MongoDB C++:mongocxx :: pool线程安全吗？

Question

获取连接时是否必须手动锁定mongocxx :: pool？

即这样安全吗？(从Mongo网站复制的例子)

mongocxx::instance instance{};
mongocxx::pool pool {mongocxx::uri{}};

using mongocxx::pool::entry = std::unique_ptr<client, std::function<void (client*)>>

auto threadfunc = [](mongocxx::client &client, stdx::string_view dbname) {
    client[dbname]["col"].insert({});
}
// don't even bother sharing clients. Just give each thread its own,
std::thread([]() {
    // pool.acquire() returns a mongo::pool::entry type
    mongocxx::client *c= pool.acquire().get();
    threadfunc(*c, "db1");
    threadfunc(*c, "db2");
});

std::thread([]() {
    mongocxx::client *c = pool.acquire().get();;
    threadfunc(*c, "db2");
    threadfunc(*c, "db1");
});

Answer 1

是的,mongocxx::pool是线程安全的.您可以从多个线程同时访问它.但是,mongocxx::client从池返回的单个对象不是线程安全的,从属对象也不喜欢collection或database从中获取client- 您不能在线程之间共享它们.

另请注意,您的示例(不是从网站逐字复制,而是从其中一个示例中修改)包含严重的编程错误.

这一行:

   mongocxx::client *c= pool.acquire().get();

将获取一个池条目,然后从中提取一个裸指针.但是,池条目将在语句末尾被销毁,导致底层client对象返回到池中,允许另一个线程在您继续使用它时可能会将其拾取.

你应该把它写成:

mongocxx::instance instance{};
mongocxx::pool pool {mongocxx::uri{}};

auto threadfunc = [](mongocxx::client &client, stdx::string_view dbname) {
    client[dbname]["col"].insert({});
}
// don't even bother sharing clients. Just give each thread its own,
std::thread([]() {
    // pool.acquire() returns a mongo::pool::entry type
    auto c = pool.acquire();
    threadfunc(*c, "db1");
    threadfunc(*c, "db2");
});

std::thread([]() {
    auto c = pool.acquire();
    threadfunc(*c, "db2");
    threadfunc(*c, "db1");
});

这样,每个线程都会保留池条目,直到它完成使用它为止,此时它将在unique_ptrin c被销毁时自动返回.