TST: Regression test against non-deterministic accidental object reuse

Full credit to sjoerdjob for this contribution via #1995

See #1788

Co-authored-by: Sjoerd Job Postmus <sjoerdjob@sjec.nl>

tests/test_writer.py

@@ -1859,3 +1859,31 @@ def test_object_contains_indirect_reference_to_self():
     outpage = writer.add_blank_page(width, height)
     outpage.merge_page(reader.pages[6])
     writer.append(reader)
+
+
+def test_merging_many_temporary_files():
+    def create_number_pdf(n) -> BytesIO:
+        from fpdf import FPDF
+
+        pdf = FPDF()
+        pdf.add_page()
+        pdf.set_font("helvetica", "B", 16)
+        pdf.cell(40, 10, str(n))
+        byte_string = pdf.output()
+        return BytesIO(byte_string)
+
+    writer = PdfWriter()
+    for n in range(100):
+        reader = PdfReader(create_number_pdf(n))
+        for page in reader.pages:
+            # Should only be one page.
+            writer.add_page(page)
+
+    out = BytesIO()
+    writer.write(out)
+
+    out.seek(0)
+    reader = PdfReader(out)
+    for n, page in enumerate(reader.pages):
+        text = page.extract_text()
+        assert text == str(n)