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blocking

Build Rustc version License Cargo Documentation Chat

Block on async code or await blocking code.

To convert async to blocking, block on async code with block_on(), block_on!, or BlockOn.

To convert blocking to async, unblock blocking code with unblock(), unblock!, or Unblock.

Thread pool

Sometimes there's no way to avoid blocking I/O in async programs. Consider files or stdin, which have weak async support on modern operating systems. While IOCP, AIO, and io_uring are possible solutions, they're not always available or ideal.

Since blocking is not allowed inside futures, we must move blocking I/O onto a special thread pool provided by this crate. The pool dynamically spawns and stops threads depending on the current number of running I/O jobs.

Note that there is a limit on the number of active threads. Once that limit is hit, a running job has to finish before others get a chance to run. When a thread is idle, it waits for the next job or shuts down after a certain timeout.

Examples

Await a blocking file read with unblock!:

use blocking::{block_on, unblock};
use std::{fs, io};

block_on(async {
    let contents = unblock!(fs::read_to_string("file.txt"))?;
    println!("{}", contents);
    io::Result::Ok(())
});

Read a file and pipe its contents to stdout:

use blocking::{block_on, Unblock};
use std::fs::File;
use std::io::{self, stdout};

block_on(async {
    let input = Unblock::new(File::open("file.txt")?);
    let mut output = Unblock::new(stdout());

    futures::io::copy(input, &mut output).await?;
    io::Result::Ok(())
});

Iterate over the contents of a directory:

use blocking::{block_on, Unblock};
use futures::prelude::*;
use std::{fs, io};

block_on(async {
    let mut dir = Unblock::new(fs::read_dir(".")?);
    while let Some(item) = dir.next().await {
        println!("{}", item?.file_name().to_string_lossy());
    }
    io::Result::Ok(())
});

Convert a stream into an iterator:

use blocking::BlockOn;
use futures::stream;

let stream = stream::once(async { 7 });
let mut iter = BlockOn::new(Box::pin(stream));

assert_eq!(iter.next(), Some(7));
assert_eq!(iter.next(), None);

License

Licensed under either of

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.