items: fix +index crash on unmatched itemid

src/moin/items/__init__.py

@@ -39,7 +39,7 @@
 
 from urllib.parse import unquote
 
-from whoosh.query import Term, Prefix, And, Or, Not
+from whoosh.query import Term, Prefix, And, Or, Not, NullQuery
 
 from moin import current_app, flaskg, log
 from moin.constants.chartypes import CHARS_UPPER, CHARS_LOWER
@@ -1425,8 +1425,15 @@ def build_index_query(self, startswith=None, selected_groups=None, isglobalindex
            isglobalindex: ???
 
         Output:
-          Returns a whoosh.query.Prefix object for the input parameters
+          Returns a whoosh.query.Prefix object for the input parameters,
+          or a NullQuery matching nothing when the item has no subitem
+          prefixes (e.g. an index rooted at an itemid that matches nothing).
         """
+        if not isglobalindex and not self.subitem_prefixes:
+            # an item addressed by something other than a name, such as a
+            # nonexistent itemid, has no names and thus no subitem prefixes;
+            # no stored item can be one of its subitems
+            return NullQuery()
         prefix = "" if isglobalindex else self.subitem_prefixes[0]
         if startswith:
             query = Prefix(NAME_EXACT, prefix + startswith) | Prefix(NAME_EXACT, prefix + startswith.swapcase())

src/moin/items/_tests/test_Item.py

@@ -215,6 +215,15 @@ def testIndex(self):
         fix_files, fix_builds = fix_meta(files, builds)
         assert fix_files == fix_builds
 
+    def testIndexMissingItemid(self):
+        # an index rooted at an itemid that matches nothing has no names,
+        # so there are no subitem prefixes; it must yield an empty index
+        # rather than crash on subitem_prefixes[0]
+        item = Item.create("@itemid/" + "0" * 32)
+        assert isinstance(item, NonExistent)
+        assert item.names == []
+        assert item.get_index() == ([], [])
+
     def test_meta_filter(self):
         name = "Test_item"
         contenttype = "text/plain;charset=utf-8"