multifilesystem: Improve performance of PROPFIND

The PROPFIND does not need to check the filesystem for the file existence,
because the list comes from the directory listing, similar as the other
parts already do it. That was highly visible with large collections (10k+ items),
causing massive delays.

Also avoid repeated stat() calls on the same path.

radicale/tests/test_storage.py

@@ -344,6 +344,41 @@ class TestMultiFileSystem(BaseTest):
             logs = caplog.messages
             assert len([log for log in logs if "File name collision" in log]) == 2
 
+    def test_propfind_depth1_does_not_check_collisions_per_item(self) -> None:
+        """Listing a collection (PROPFIND depth:1) must not re-run the
+        filesystem collision check (path_to_filesystem) once per item.
+        """
+        self.mkcalendar("/calendar.ics/")
+        item_count = 20
+        for index in range(item_count):
+            uid = "event%d" % index
+            event = ("BEGIN:VCALENDAR\r\nVERSION:2.0\r\n"
+                     "BEGIN:VEVENT\r\nUID:%s\r\nSUMMARY:Event %d\r\n"
+                     "DTSTART:20130901T180000Z\r\nEND:VEVENT\r\n"
+                     "END:VCALENDAR\r\n" % (uid, index))
+            self.put("/calendar.ics/%s.ics" % uid, event)
+
+        call_count = 0
+        original_path_to_filesystem = pathutils.path_to_filesystem
+
+        def counting_path_to_filesystem(*args, **kwargs):
+            nonlocal call_count
+            call_count += 1
+            return original_path_to_filesystem(*args, **kwargs)
+
+        with pytest.MonkeyPatch.context() as monkeypatch:
+            monkeypatch.setattr(pathutils, "path_to_filesystem",
+                                counting_path_to_filesystem)
+            _, responses = self.propfind("/calendar.ics/", """\
+<?xml version="1.0" encoding="utf-8" ?>
+<D:propfind xmlns:D="DAV:">
+  <D:prop><D:getetag/></D:prop>
+</D:propfind>
+""", HTTP_DEPTH="1")
+
+        assert len(responses) == item_count + 1
+        assert call_count < item_count
+
     @pytest.mark.skipif(not shutil.which("flock"), reason="flock command not found")
     @pytest.mark.skipif(radicale.log.logger.getEffectiveLevel() == logging.INFO, reason="requires loglevel DEBUG")
     def test_hook_placeholders_PUT(self, caplog) -> None: