Handle errors from publish_parts

- Fixes: #3892

readthedocs/core/templatetags/core_tags.py

@@ -46,6 +46,7 @@ def make_document_url(project, version=None, page=''):
 def restructuredtext(value, short=False):
     try:
         from docutils.core import publish_parts
+        from docutils import ApplicationError
     except ImportError:
         if settings.DEBUG:
             raise template.TemplateSyntaxError(
@@ -59,8 +60,15 @@ def restructuredtext(value, short=False):
             'file_insertion_enabled': False,
         }
         docutils_settings.update(getattr(settings, 'RESTRUCTUREDTEXT_FILTER_SETTINGS', {}))
-        parts = publish_parts(source=force_bytes(value), writer_name="html4css1",
-                              settings_overrides=docutils_settings)
+        try:
+            parts = publish_parts(
+                source=force_bytes(value),
+                writer_name="html4css1",
+                settings_overrides=docutils_settings,
+            )
+        except ApplicationError:
+            return force_text(value)
+
         out = force_text(parts["fragment"])
         try:
             if short:

readthedocs/rtd_tests/tests/test_core_tags.py

@@ -180,3 +180,17 @@ class CoreTagsTests(TestCase):
         proj.documentation_type = 'mkdocs'
         url = core_tags.make_document_url(proj, LATEST, 'index.html')
         self.assertEqual(url, 'http://readthedocs.org/docs/pip/en/latest/')
+
+    def test_restructured_text(self):
+        value = '*test*'
+        result = core_tags.restructuredtext(value)
+        self.assertIn('<em>test</em>', result)
+
+    def test_restructured_text_invalid(self):
+        value = (
+            '*******\n'
+            'Test\n'
+            '****\n\n'
+        )
+        result = core_tags.restructuredtext(value)
+        self.assertEqual(result, value)