = data.indirect_reference.get_object()
     obj["/DecodeParms"][NameObject("/Columns")] = NumberObject(1000)
     obj.decoded_self = None
-    with pytest.raises(PdfReadError) as exc:
-        reader.pages[0].images[0]
-    assert exc.value.args[0] == "Image data is not rectangular"
+    with pytest.raises(expected_exception=PdfReadError, match="^Unsupported PNG filter 244$"):
+        _ = reader.pages[0].images[0]
 
 
 @pytest.mark.enable_socket
@@ -672,3 +671,22 @@ def test_flate_decode__image_is_none_due_to_size_limit(caplog):
         "Failed loading image: Image size (180000000 pixels) exceeds limit of "
         "178956970 pixels, could be decompression bomb DOS attack."
     ) in caplog.messages
+
+
+@pytest.mark.enable_socket
+def test_flate_decode__not_rectangular(caplog):
+    url = "https://github.com/user-attachments/files/19663603/issue3241_compressed.txt"
+    name = "issue3241.txt"
+    data = get_data_from_url(url, name=name)
+    decode_parms = DictionaryObject()
+    decode_parms[NameObject("/Predictor")] = NumberObject(15)
+    decode_parms[NameObject("/Columns")] = NumberObject(4881)
+    actual = FlateDecode.decode(data=data, decode_parms=decode_parms)
+    actual_image = BytesIO()
+    Image.frombytes(mode="1", size=(4881, 81), data=actual).save(actual_image, format="png")
+
+    url = "https://github.com/user-attachments/assets/c5695850-c076-4255-ab72-7c86851a4a04"
+    name = "issue3241.png"
+    expected = get_data_from_url(url, name=name)
+    assert actual_image.getvalue() == expected
+    assert caplog.messages == ["Image data is not rectangular. Adding padding."]