BUG: Fix test_watermarking_reportlab_rendering() (#2203)

This fixes the issue spotted in #2191

The solution was to re-introduce calls to `PageObject._push_pop_gs()`,
in `PageObject._merge_page` & `PageObject._merge_page_writer()`,
but to optimize `PageObject._push_pop_gs()` by introducing a `ContentsStream.isolate_graphics_state()` method.

pypdf/_page.py

@@ -880,23 +880,6 @@ def _content_stream_rename(
                 raise KeyError(f"type of operands is {type(operands)}")
         return stream
 
-    @staticmethod
-    def _push_pop_gs(
-        contents: Any,
-        pdf: Union[None, PdfReaderProtocol, PdfWriterProtocol],
-        use_original: bool = True,
-    ) -> ContentStream:
-        # adds a graphics state "push" and "pop" to the beginning and end
-        # of a content stream.  This isolates it from changes such as
-        # transformation matricies.
-        if use_original:
-            stream = contents
-        else:
-            stream = ContentStream(contents, pdf)
-        stream.operations.insert(0, ([], "q"))
-        stream.operations.append(([], "Q"))
-        return stream
-
     @staticmethod
     def _add_transformation_matrix(
         contents: Any,
@@ -1127,6 +1110,7 @@ def _merge_page(
         new_content_array = ArrayObject()
         original_content = self.get_contents()
         if original_content is not None:
+            original_content.isolate_graphics_state()
             new_content_array.append(original_content)
 
         page2content = page2.get_contents()
@@ -1154,7 +1138,7 @@ def _merge_page(
             page2content = PageObject._content_stream_rename(
                 page2content, rename, self.pdf
             )
-            page2content = PageObject._push_pop_gs(page2content, self.pdf)
+            page2content.isolate_graphics_state()
             if over:
                 new_content_array.append(page2content)
             else:
@@ -1262,9 +1246,9 @@ def _merge_page_writer(
                     pass
 
         new_content_array = ArrayObject()
-
         original_content = self.get_contents()
         if original_content is not None:
+            original_content.isolate_graphics_state()
             new_content_array.append(original_content)
 
         page2content = page2.get_contents()
@@ -1292,7 +1276,7 @@ def _merge_page_writer(
             page2content = PageObject._content_stream_rename(
                 page2content, rename, self.pdf
             )
-            page2content = PageObject._push_pop_gs(page2content, self.pdf)
+            page2content.isolate_graphics_state()
             if over:
                 new_content_array.append(page2content)
             else:
@@ -1642,7 +1626,7 @@ def add_transformation(
         content = self.get_contents()
         if content is not None:
             content = PageObject._add_transformation_matrix(content, self.pdf, ctm)
-            content = PageObject._push_pop_gs(content, self.pdf)
+            content.isolate_graphics_state()
             self.replace_contents(content)
         # if expanding the page to fit a new page, calculate the new media box size
         if expand:

pypdf/_utils.py

@@ -328,11 +328,11 @@ def mark_location(stream: StreamType) -> None:
 
 
 def b_(s: Union[str, bytes]) -> bytes:
+    if isinstance(s, bytes):
+        return s
     bc = B_CACHE
     if s in bc:
         return bc[s]
-    if isinstance(s, bytes):
-        return s
     try:
         r = s.encode("latin-1")
         if len(s) < 2:

pypdf/generic/_data_structures.py

@@ -1229,6 +1229,13 @@ def operations(self, operations: List[Tuple[Any, Any]]) -> None:
         self._operations = operations
         self._data = b""
 
+    def isolate_graphics_state(self) -> None:
+        if self._operations:
+            self._operations.insert(0, ([], "q"))
+            self._operations.append(([], "Q"))
+        elif self._data:
+            self._data = b"q\n" + b_(self._data) + b"Q\n"
+
     # This overrides the parent method:
     def write_to_stream(
         self, stream: StreamType, encryption_key: Union[None, str, bytes] = None

tests/test_writer.py

@@ -1619,10 +1619,9 @@ def test_watermark_rendering(tmp_path):
     assert image_similarity(png_path, target_png_path) >= 0.95
 
 
-@pytest.mark.xfail(reason="issue introduced with pypdf==3.15.4")
 def test_watermarking_reportlab_rendering(tmp_path):
     """
-    This test shows that the merged page is rotated+mirrored.
+    This test is showing a rotated+mirrored watermark in pypdf==3.15.4.
 
     Replacing the generate_base with e.g. the crazyones did not show the issue.
     """
@@ -1637,9 +1636,6 @@ def test_watermarking_reportlab_rendering(tmp_path):
     base_page.merge_page(watermark)
     writer.add_page(base_page)
 
-    for page in writer.pages:
-        page.compress_content_streams()
-
     target_png_path = RESOURCE_ROOT / "test_watermarking_reportlab_rendering.png"
     pdf_path = tmp_path / "out.pdf"
     png_path = tmp_path / "test_watermarking_reportlab_rendering.png"