Ignore numbering levels that use numStyleLink to refer to themselves

3 files changed, 35 insertions, 8 deletions

diff --git a/NEWS b/NEWS
index 66e134e..cce165e 100644
--- a/NEWS
+++ b/NEWS
@@ -3,6 +3,9 @@
 * Fix: on Windows, when an image's content type includes a backslash in the
   subpart, files may be written outside of the directory set by --output-dir.
 
+* Detect and ignore numbering levels that use numStyleLink to refer to
+  themselves.
+
 # 1.12.0
 
 * Handle hyperlinked wp:anchor and wp:inline elements.
diff --git a/mammoth/docx/numbering_xml.py b/mammoth/docx/numbering_xml.py
index 0bd1952..22fa28f 100644
--- a/mammoth/docx/numbering_xml.py
+++ b/mammoth/docx/numbering_xml.py
@@ -101,18 +101,25 @@ class Numbering(object):
         self._styles = styles
 
     def find_level(self, num_id, level):
+        return self._find_level_with_seen_num_ids(num_id, level, seen_num_ids=set())
+
+    def _find_level_with_seen_num_ids(self, num_id, level, seen_num_ids):
+        if num_id in seen_num_ids:
+            return None
+        seen_num_ids.add(num_id)
+
         num = self._nums.get(num_id)
         if num is None:
             return None
+
+        abstract_num = self._abstract_nums.get(num.abstract_num_id)
+        if abstract_num is None:
+            return None
+        elif abstract_num.num_style_link is None:
+            return self._to_numbering_level(abstract_num.levels.get(level))
         else:
-            abstract_num = self._abstract_nums.get(num.abstract_num_id)
-            if abstract_num is None:
-                return None
-            elif abstract_num.num_style_link is None:
-                return self._to_numbering_level(abstract_num.levels.get(level))
-            else:
-                style = self._styles.find_numbering_style_by_id(abstract_num.num_style_link)
-                return self.find_level(style.num_id, level)
+            style = self._styles.find_numbering_style_by_id(abstract_num.num_style_link)
+            return self._find_level_with_seen_num_ids(style.num_id, level, seen_num_ids=seen_num_ids)
 
     def find_level_by_paragraph_style_id(self, style_id):
         return self._levels_by_paragraph_style_id.get(style_id)
diff --git a/tests/docx/numbering_xml_tests.py b/tests/docx/numbering_xml_tests.py
index 6cc779f..52aad5d 100644
--- a/tests/docx/numbering_xml_tests.py
+++ b/tests/docx/numbering_xml_tests.py
@@ -154,6 +154,23 @@ def test_when_abstract_num_has_num_style_link_then_style_is_used_to_find_num():
     assert_equal(True, numbering.find_level("201", "0").is_ordered)
 
 
+def test_when_abstract_num_has_self_recursive_num_style_link_then_level_is_not_found():
+    numbering = _read_numbering_xml_element(
+        xml_element("w:numbering", {}, [
+            xml_element("w:abstractNum", {"w:abstractNumId": "100"}, [
+                xml_element("w:numStyleLink", {"w:val": "List1"}),
+            ]),
+            xml_element("w:num", {"w:numId": "200"}, [
+                xml_element("w:abstractNumId", {"w:val": "100"}),
+            ]),
+        ]),
+        styles=Styles.create(numbering_styles={
+            "List1": NumberingStyle(style_id="List1", num_id="200"),
+        }),
+    )
+    assert_equal(None, numbering.find_level("200", "0"))
+
+
 # See: 17.9.23 pStyle (Paragraph Style's Associated Numbering Level) in ECMA-376, 4th Edition
 def test_numbering_level_can_be_found_by_paragraph_style_id():
     numbering = _read_numbering_xml_element(