fix: skip scratch downloads in the restore scan

scripts/restore_large_files.py

@@ -40,6 +40,11 @@ def index_cache(cache_dir: str) -> dict[str, str]:
     """Map SHA1 to cached file path for every asset in the cache."""
     index: dict[str, str] = {}
     for name in sorted(os.listdir(cache_dir)):
+        # A download in progress writes `<asset>.<random>.tmp` beside the
+        # cache and renames it when done: it is partial, and may be gone by
+        # the time it is opened.
+        if name.endswith(".tmp"):
+            continue
         path = os.path.join(cache_dir, name)
         if not os.path.isfile(path):
             continue

tests/test_atomic_artifacts.py

@@ -214,5 +214,34 @@ class ScratchFilesGetThePlainMode(unittest.TestCase):
         self.assertEqual(unset, [])
 
 
+
+class RestoreSkipsDownloadsInProgress(unittest.TestCase):
+    """A scratch file another run is still writing is neither hashed nor read."""
+
+    def test_a_scratch_promoted_mid_scan_does_not_stop_the_restore(self) -> None:
+        import restore_large_files
+
+        with tempfile.TemporaryDirectory() as tmp:
+            cache = Path(tmp)
+            (cache / "done.bin").write_bytes(b"whole")
+            scratch = cache / "big.bin.ab12.tmp"
+            scratch.write_bytes(b"part")
+            real_isfile = os.path.isfile
+
+            def check_then_promote(path: str) -> bool:
+                found = real_isfile(path)
+                # The concurrent fetch renames its scratch file right after
+                # the check, before the open.
+                if path == str(scratch) and found:
+                    os.replace(scratch, cache / "big.bin")
+                return found
+
+            with mock.patch.object(restore_large_files.os.path, "isfile",
+                                   side_effect=check_then_promote):
+                index = restore_large_files.index_cache(str(cache))
+            self.assertNotIn(str(scratch), index.values())
+            self.assertIn(str(cache / "done.bin"), index.values())
+
+
 if __name__ == "__main__":
     unittest.main()