asynchronous - 如何使用 tokio 0.1 和异步函数递归删除目录？

Question

我想编写一个程序，在 Rust 1.39 中使用异步函数递归删除目录。

作为第一步，我尝试了以下代码，但它没有编译:

use std::env;
use failure::Error;
use futures::Future;
use std::path::PathBuf;
use tokio::prelude::*;

fn walk(path: PathBuf) -> Box<dyn Future<Item = (), Error = Error> + Send> {
    let task = tokio::fs::read_dir(path)
        .flatten_stream()
        .for_each(move |entry| {
            let filepath = entry.path();
            if filepath.is_dir() {
                future::Either::A(walk(filepath))
            } else {
                println!("File: {:?}", filepath);
                future::Either::B(future::ok(()))
            }
        })
        .map_err(Error::from)
        .and_then(|_| {
            println!("All tasks done");
        });
    Box::new(task)
}

fn main() -> Result<(), std::io::Error> {
    let args: Vec<String> = env::args().collect();
    let dir = &args[1];
    let t = walk(PathBuf::from(&dir)).map_err(drop);
    tokio::run(t);
    Ok(())
}

当我运行 cargo build 时，我得到以下输出:

error[E0220]: associated type `Item` not found for `core::future::future::Future`
  --> src\main.rs:10:42
   |
10 | fn walk(path: PathBuf) -> Box<dyn Future<Item = (), Error = Error> + Send> {
   |                                          ^^^^^^^^^ associated type `Item` not found

error[E0220]: associated type `Error` not found for `core::future::future::Future`
  --> src\main.rs:10:53
   |
10 | fn walk(path: PathBuf) -> Box<dyn Future<Item = (), Error = Error> + Send> {
   |                                                     ^^^^^^^^^^^^^ associated type `Error` not found

error[E0191]: the value of the associated type `Output` (from the trait `core::future::future::Future`) must be specified
  --> src\main.rs:10:31
   |
10 | fn walk(path: PathBuf) -> Box<dyn Future<Item = (), Error = Error> + Send> {
   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated type `Output` must be specified

cargo .toml:

[dependencies]
async-std = "1.0.1"
failure = "0.1.6"
futures = "0.3.1"
tokio = "0.1.22"

有什么帮助吗？

Answer 1

您正在使用 futures 0.3，tokio 0.1 尚不支持。相反，导入 Tokio 并使用其 prelude 模块，该模块重新导出它支持的 futures 版本:

use std::{env, path::PathBuf};
use tokio::prelude::*; // 0.1.22

type Error = Box<dyn std::error::Error + Send + Sync>;

fn walk(path: PathBuf) -> Box<dyn Future<Item = (), Error = Error> + Send> {
    let task = tokio::fs::read_dir(path)
        .flatten_stream()
        .map_err(Error::from)
        .for_each(|entry| {
            let filepath = entry.path();
            if filepath.is_dir() {
                future::Either::A(walk(filepath))
            } else {
                println!("File: {:?}", filepath);
                future::Either::B(future::ok(()))
            }
        })
        .inspect(|_| {
            println!("All tasks done");
        });
    Box::new(task)
}

fn main() -> Result<(), std::io::Error> {
    let args: Vec<String> = env::args().collect();
    let dir = &args[1];
    let t = walk(PathBuf::from(&dir)).map_err(drop);
    tokio::run(t);
    Ok(())
}

您还需要在 flatten_stream 之后转换错误，并且不能将 and_then 与 println! 一起使用，因为它不会返回 future 。使用 inspect 进行副作用调试。

按照惯例，您不需要收集所有参数来使用一个参数(使用 Iterator::nth)，也不需要要求 PathBuf作为论据。不清楚为什么要从 main 返回 Result，因为它永远不可能是 Err。

另见:

• Why is a trait not implemented for a type that clearly has it implemented?
• How do I import multiple versions of the same crate?
• How to async explore a directory and its sub-directories