settings: safer writing (#3644)

* settings: clean up imports

* settings: try to use atomic rename

* settings: flush, sync and validate new yaml

before replacing the old one

* settings: add test for Settings.save

settings.py

@@ -3,6 +3,7 @@ Application settings / host.yaml interface using type hints.
 This is different from player options.
 """
 
+import os
 import os.path
 import shutil
 import sys
@@ -11,7 +12,6 @@ import warnings
 from enum import IntEnum
 from threading import Lock
 from typing import cast, Any, BinaryIO, ClassVar, Dict, Iterator, List, Optional, TextIO, Tuple, Union, TypeVar
-import os
 
 __all__ = [
     "get_settings", "fmt_doc", "no_gui",
@@ -798,6 +798,7 @@ class Settings(Group):
             atexit.register(autosave)
 
     def save(self, location: Optional[str] = None) -> None:  # as above
+        from Utils import parse_yaml
         location = location or self._filename
         assert location, "No file specified"
         temp_location = location + ".tmp"  # not using tempfile to test expected file access
@@ -807,10 +808,18 @@ class Settings(Group):
         # can't use utf-8-sig because it breaks backward compat: pyyaml on Windows with bytes does not strip the BOM
         with open(temp_location, "w", encoding="utf-8") as f:
             self.dump(f)
-        # replace old with new
-        if os.path.exists(location):
+            f.flush()
+            if hasattr(os, "fsync"):
+                os.fsync(f.fileno())
+        # validate new file is valid yaml
+        with open(temp_location, encoding="utf-8") as f:
+            parse_yaml(f.read())
+        # replace old with new, try atomic operation first
+        try:
+            os.rename(temp_location, location)
+        except (OSError, FileExistsError):
             os.unlink(location)
-        os.rename(temp_location, location)
+            os.rename(temp_location, location)
         self._filename = location
 
     def dump(self, f: TextIO, level: int = 0) -> None:
@@ -832,7 +841,6 @@ def get_settings() -> Settings:
     with _lock:  # make sure we only have one instance
         res = getattr(get_settings, "_cache", None)
         if not res:
-            import os
             from Utils import user_path, local_path
             filenames = ("options.yaml", "host.yaml")
             locations: List[str] = []

test/general/test_host_yaml.py

@@ -1,11 +1,12 @@
 import os
+import os.path
 import unittest
 from io import StringIO
-from tempfile import TemporaryFile
+from tempfile import TemporaryDirectory, TemporaryFile
 from typing import Any, Dict, List, cast
 
 import Utils
-from settings import Settings, Group
+from settings import Group, Settings, ServerOptions
 
 
 class TestIDs(unittest.TestCase):
@@ -80,3 +81,27 @@ class TestSettingsDumper(unittest.TestCase):
                 self.assertEqual(value_spaces[2], value_spaces[0])  # start of sub-list
                 self.assertGreater(value_spaces[3], value_spaces[0],
                                    f"{value_lines[3]} should have more indentation than {value_lines[0]} in {lines}")
+
+
+class TestSettingsSave(unittest.TestCase):
+    def test_save(self) -> None:
+        """Test that saving and updating works"""
+        with TemporaryDirectory() as d:
+            filename = os.path.join(d, "host.yaml")
+            new_release_mode = ServerOptions.ReleaseMode("enabled")
+            # create default host.yaml
+            settings = Settings(None)
+            settings.save(filename)
+            self.assertTrue(os.path.exists(filename),
+                            "Default settings could not be saved")
+            self.assertNotEqual(settings.server_options.release_mode, new_release_mode,
+                                "Unexpected default release mode")
+            # update host.yaml
+            settings.server_options.release_mode = new_release_mode
+            settings.save(filename)
+            self.assertFalse(os.path.exists(filename + ".tmp"),
+                             "Temp file was not removed during save")
+            # read back host.yaml
+            settings = Settings(filename)
+            self.assertEqual(settings.server_options.release_mode, new_release_mode,
+                             "Settings were not overwritten")