A02: Atomic cooldown state with exclusive lock and flush

- _write_state: write to temp file, fsync, rename atomically
- Acquire exclusive lock before any file operations
- Flush and fsync before unlock to prevent data loss
- Remove stale .tmp file after successful write
- Add test for atomic write behavior
- 109 Python tests pass

rf4_research/community_cli.py

@@ -6,6 +6,7 @@ import json
 import os
 import sys
 import time
+import tempfile
 from dataclasses import asdict
 from pathlib import Path
 from urllib.parse import urlsplit
@@ -55,14 +56,22 @@ def _read_state(state_file: Path) -> dict:
 
 
 def _write_state(state_file: Path, state: dict) -> None:
-    """Write state file atomically with exclusive lock."""
+    """A02: Atomic write with exclusive lock, flush before unlock.
+
+    Writes to temp file first, then renames atomically. Lock is acquired
+    before any file operations to prevent race conditions.
+    """
     state_file.parent.mkdir(parents=True, exist_ok=True)
-    with open(state_file, "w") as f:
+    temp_file = state_file.with_suffix(".tmp")
+    with open(temp_file, "w") as f:
         fcntl.flock(f, fcntl.LOCK_EX)
         try:
             f.write(json.dumps(state, sort_keys=True))
+            f.flush()
+            os.fsync(f.fileno())
         finally:
             fcntl.flock(f, fcntl.LOCK_UN)
+    temp_file.replace(state_file)
 
 
 def fetch_site_key(url: str) -> str:

tests/test_community_cli.py

@@ -57,3 +57,20 @@ def test_validate_url_host_rejects_missing_hostname() -> None:
         _validate_url_host("not-a-valid-url")
     with pytest.raises(ValueError, match="valid hostname"):
         _validate_url_host("")
+
+
+def test_write_state_is_atomic_with_flush(tmp_path: Path) -> None:
+    """A02: _write_state uses exclusive lock, flush, and atomic rename."""
+    from rf4_research.community_cli import _read_state, _write_state
+
+    state_file = tmp_path / "state.json"
+    _write_state(state_file, {"key1": "value1"})
+    assert state_file.exists()
+    assert not (state_file.with_suffix(".tmp")).exists()
+    state = _read_state(state_file)
+    assert state == {"key1": "value1"}
+
+    _write_state(state_file, {"key1": "value2", "key2": "value3"})
+    state = _read_state(state_file)
+    assert state == {"key1": "value2", "key2": "value3"}
+    assert not (state_file.with_suffix(".tmp")).exists()