6

我正在使用 Rocket State,它传递给 HTTP 请求。这个结构包含一个Mutex<DatastoreInstance>可以访问 SQLite 数据库并被互斥锁锁定以确保读写安全的结构。

pub struct DatastoreInstance {
    conn: Connection,
}

DatastoreInstance结构看起来像这样时,只有一个 SQLite 连接一切正常,但我还想在这个结构中添加一个事务对象:

pub struct DatastoreInstance {
    conn: Connection,
    events_transaction: Transaction,
}

这没有编译,因为Transaction对象需要引用一个Connection应该具有它知道的生命周期的对象。我正在使用的 rusqlite 中的Connectionand对象定义如下:Transaction

pub struct Connection {
    db: RefCell<InnerConnection>,
    cache: StatementCache,
    path: Option<PathBuf>,
}

pub struct Transaction<'conn> {
    conn: &'conn Connection,
    drop_behavior: DropBehavior,
}

为了解决生命周期问题,我必须添加这些生命周期参数才能使其正常工作:

pub struct DatastoreInstance<'a> {
    conn: Connection,
    events_transaction: Transaction<'a>,
}

这是结果,应该根据我对生命周期和互斥锁的理解工作,但现在我得到一个编译器错误告诉我:

`std::cell::RefCell<lru_cache::LruCache<std::string::String, rusqlite::raw_statement::RawStatement>>` cannot be shared between threads safely
    |                                                                                                            
    = help: within `rusqlite::Connection`, the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<lru_cache::LruCache<std::string::String, rusqlite::raw_statement::RawStatement>>`
    = note: required because it appears within the type `rusqlite::cache::StatementCache`                        
    = note: required because it appears within the type `rusqlite::Connection`                                   
    = note: required because of the requirements on the impl of `std::marker::Send` for `&rusqlite::Connection`  
    = note: required because it appears within the type `datastore::DatastoreInstance<'_>`                       
    = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Mutex<datastore::DatastoreInstance<'_>>`
    = note: required because it appears within the type `endpoints::ServerState<'_>`                             
    = note: required by `rocket::State`

根据我对互斥体的理解,这段代码应该是有效的,因为整个DatastoreInstance结构都被包裹在一个Mutex应该保证一次只有一个线程引用这个对象的情况下。

我错过了什么?

为什么编译器在 a 内引用而不是仅在 a内引用RefCell后发现不再安全?ConnectionTransactionConnection

我对互斥锁的工作原理有不好的理解吗?我的生命周期是否无效并以某种方式破坏了读/写安全?将ConnectionandTransaction放在同一个结构中的设计是否会破坏读/写安全性?我是否需要以某种方式重新设计我的数据结构以确保安全?还是我只是错过了一些非常明显的东西?

4

1 回答 1

6

A Mutexis onlySend或者Sync 如果它包含的值是它本身Send

impl<T: ?Sized + Send> Send for Mutex<T>    
impl<T: ?Sized + Send> Sync for Mutex<T>

A&TSend TSync时:

impl<'a, T> Send for &'a T
where
    T: Sync + ?Sized, 

而 aRefCell 永远不会Sync

impl<T> !Sync for RefCell<T>
where
    T: ?Sized, 

如错误消息所述,您的交易包含对 a 的引用RefCell。有一个互斥锁并不重要,跨线程共享它本质上不是内存安全的。一个简单的再现:

use std::{cell::RefCell, sync::Mutex};

struct Connection(RefCell<i32>);
struct Transaction<'a>(&'a Connection);

fn is_send<T: Send>(_: T) {}

fn main() {
    let c = Connection(RefCell::new(42));
    let t = Transaction(&c);
    let m = Mutex::new(t);

    is_send(m);
}
error[E0277]: `std::cell::RefCell<i32>` cannot be shared between threads safely
  --> src/main.rs:13:5
   |
13 |     is_send(m);
   |     ^^^^^^^ `std::cell::RefCell<i32>` cannot be shared between threads safely
   |
   = help: within `Connection`, the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<i32>`
   = note: required because it appears within the type `Connection`
   = note: required because of the requirements on the impl of `std::marker::Send` for `&Connection`
   = note: required because it appears within the type `Transaction<'_>`
   = note: required because of the requirements on the impl of `std::marker::Send` for `std::sync::Mutex<Transaction<'_>>`
note: required by `is_send`
  --> src/main.rs:6:1
   |
6  | fn is_send<T: Send>(_: T) {}
   | ^^^^^^^^^^^^^^^^^^^^^^^^^

为什么编译器在 a 内引用而不是仅在 a内引用RefCell后发现不再安全?ConnectionTransactionConnection

RefCell很好,它是对 a的引用RefCell,那不是。

ConnectionandTransaction包含在同一个结构中的设计是一个糟糕的设计 [...] 我需要重新设计我的数据结构吗

是的。

于 2018-10-14T14:08:46.740 回答