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)

tests/test_community_cli.py

@@ -8,13 +8,19 @@ from rf4_research.community_cli import enforce_fetch_interval, fetch_site_key, m
 
 
 def test_fetch_cooldown_is_persistent_per_source(tmp_path: Path) -> None:
-    state_file = tmp_path / "fetch-state.json"
-    mark_fetch("rf4map-point", state_file=state_file, now=1_000)
+    """A02: check_and_reserve is atomic — one reservation per interval per source."""
+    from rf4_research.community_cli import check_and_reserve
 
+    state_file = tmp_path / "fetch-state.json"
+    check_and_reserve("rf4map-point", state_file=state_file, now=1_000)
+
+    # Second call for same source within interval should fail
     with pytest.raises(RuntimeError, match="retry in 1800 seconds"):
-        enforce_fetch_interval("rf4map-point", state_file=state_file, now=1_000)
-    enforce_fetch_interval("rf4posts-spot", state_file=state_file, now=1_000)
-    enforce_fetch_interval("rf4map-point", state_file=state_file, now=2_800)
+        check_and_reserve("rf4map-point", state_file=state_file, now=1_000)
+    # Different source should succeed
+    check_and_reserve("rf4posts-spot", state_file=state_file, now=1_000)
+    # After interval expires, should succeed again
+    check_and_reserve("rf4map-point", state_file=state_file, now=2_800)
 
 
 def test_fetch_site_key_groups_endpoints_and_normalizes_www() -> None:
@@ -77,3 +83,61 @@ def test_write_state_is_atomic_with_flush(tmp_path: Path) -> None:
     state = _read_state(state_file)
     assert state == {"key1": "value2", "key2": "value3"}
     assert not (state_file.with_suffix(".tmp")).exists()
+
+
+def test_check_and_reserve_allows_only_one_per_interval(tmp_path: Path) -> None:
+    """A02: Atomic check-and-reserve — only one success per interval."""
+    from rf4_research.community_cli import check_and_reserve
+
+    state_file = tmp_path / "state.json"
+    base_time = 1000.0
+
+    # First call should succeed
+    check_and_reserve("test-source", state_file=state_file, now=base_time)
+    state = json.loads(state_file.read_text(encoding="utf-8"))
+    assert state["test-source"] == base_time
+
+    # Second call within interval should fail
+    with pytest.raises(RuntimeError, match="retry in 1799"):
+        check_and_reserve("test-source", state_file=state_file, now=base_time + 1)
+
+    # After interval expires, should succeed again
+    check_and_reserve("test-source", state_file=state_file, now=base_time + 1800)
+
+
+def _try_reserve_for_test(args: tuple) -> tuple:
+    """Helper for multiprocessing — must be at module level."""
+    pid, state_file_str, results_list = args
+    from rf4_research.community_cli import check_and_reserve
+    from pathlib import Path
+    try:
+        check_and_reserve("shared-source", state_file=Path(state_file_str), now=1000.0)
+        results_list.append((pid, "ok"))
+    except RuntimeError as e:
+        results_list.append((pid, str(e)))
+    return (pid, "ok")
+
+
+def test_check_and_reserve_atomic_under_concurrent_access(tmp_path: Path) -> None:
+    """A02: Real multi-process test — concurrent processes get at most one reservation."""
+    import multiprocessing
+
+    state_file = tmp_path / "concurrent.json"
+    results = multiprocessing.Manager().list()
+
+    # Launch 3 processes simultaneously
+    processes = []
+    for i in range(3):
+        args = (i, str(state_file), results)
+        p = multiprocessing.Process(target=_try_reserve_for_test, args=(args,))
+        processes.append(p)
+    for p in processes:
+        p.start()
+    for p in processes:
+        p.join(timeout=10)
+
+    # At most one should succeed
+    ok_count = sum(1 for _, r in results if r == "ok")
+    assert ok_count == 1, f"Expected exactly 1 ok, got {ok_count}: {results}"
+    denied_count = sum(1 for _, r in results if "cooldown" in r)
+    assert denied_count == 2, f"Expected 2 denied, got {denied_count}: {results}"