Force explicit allowing initialization in child process.

Change: in current implementation, pools created in child processes COULD NOT share data with main process. Thus an exception will be raised by default when attemp to create pool in a child process, which can be suppressed with `allow_child_init` parameter.

test/test_child_process_creation.py

@@ -0,0 +1,123 @@
+"""
+子进程创建 pool 的行为控制测试
+"""
+
+import multiprocessing
+import pytest
+from mpsp.mpsp import MultiProcessingSharedPool
+
+
+# 子进程创建 pool 相关的测试辅助函数
+def worker_create_pool_in_child(result_queue):
+    """子进程：尝试创建 pool（应该失败）"""
+    try:
+        # 重置单例以模拟子进程首次创建
+        MultiProcessingSharedPool._instance = None
+        pool = MultiProcessingSharedPool()
+        pool.put("child_key", "child_value")
+        result_queue.put(("success", "created"))
+    except RuntimeError as e:
+        result_queue.put(("error", str(e)))
+
+
+def worker_create_pool_with_allow(result_queue):
+    """子进程：使用 allow_child_init=True 创建 pool"""
+    try:
+        # 重置单例以模拟子进程首次创建
+        MultiProcessingSharedPool._instance = None
+        pool = MultiProcessingSharedPool(allow_child_init=True)
+        pool.put("child_key", "child_value")
+        result_queue.put(("success", "created_with_allow"))
+    except RuntimeError as e:
+        result_queue.put(("error", str(e)))
+
+
+def worker_get_data(key, result_queue):
+    """子进程：从共享池读取数据"""
+    pool = MultiProcessingSharedPool()
+    value = pool.get(key)
+    result_queue.put(value)
+
+
+class TestChildProcessCreation:
+    """测试子进程中创建 pool 的行为控制"""
+
+    def test_child_process_creation_blocked_by_default(self):
+        """测试默认情况下子进程创建 pool 会被阻止"""
+        pool = MultiProcessingSharedPool()
+        pool.clear()
+
+        result_queue = multiprocessing.Queue()
+        p = multiprocessing.Process(
+            target=worker_create_pool_in_child, args=(result_queue,)
+        )
+        p.start()
+        p.join()
+
+        status, message = result_queue.get()
+        assert status == "error"
+        assert (
+            "Cannot initialize MultiProcessingSharedPool in a child process" in message
+        )
+
+    def test_child_process_creation_allowed_with_flag(self):
+        """测试使用 allow_child_init=True 允许子进程创建 pool"""
+        pool = MultiProcessingSharedPool()
+        pool.clear()
+
+        result_queue = multiprocessing.Queue()
+        p = multiprocessing.Process(
+            target=worker_create_pool_with_allow, args=(result_queue,)
+        )
+        p.start()
+        p.join()
+
+        status, message = result_queue.get()
+        assert status == "success"
+        assert message == "created_with_allow"
+
+    def test_allow_child_init_parameter(self):
+        """测试 allow_child_init 参数可以被正确设置"""
+        # 重置单例
+        MultiProcessingSharedPool._instance = None
+
+        # 默认情况下不允许
+        pool1 = MultiProcessingSharedPool()
+        assert MultiProcessingSharedPool._allow_child_init is False
+
+        # 重置单例
+        MultiProcessingSharedPool._instance = None
+        MultiProcessingSharedPool._allow_child_init = False
+
+        # 显式设置为允许
+        pool2 = MultiProcessingSharedPool(allow_child_init=True)
+        assert MultiProcessingSharedPool._allow_child_init is True
+
+        # 清理
+        MultiProcessingSharedPool._instance = None
+        MultiProcessingSharedPool._allow_child_init = False
+
+    def test_best_practice_main_process_init(self):
+        """测试最佳实践：主进程先初始化，子进程后使用"""
+        # 重置单例
+        MultiProcessingSharedPool._instance = None
+        MultiProcessingSharedPool._allow_child_init = False
+
+        # 主进程先初始化
+        main_pool = MultiProcessingSharedPool()
+        main_pool._ensure_initialized()
+        main_pool.put("shared_key", "shared_value")
+
+        # 子进程使用（不创建新的）
+        result_queue = multiprocessing.Queue()
+        p = multiprocessing.Process(
+            target=worker_get_data, args=("shared_key", result_queue)
+        )
+        p.start()
+        p.join()
+
+        result = result_queue.get()
+        assert result == "shared_value"
+
+        # 清理
+        MultiProcessingSharedPool._instance = None