我需要浏览目录及其所有子目录。我可以通过同步方式轻松地通过递归浏览目录：
use failure::Error;
use std::fs;
use std::path::Path;
fn main() -> Result<(), Error> {
visit(Path::new("."))
}
fn visit(path: &Path) -> Result<(), Error> {
for e in fs::read_dir(path)? {
let e = e?;
let path = e.path();
if path.is_dir() {
visit(&path)?;
} else if path.is_file() {
println!("File: {:?}", path);
}
}
Ok(())
}
[当我尝试使用tokio_fs以异步方式执行相同操作时：
use failure::Error; // 0.1.6
use futures::Future; // 0.1.29
use std::path::PathBuf;
use tokio::{fs, prelude::*}; // 0.1.22
fn visit(path: PathBuf) -> impl Future<Item = (), Error = Error> {
let task = fs::read_dir(path)
.flatten_stream()
.for_each(|entry| {
println!("{:?}", entry.path());
let path = entry.path();
if path.is_dir() {
let task = visit(entry.path());
tokio::spawn(task.map_err(drop));
}
future::ok(())
})
.map_err(Error::from);
task
}
我收到以下错误：
error[E0391]: cycle detected when processing `visit::{{opaque}}#0`
--> src/lib.rs:6:28
|
6 | fn visit(path: PathBuf) -> impl Future<Item = (), Error = Error> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: ...which requires processing `visit`...
--> src/lib.rs:6:1
|
6 | fn visit(path: PathBuf) -> impl Future<Item = (), Error = Error> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: ...which requires evaluating trait selection obligation `futures::future::map_err::MapErr<impl futures::future::Future, fn(failure::error::Error) {std::mem::drop::<failure::error::Error>}>: std::marker::Send`...
= note: ...which again requires processing `visit::{{opaque}}#0`, completing the cycle
note: cycle used when checking item types in top-level module
--> src/lib.rs:1:1
|
1 | / use failure::Error; // 0.1.6
2 | | use futures::Future; // 0.1.29
3 | | use std::path::PathBuf;
4 | | use tokio::{fs, prelude::*}; // 0.1.22
... |
20| | task
21| | }
| |_^
error[E0391]: cycle detected when processing `visit::{{opaque}}#0`
--> src/lib.rs:6:28
|
6 | fn visit(path: PathBuf) -> impl Future<Item = (), Error = Error> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
note: ...which requires processing `visit`...
--> src/lib.rs:6:1
|
6 | fn visit(path: PathBuf) -> impl Future<Item = (), Error = Error> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
= note: ...which again requires processing `visit::{{opaque}}#0`, completing the cycle
note: cycle used when checking item types in top-level module
--> src/lib.rs:1:1
|
1 | / use failure::Error; // 0.1.6
2 | | use futures::Future; // 0.1.29
3 | | use std::path::PathBuf;
4 | | use tokio::{fs, prelude::*}; // 0.1.22
... |
20| | task
21| | }
| |_^
传播所有错误时异步浏览目录及其子目录的正确方法是什么？
您的代码有两个错误：
首先，返回impl Trait的函数当前无法递归，因为返回的实际类型将取决于自身。
要使示例正常工作，您需要返回一个大小类型。简单的候选者是特征对象，即Box<dyn Future<...>>：
fn visit(path: PathBuf) -> Box<dyn Future<Item = (), Error = Error>> {
// ...
let task = visit(entry.path());
tokio::spawn(task.map_err(drop));
// ...
Box::new(task)
}
还有第二个错误：
error[E0277]: `dyn futures::future::Future<Item = (), Error = failure::error::Error>` cannot be sent between threads safely
--> src/lib.rs:14:30
|
14 | tokio::spawn(task.map_err(drop));
| ^^^^^^^^^^^^^^^^^^ `dyn futures::future::Future<Item = (), Error = failure::error::Error>` cannot be sent between threads safely
|
::: /root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.1.22/src/executor/mod.rs:131:52
|
131 | where F: Future<Item = (), Error = ()> + 'static + Send
| ---- required by this bound in `tokio::executor::spawn`
|
= help: the trait `std::marker::Send` is not implemented for `dyn futures::future::Future<Item = (), Error = failure::error::Error>`
= note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<dyn futures::future::Future<Item = (), Error = failure::error::Error>>`
= note: required because it appears within the type `std::boxed::Box<dyn futures::future::Future<Item = (), Error = failure::error::Error>>`
= note: required because it appears within the type `futures::future::map_err::MapErr<std::boxed::Box<dyn futures::future::Future<Item = (), Error = failure::error::Error>>, fn(failure::error::Error) {std::mem::drop::<failure::error::Error>}>`
这意味着您的特征对象不是Send，因此无法计划使用tokio::spawn()在另一个线程中执行它。幸运的是，这很容易解决：只需将+ Send添加到特征对象中即可：
fn visit(path: PathBuf) -> Box<dyn Future<Item = (), Error = Error> + Send> {
//...
}
请参阅Playground中的完整代码。
我将对rodrigo's existing answer进行几处修改：
Stream，允许调用者通过给定的文件条目执行他们需要的操作。impl Stream，而不是Box<dyn Stream>。这为实现中的更大灵活性留出了空间。例如，可以创建一个使用内部堆栈而不是效率较低的递归类型的自定义类型。io::Error，以允许用户处理任何错误。impl Into<PathBuf>以允许使用更好的API。use std::path::PathBuf;
use tokio::{fs, prelude::*}; // 0.1.22
use tokio_fs::DirEntry; // 1.0.6
fn visit(
path: impl Into<PathBuf>,
) -> impl Stream<Item = DirEntry, Error = std::io::Error> + Send + 'static {
fn visit_inner(
path: PathBuf,
) -> Box<dyn Stream<Item = DirEntry, Error = std::io::Error> + Send + 'static> {
Box::new({
fs::read_dir(path)
.flatten_stream()
.map(|entry| {
let path = entry.path();
if path.is_dir() {
// Optionally include `entry` if you want to
// include directories in the resulting
// stream.
visit_inner(path)
} else {
Box::new(stream::once(Ok(entry)))
}
})
.flatten()
})
}
visit_inner(path.into())
}
fn main() {
tokio::run({
let root_path = std::env::args().nth(1).expect("One argument required");
let paths = visit(root_path);
paths
.then(|entry| {
match entry {
Ok(entry) => println!("visiting {:?}", entry),
Err(e) => eprintln!("encountered an error: {}", e),
};
Ok(())
})
.for_each(|_| Ok(()))
});
}
另请参见：