BUG: Fix object count for incremental writer (#3117)

Closes #3116.

---------

Co-authored-by: m32 <grzegorz.makarewicz@gmail.com>

pypdf/_writer.py

@@ -1190,8 +1190,8 @@ def clone_reader_document_root(self, reader: PdfReader) -> None:
         """
         self._info_obj = None
         if self.incremental:
-            self._objects = [None] * cast(int, reader.trailer["/Size"])
-            for i in range(len(self._objects) - 1):
+            self._objects = [None] * (cast(int, reader.trailer["/Size"]) - 1)
+            for i in range(len(self._objects)):
                 o = reader.get_object(i + 1)
                 if o is not None:
                     self._objects[i] = o.replicate(self)

tests/test_writer.py

@@ -2618,3 +2618,37 @@ def test_insert_filtered_annotations__annotations_are_none():
     assert writer._insert_filtered_annotations(
         annots=None, page=PageObject(), pages={}, reader=reader
     ) == []
+
+
+def test_incremental_read():
+    """Test for #3116"""
+    writer = PdfWriter()
+    writer.add_blank_page(72, 72)
+    stream0 = BytesIO()
+    writer.write(stream0)
+
+    reader = PdfReader(stream0)
+    # 1 = Catalog, 2 = Pages, 3 = New Page, 4 = Info, Size == 5
+    assert reader.trailer["/Size"] == 5
+
+    stream0.seek(0, 0)
+    writer = PdfWriter(stream0, incremental=True)
+    assert len(writer._objects) == 4
+    assert writer._objects[-1] is not None
+    stream1 = BytesIO()
+    writer.write(stream1)
+
+    # nothing modified, so nothing added = ideal situation
+    assert stream1.getvalue() == stream1.getvalue()
+
+    stream0.seek(0, 0)
+    writer = PdfWriter(stream0, incremental=True)
+    assert len(writer._objects) == 4
+    assert writer._objects[-1] is not None
+    writer.add_blank_page(72, 72)
+    assert len(writer._objects) == 5
+    stream1 = BytesIO()
+    writer.write(stream1)
+    # 2 = Pages, 5 = New Page, 6 = XRef, Size == 7
+    # XRef is created on write and not counted
+    assert len(writer._objects) == 5