keep config owner and handle file bind mounts in migration writes

os.replace failed with EBUSY when config.yml was a file bind mount, as in the documented --validate-config command, and the replaced file lost its original owner when Frigate ran as root. The write now copies in place on EBUSY and restores the original owner when permitted. Tests cover a failed dump leaving the original config and the bind-mount fallback.

frigate/test/test_config_migration.py

@@ -1,5 +1,6 @@
 """Tests for migrating detectors and model into the models list."""
 
+import errno
 import logging
 import os
 import tempfile
@@ -240,6 +241,30 @@ class TestMigrateConfigFile(unittest.TestCase):
         with open(self.config_file) as f:
             return YAML().load(f)
 
+    def test_a_failed_write_leaves_the_original_config(self):
+        original = "mqtt:\n  enabled: false\ncameras: {}\n"
+        with open(self.config_file, "w") as f:
+            f.write(original)
+
+        with (
+            patch.object(YAML, "dump", side_effect=RuntimeError),
+            self.assertRaises(RuntimeError),
+        ):
+            migrate_frigate_config(self.config_file)
+
+        with open(self.config_file) as f:
+            self.assertEqual(f.read(), original)
+
+    def test_a_file_bind_mount_is_written_in_place(self):
+        with patch(
+            "frigate.util.config.os.replace",
+            side_effect=OSError(errno.EBUSY, "Device or resource busy"),
+        ):
+            migrated = self._migrate("mqtt:\n  enabled: false\ncameras: {}\n")
+
+        self.assertEqual(migrated["version"], CURRENT_CONFIG_VERSION)
+        self.assertFalse(os.path.exists(f"{self.config_file}.tmp"))
+
     def test_migrates_a_config_already_stamped_with_the_current_version(self):
         # 0.19 is unreleased, so a dev config can be current and still use
         # the pre-models keys

frigate/util/config.py

@@ -1,6 +1,8 @@
 """configuration utils."""
 
 import asyncio
+import contextlib
+import errno
 import ipaddress
 import logging
 import os
@@ -213,11 +215,24 @@ def find_config_file() -> str:
 
 
 def _write_config(config_file: str, yaml: YAML, config: dict[str, Any]) -> None:
+    st = os.stat(config_file)
     tmp_file = f"{config_file}.tmp"
     with open(tmp_file, "w") as f:
         yaml.dump(config, f)
-    shutil.copymode(config_file, tmp_file)
-    os.replace(tmp_file, config_file)
+    os.chmod(tmp_file, st.st_mode)
+
+    with contextlib.suppress(PermissionError):
+        os.chown(tmp_file, st.st_uid, st.st_gid)
+
+    try:
+        os.replace(tmp_file, config_file)
+    except OSError as e:
+        if e.errno != errno.EBUSY:
+            raise
+
+        # a file bind mount can't be renamed over, so write it in place
+        shutil.copyfile(tmp_file, config_file)
+        os.remove(tmp_file)
 
 
 def migrate_frigate_config(config_file: str):