From d7b3cec727457e1f75d1e67d385c9c5d962cca64 Mon Sep 17 00:00:00 2001
From: Christian Zosel <christian@zosel.ch>
Date: Mon, 22 Sep 2025 15:58:22 +0200
Subject: [PATCH] fix(docx-template): corruption of libreoffice-originated
 templates

Certain versions of python-docx and python-docxtemplate cause corruption
of files that were originally created with LibreOffice. One effect of that
corruption is a duplicate entry in the document-internal files; there
are two docProps/core.xml files in the resulting document.

Co-Authored-By: David Vogt <david.vogt@adfinis.com>
---
 document_merge_service/api/apps.py            |  55 ++++++++++++++++++
 .../api/data/created_with_libreoffice.docx    | Bin 0 -> 4989 bytes
 .../api/tests/test_template.py                |  37 +++++++++++-
 3 files changed, 91 insertions(+), 1 deletion(-)
 create mode 100644 document_merge_service/api/data/created_with_libreoffice.docx

diff --git a/document_merge_service/api/apps.py b/document_merge_service/api/apps.py
index 68a27710..78326e0e 100644
--- a/document_merge_service/api/apps.py
+++ b/document_merge_service/api/apps.py
@@ -10,3 +10,58 @@ class DefaultConfig(AppConfig):
     def ready(self):
         if "sqlite3" in settings.DATABASES["default"]["ENGINE"]:  # pragma: no cover
             TextField.register_lookup(IContains, lookup_name="search")
+        mitigate_docxtpl_corruption_bug()
+
+
+def mitigate_docxtpl_corruption_bug():
+    # This is basically monkey-patching this PR:
+    # https://github.com/python-openxml/python-docx/pull/1436
+
+    # Hold my beer!
+    from docx.opc.constants import RELATIONSHIP_TYPE
+
+    if hasattr(RELATIONSHIP_TYPE, "CORE_PROPERTIES_OFFICEDOCUMENT"):  # pragma: no cover
+        raise Exception(
+            "The docxtpl mitigation is no longer required, please remove the monkeypatch code"
+        )
+
+    RELATIONSHIP_TYPE.CORE_PROPERTIES_OFFICEDOCUMENT = (
+        "http://schemas.openxmlformats.org/officedocument/2006/relationships"
+        "/metadata/core-properties"
+    )
+
+    from docx.opc.package import RT, CorePropertiesPart, OpcPackage, cast
+
+    @property
+    def _core_properties_part(self) -> CorePropertiesPart:
+        """|CorePropertiesPart| object related to this package.
+
+        Creates a default core properties part if one is not present (not common).
+        """
+        try:
+            return cast(CorePropertiesPart, self.part_related_by(RT.CORE_PROPERTIES))
+        except KeyError:
+            try:
+                office_document_part = self.part_related_by(
+                    RT.CORE_PROPERTIES_OFFICEDOCUMENT  # type: ignore
+                )
+                rel = self.relate_to(
+                    office_document_part,
+                    RT.CORE_PROPERTIES_OFFICEDOCUMENT,  # type: ignore
+                )
+                self.rels[rel].reltype = RT.CORE_PROPERTIES
+                return cast(CorePropertiesPart, office_document_part)
+            except KeyError:
+                core_properties_part = CorePropertiesPart.default(self)
+                self.relate_to(core_properties_part, RT.CORE_PROPERTIES)
+                return core_properties_part
+
+    OpcPackage._core_properties_part = _core_properties_part
+
+    from docx.opc.rel import _Relationship
+
+    @_Relationship.reltype.setter
+    def reltype(self, value: str):
+        self._reltype = value
+
+    _Relationship.reltype = reltype
diff --git a/document_merge_service/api/data/created_with_libreoffice.docx b/document_merge_service/api/data/created_with_libreoffice.docx
new file mode 100644
index 0000000000000000000000000000000000000000..39f4956e5193407484a1537d912f1d19b4fd09ae
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diff --git a/document_merge_service/api/tests/test_template.py b/document_merge_service/api/tests/test_template.py
index d0964383..cb397a6d 100644
--- a/document_merge_service/api/tests/test_template.py
+++ b/document_merge_service/api/tests/test_template.py
@@ -2,7 +2,9 @@
 import json
 import os
 import re
-from collections import namedtuple
+import tempfile
+import zipfile
+from collections import Counter, namedtuple
 
 import openpyxl
 import pytest
@@ -916,3 +918,36 @@ def test_placeholder_with_unsupported_operand(
     with pytest.raises(exceptions.ValidationError) as exc_info:
         serializer.validate({"data": {"E_BAU_NUMBER": 12345}})
     assert exc_info.value.args[0] == expected_error
+
+
+def test_template_merge_docx_libreoffice_bug(
+    db, client, mock_filefield_name_validation, template, snapshot
+):
+    """Verify a certain docx corruption bug does not occur.
+
+    Certain versions of python-docx and python-docxtemplate cause corruption
+    of files that were originally created with LibreOffice. One effect of that
+    corruption is a duplicate entry in the document-internal files; there
+    are two docProps/core.xml files in the resulting document.
+    """
+    file = django_file("created_with_libreoffice.docx")
+    template.template.save(os.path.basename(file.name), file)
+    template.engine = "docx-template"
+    template.save()
+    url = reverse("template-merge", args=[template.pk])
+
+    response = client.post(url, data={"data": {"test": "Test input"}}, format="json")
+
+    with tempfile.NamedTemporaryFile(suffix=".docx") as tmp:
+        tmp.write(response.content)
+        tmp.seek(0)
+
+        zzz = zipfile.ZipFile(tmp.name)
+        name_counter = Counter()
+        name_counter.update([f.filename for f in zzz.filelist])
+
+    problematic_names = {
+        name: count for name, count in name_counter.most_common() if count > 1
+    }
+
+    assert problematic_names == {}, "Duplicate entry in docx file's internal structure"