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Implement vectored write functionality for files #5958

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merged 6 commits into from
Aug 29, 2023

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maminrayej
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The poll_write_vectored function for File currently uses the default implementation in AsyncWrite, which only writes the first non-empty buffer in bufs. This PR adds proper vectored write support for files.

Motivation

Solution

This implementation copies the maximum permissible data (as determined by the MAX_BUF constant) from the buffers to the internal buffer of File. All other behavior aligns with the existing poll_write method.

Resolves Issue #5949.

@Darksonn Darksonn added A-tokio Area: The main tokio crate M-fs Module: tokio/fs labels Aug 29, 2023
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Overall, looks reasonable to me. I have a few nits:

.write_vectored(&[IoSlice::new(HELLO), IoSlice::new(HELLO)])
.await
.unwrap();
assert_eq!(ret, HELLO.bytes().count() * 2);
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This is in the test twice.

Suggested change
assert_eq!(ret, HELLO.bytes().count() * 2);
assert_eq!(ret, HELLO.len() * 2);

pub(crate) fn copy_from_bufs(&mut self, bufs: &[io::IoSlice<'_>]) -> usize {
assert!(self.is_empty());

let n = bufs.iter().map(|b| b.len()).sum::<usize>().min(MAX_BUF);
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This iterates all of the buffers every time, even if we only write a few of them. If the buffers are very long and this is called in a loop, that gives quadratic performance.

We should be able to embed this logic inside the for loop instead to avoid that.

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You mean b.len() is O(n) and causes the bufs.iter() to be O(n^2)? I thought since it's a Deref to &[u8], it's O(1).
I've provided an alternate implementation that doesn't use bufs.iter().

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No, b.len() is constant time. Instead, it's O(n) in the length of bufs, which you iterate over.

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Looks good to me.

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@Darksonn Darksonn merged commit 37bb47c into tokio-rs:master Aug 29, 2023
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