ROB: Do not crash when the AcroForm fields entry is not an array

pypdf/_doc_common.py

@@ -584,7 +584,14 @@ def get_fields(
             return retval
         assert stack is not None
         if "/Fields" in tree:
-            fields = cast(ArrayObject, tree["/Fields"])
+            fields = tree["/Fields"].get_object()
+            if not isinstance(fields, ArrayObject):
+                logger_warning(
+                    "AcroForm /Fields is not an array: %(fields)s",
+                    source=__name__,
+                    fields=fields,
+                )
+                return retval
             for f in fields:
                 field = f.get_object()
                 self._build_field(field, retval, fileobj, field_attributes, stack)

tests/test_doc_common.py

@@ -1035,3 +1035,32 @@ def test_get_outline__entry_count():
                 expected_exception=LimitReachedError, match=r"^Maximum outline entry limit reached: 1001 > 1000\.$"
             ):
         writer._get_outline()
+
+
+@pytest.mark.parametrize(
+    ("fields", "expected"),
+    [
+        pytest.param(
+            NumberObject(1), "AcroForm /Fields is not an array: 1", id="number"
+        ),
+        pytest.param(
+            TextStringObject("x"), "AcroForm /Fields is not an array: x", id="string"
+        ),
+        pytest.param(
+            DictionaryObject(), "AcroForm /Fields is not an array: {}", id="dictionary"
+        ),
+    ],
+)
+def test_get_fields__fields_is_not_an_array(caplog, fields, expected):
+    """A malformed /Fields raised a TypeError when the loop tried to iterate it."""
+    writer = PdfWriter()
+    writer.add_blank_page(width=72, height=72)
+    writer.root_object[NameObject("/AcroForm")] = DictionaryObject(
+        {NameObject("/Fields"): fields}
+    )
+    stream = BytesIO()
+    writer.write(stream)
+    stream.seek(0)
+
+    assert PdfReader(stream).get_fields() == {}
+    assert expected in caplog.text