test(images): reproduce the newest-order race and fix a Windows clock flake (#348, PR 2)

- The race test now has the newer build step from the values the cached
  order was built from, which is the interleaving that inserted a file
  twice; it fails without the parts check in _newest_step.
- test_a_failed_walk_records_its_timing_and_clears_dirty compared two
  monotonic readings that Windows' ~15.6 ms clock can make equal; the
  previous start is moved back instead.
- TESTING.md: os.access ignores ACLs on Windows.

TESTING.md

@@ -350,7 +350,8 @@ hold a walk open with a gated scan instead of sleeping.
   (`_patch_listing`): `_settled_entry` applies the walk's filters
   (not a symlink or folder by `lstat`, `_name_suffix` in `MEDIA_EXTENSIONS`,
   no `thumbnails` folder, at most `MAX_SCAN_DEPTH` folders, every folder on
-  the way listable by `os.access`). Where names fold case the stored
+  the way listable by `os.access`, which on Windows ignores ACLs: a folder
+  whose ACL denies listing is added there and corrected by the next walk). Where names fold case the stored
   spelling must match first, as the filters compare it: Windows through
   `nt._getfinalpathname` (its `realpath` keeps the given case when it cannot
   open the file), macOS through `F_GETPATH` on a non-blocking open (a FIFO

tests/test_api_images.py

@@ -3229,6 +3229,10 @@ class TestSharedListing(ImageAPITestCase):
         self.api.mark_root_dirty(self._output("b.png"))
         self.assertTrue(record.dirty)
         self.api._scan_gallery_files_sync = _raise
+        # Moved back, so the failed walk's start is later even on a coarse
+        # monotonic clock (Windows ticks every ~15.6 ms).
+        record.last_started_at -= 1000
+        record.last_duration = None
         started = record.last_started_at
         with self.assertLogs(self.api.logger, "WARNING"):
             self._fire(record)
@@ -4097,7 +4101,8 @@ class TestSharedListing(ImageAPITestCase):
         f1 = gallery_listing.insert_entry(f0, (root / "n1.png", 5000.0), 100)
         f2 = gallery_listing.insert_entry(f1, (root / "n2.png", 6000.0), 100)
         self.api._workspace_orders([(0, root, f0)])
-        self.api._workspace_listed(0, root, f1)  # the warm build of add 1
+        key = (0, str(root))
+        built_from_f0 = self.api._workspace_listed_cache[key]
         real = self.api._workspace_listed
         armed = [True]
 
@@ -4105,15 +4110,18 @@ class TestSharedListing(ImageAPITestCase):
             values = real(index, part_root, files)
             if armed[0] and files is f1:
                 armed[0] = False
-                # Another executor thread reads f2 in full meanwhile.
-                self.api._workspace_orders([(0, root, f2)])
+                # Another executor thread read the f0 values before this one
+                # stored f1's, and stores its f2 step (from f0) only now.
+                self.api._workspace_listed_cache[key] = built_from_f0
+                real(index, part_root, f2)
             return values
 
         with patch.object(self.api, "_workspace_listed", side_effect=listed):
             stale = self.api._workspace_orders([(0, root, f1)])["newest"]
         self.assertEqual([entry.path for entry in stale], [p for p, _m in f1])
-        newest = self.api._workspace_orders([(0, root, f2)])["newest"]
-        self.assertEqual([entry.path for entry in newest], [p for p, _m in f2])
+        for files in (f1, f2, f1, f2):
+            newest = self.api._workspace_orders([(0, root, files)])["newest"]
+            self.assertEqual([entry.path for entry in newest], [p for p, _m in files])
 
     def test_a_roots_change_during_an_order_build_sorts_in_full(self):
         root = self.output_dir