Calculating sha1 for files using tokio::io::copy

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2

One option is to create an AsyncRead wrapper that calls the underlying AsyncRead type to do the read and then use any new data to update the hasher

use pin_project_lite::pin_project;
use sha1::{Digest, Sha1};
use std::{error::Error, io::Read, task::Poll};
use tokio::{
    fs::{File, OpenOptions},
    io::AsyncRead,
};

// Using pin_project_lite to avoid the unsafe pin manipulation
pin_project! {
    pub struct HashRead<T> {
        #[pin]
        read: T,
        // CAUTION: Sha1 is considered broken, don't use it where you need strong
        // cryptographic guarantees from a hash function
        hasher: Sha1,
    }
}

impl<T> HashRead<T> {
    pub fn new(read: T) -> Self {
        Self {
            read,
            hasher: Sha1::new(),
        }
    }

    pub fn hash(self) -> Vec<u8> {
        self.hasher.finalize().as_slice().into()
    }
}

impl<T> AsyncRead for HashRead<T>
where
    T: AsyncRead,
{
    fn poll_read(
        self: std::pin::Pin<&mut Self>,
        cx: &mut std::task::Context<'_>,
        buf: &mut tokio::io::ReadBuf<'_>,
    ) -> std::task::Poll<std::io::Result<()>> {
        let this = self.project();
        let before_len = buf.filled().len();

        // Pass on the Poll result, updating the hasher if some new data was written to the buffer.
        match this.read.poll_read(cx, buf) {
            Poll::Pending => Poll::Pending,
            Poll::Ready(Err(e)) => Poll::Ready(Err(e)),
            Poll::Ready(Ok(())) => {
                let filled = buf.filled();
                let after_len = filled.len();

                if after_len > before_len {
                    // new data was placed in the buffer, update the hasher with newly written data.
                    let new = &filled[before_len..];
                    this.hasher.update(new);
                }

                Poll::Ready(Ok(()))
            }
        }
    }
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
    let file_path = std::env::args().nth(1).unwrap();
    let mut src = HashRead::new(File::open(&file_path).await?);
    let mut dest = OpenOptions::new()
        .create(true)
        .write(true)
        .open("foo")
        .await?;
    tokio::io::copy(&mut src, &mut dest).await.unwrap();

    let hash = src.hash();
    println!("{hash:?}");

    // Check that the hash we calculated incrementally matches the hash if we just read the whole file in one operation and feed it directly to the hasher.
    assert_eq!(hash, {
        use std::fs::File;

        let mut buffer = Vec::new();
        File::open(file_path)
            .unwrap()
            .read_to_end(&mut buffer)
            .unwrap();

        let mut hasher = Sha1::new();
        hasher.update(&buffer);

        std::convert::identity::<Vec<u8>>(hasher.finalize().as_slice().into())
    });

    Ok(())
}

You can run that with

cargo run path/to/big/file
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