multithreading - Send 只能为结构/枚举类型实现，不能为特征实现

Question

我在为特征实现Send 时遇到困难。 Full code in the playground .

我有一个 Storage 特征:

pub trait Storage {
    fn get_value(&self, key: &str) -> Result<Vec<u8>, Error>;
    fn put_value(&mut self, key: &str, value: &[u8]) -> Result<(), Error>;
    fn key_exists(&self, key: &str) -> bool;
    fn delete_key(&mut self, key: &str) -> Result<(), Error>;
}

它是 Consistency 结构的一部分:

pub struct Consistency<'a> {
    storage: &'a mut Storage,
}

我已经实现了一个 MemoryStorage 结构:

#[derive(Debug)]
struct MemoryStorage {
    data: HashMap<String, Vec<u8>>,
}

impl Storage for MemoryStorage {
    fn get_value(&self, key: &str) -> Result<Vec<u8>, Error> {
        self.data.get(key).map_or(
            Err(Error::new(ErrorKind::NotFound, "Key does not exists")),
            |v| Ok(v.clone()),
        )
    }

    fn put_value(&mut self, key: &str, value: &[u8]) -> Result<(), Error> {
        self.data.insert(key.to_string(), Vec::from(value));
        Ok(())
    }

    fn key_exists(&self, key: &str) -> bool {
        self.data.contains_key(key)
    }

    fn delete_key(&mut self, key: &str) -> Result<(), Error> {
        self.data.remove(key).map_or(
            Err(Error::new(ErrorKind::NotFound, "Key does not exists")),
            |v| Ok(()),
        )
    }
}

我写了一个测试:

#[test]
fn multiple_threads_bombing_storage() {
    const parallels: usize = 10;
    let mut storage = MemoryStorage {
        data: HashMap::new(),
    };
    let mut consistency = Consistency::new(&mut storage);

    // create all keys.
    let mut keys = Vec::<String>::new();
    for k in 1..parallels {
        keys.push(k.to_string());
    }

    let zero = "0".as_bytes();
    // put default values in files.
    for key in keys.clone() {
        consistency.put_value(&key, zero).unwrap();
    }

    // test write threads
    let write_thread = |to_write: Vec<u8>, keys: &mut Vec<String>| {
        let mut rng = rand::thread_rng();
        rng.shuffle(keys);

        for key in keys {
            let mut val = consistency.get_value(key).unwrap();
            val.append(to_write.clone().as_mut());
            consistency.put_value(key, &val).unwrap();
        }
    };

    // start parallels threads..
    for t in 1..parallels {
        let handle = thread::spawn(move || {
            write_thread(t.to_string().into_bytes(), &mut keys);
        });
    }

    // wait for all threads..

    // check if all parallels keys have all numbers in any order.
}

这给了我一个错误:

error[E0277]: the trait bound `fileapi::Storage: std::marker::Send` is not satisfied
   --> src/consistency.rs:158:26
    |
158 |             let handle = thread::spawn(move || {
    |                          ^^^^^^^^^^^^^ `fileapi::Storage` cannot be sent between threads safely
    |
    = help: the trait `std::marker::Send` is not implemented for `fileapi::Storage`
    = note: required because of the requirements on the impl of `std::marker::Send` for `&mut fileapi::Storage`
    = note: required because it appears within the type `consistency::Consistency<'_>`
    = note: required because of the requirements on the impl of `std::marker::Send` for `&mut consistency::Consistency<'_>`
    = note: required because it appears within the type `[closure@src/consistency.rs:145:28: 154:10 consistency:&mut consistency::Consistency<'_>]`
    = note: required because it appears within the type `[closure@src/consistency.rs:158:40: 160:14 write_thread:[closure@src/consistency.rs:145:28: 154:10 consistency:&mut consistency::Consistency<'_>], t:usize, keys:std::vec::Vec<std::string::String>]`
    = note: required by `std::thread::spawn`

添加 unsafe impl Send for Storage{} 给我一个矛盾的错误，即 Send 无法为特征实现:

cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `fileapi::Storage + 'static`

我不明白如何解决这个问题。

Answer 1

您有一个实现特征的引用(trait object)，但不能保证您的特征能够在线程之间发送。

您需要确保它是。你可以要求你的特征的所有实现者都是 Send:

pub trait Storage: Send {
    // ...
}

或者这个特定的特征对象实现了Send:

pub struct Consistency<'a> {
    storage: &'a mut (Storage + Send),
}

您的代码还有许多其他问题，因此仍然无法编译。其中:

1. 您试图将堆栈分配的变量传递给线程。
2. 您尝试在多个线程之间共享可变引用。

你们很多人需要回去重新阅读The Rust Programming Language ，尤其是 chapter on concurrency .

另见:

• Can I have a static borrowed reference to a trait object?
• How can I pass a reference to a stack variable to a thread?
• Share mutable object between threads