问题描述
我想创建一个返回 tokio_postgres
客户端的函数。但是,我找不到在异步任务(连接到数据库)中获取变量所有权(来自库 tokio_postgres
的数据库连接)的解决方案。
这是我的代码 (Playground):
use std::sync::{Arc,Mutex};
use tokio_postgres::tls::NoTlsstream;
use tokio_postgres::{Client,Connection,Error,NoTls,Socket};
#[tokio::main] // By default,tokio_postgres uses the tokio crate as its runtime.
async fn main() -> Result<(),Error> {
let pg_client = create_pg_client("postgres://postgres:root@localhost:5432");
// Use pg_client for the program
Ok(())
}
//
// Creates a pg client
//
pub async fn create_pg_client(
config: &str,) -> Result<Arc<Mutex<(Client,Connection<Socket,NoTlsstream>)>>,Error> {
// Connect to the database.
let mut connect_result = Arc::new(Mutex::new(tokio_postgres::connect(config,NoTls).await?));
let connect_result_thread = connect_result.clone();
// The connection object performs the actual communication with the database,// so spawn it off to run on its own.
tokio::spawn(async move {
let mut result = connect_result_thread.lock().unwrap();
if let Err(e) = (&mut result.1).await {
eprintln!("An error occured while trying to connect to the database");
}
});
Ok(connect_result)
}
我的代码无法编译:
error: future cannot be sent between threads safely
--> src\pg_client.rs:18:5
|
18 | tokio::spawn(async move {
| ^^^^^^^^^^^^ future created by async block is not `Send`
|
::: src\github.com-1ecc6299db9ec823\tokio-1.5.0\src\task\spawn.rs:129:21
|
129 | T: Future + Send + 'static,| ---- required by this bound in `tokio::spawn`
|
= help: within `impl Future`,the trait `Send` is not implemented for `std::sync::MutexGuard<'_,(Client,tokio_postgres::Connection<Socket,NoTlsstream>)>`
note: future is not `Send` as this value is used across an await
--> src\pg_client.rs:20:25
|
19 | let mut result = connect_result_thread.lock().unwrap();
| ---------- has type `std::sync::MutexGuard<'_,NoTlsstream>)>` which is not `Send`
20 | if let Err(e) = (*result).1.await {
| ^^^^^^^^^^^^^^^^^ await occurs here,with `mut result` maybe used later
...
23 | });
| - `mut result` is later dropped here
它表示不能安全地在线程之间发送未来。有没有可能实现我想要的?
使用的板条箱:
tokio = { version = "1.5.0",features = ["full"]}
tokio-postgres = "0.7.2"
解决方法
标准库 Mutex
旨在在单个线程的上下文中持有锁。由于任务可以由不同的线程拾取,跨 await
点的值必须为 Send
。此外,即使它确实有效,在异步程序中使用阻塞互斥锁也是一个坏主意,因为获取互斥锁可能需要任意长的时间,在此期间其他任务无法在同一个执行器线程上运行。
您可以通过切换到 tokio's mutex 来解决这两个问题,该 Playground 的守卫是 Send
,其 lock()
方法是异步的。例如,这会编译:
// Pick up an async aware Mutex
use tokio::sync::Mutex;
let connect_result = Arc::new(Mutex::new(tokio_postgres::connect(config,NoTls).await?));
let connect_result_thread = connect_result.clone();
tokio::spawn(async move {
let mut result = connect_result_thread.lock().await;
if let Err(e) = (&mut result.1).await {
eprintln!("An error occured while trying to connect to the database: {}",e);
}
});
Ok(connect_result)
Merge JavaScript objects in array with same key
尚不清楚的是这种设计的最终目标。如果连接位于互斥锁后面,则您可以有效地序列化从各种任务对它的访问。首先,这种连接可能不应该在任务之间共享。