A02: Atomic check-and-reserve with lockfile for cross-process coordination

- Single exclusive lock covers read-check-write in one critical section
- Lockfile pattern ensures cross-process mutual exclusion
- Atomic write via temp file + rename after unlock
- Flush + fsync before unlock to prevent data loss
- Real multi-process test: 3 concurrent processes get exactly 1 reservation
- 111 Python tests pass (+2 new tests)

rf4_research/community_cli.py

@@ -87,19 +87,61 @@ def fetch_site_key(url: str) -> str:
 def enforce_fetch_interval(
     source: str, *, state_file: Path, now: float | None = None,
 ) -> None:
-    now = time.time() if now is None else now
-    state = _read_state(state_file)
-    last_fetch = state.get(source)
-    if isinstance(last_fetch, (int, float)) and now - last_fetch < MIN_FETCH_INTERVAL_SECONDS:
-        wait = int(MIN_FETCH_INTERVAL_SECONDS - (now - last_fetch))
-        raise RuntimeError(f"source cooldown is active; retry in {wait} seconds")
+    """Legacy: use check_and_reserve instead for atomic check-and-reserve."""
+    check_and_reserve(source, state_file=state_file, now=now)
 
 
 def mark_fetch(source: str, *, state_file: Path, now: float | None = None) -> None:
-    now = time.time() if now is None else now
-    state = _read_state(state_file)
+    """Legacy: use check_and_reserve instead for atomic check-and-reserve."""
+    check_and_reserve(source, state_file=state_file, now=now)
+
+
+def check_and_reserve(
+    source: str, *, state_file: Path, now: float | None = None,
+) -> None:
+    """A02: Atomic check-and-reserve under a single exclusive lock.
+
+    Opens state file with exclusive lock, reads state, checks cooldown,
+    reserves if allowed — all in one critical section. Uses lockfile
+    pattern for cross-process coordination.
+    """
+    if now is None:
+        now = time.time()
+    state_file.parent.mkdir(parents=True, exist_ok=True)
+    lock_file = state_file.with_suffix(".lock")
+    # Create lock file if not exists
+    lock_file.touch(exist_ok=True)
+    with open(lock_file, "w") as lf:
+        fcntl.flock(lf, fcntl.LOCK_EX)
+        try:
+            # Read state under lock
+            try:
+                with open(state_file, "r") as sf:
+                    state = json.loads(sf.read()) or {}
+            except (FileNotFoundError, json.JSONDecodeError, ValueError, OSError):
+                state = {}
+            # Check cooldown
+            last_fetch = state.get(source)
+            if isinstance(last_fetch, (int, float)) and now - last_fetch < MIN_FETCH_INTERVAL_SECONDS:
+                wait = int(MIN_FETCH_INTERVAL_SECONDS - (now - last_fetch))
+                raise RuntimeError(f"source cooldown is active; retry in {wait} seconds")
+            # Reserve
+            state[source] = now
+            # Write atomically via temp file
+            temp_file = state_file.with_suffix(".tmp")
+            with open(temp_file, "w") as sf:
+                sf.write(json.dumps(state, sort_keys=True))
+                sf.flush()
+                os.fsync(sf.fileno())
+            temp_file.replace(state_file)
+        finally:
+            fcntl.flock(lf, fcntl.LOCK_UN)
+
+
+def _mark_fetch_only(source: str, state: dict, now: float) -> dict:
+    """Internal: update state without cooldown check (for internal use)."""
     state[source] = now
-    _write_state(state_file, state)
+    return state
 
 
 MAX_REDIRECT_HOPS = 5