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io: impl AsyncSeek for BufReader/BufWriter #3491

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merged 4 commits into from May 14, 2021
Merged

io: impl AsyncSeek for BufReader/BufWriter #3491

merged 4 commits into from May 14, 2021

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taiki-e
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@taiki-e taiki-e commented Jan 31, 2021 •
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Motivation

Closes #2291

Solution

The implementation and added tests are basically based on my PRs in futures-rs. (rust-lang/futures-rs#1573, rust-lang/futures-rs#1608)

However, the implementation is complicated than futures-rs due to:

@taiki-e taiki-e added A-tokio Area: The main tokio crate M-io Module: tokio/io labels Jan 31, 2021
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@taiki-e taiki-e marked this pull request as draft February 2, 2021 00:18
taiki-e
taiki-e commented Feb 6, 2021
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tokio/src/io/util/buf_reader.rs
Comment on lines +190 to +193
// 1.x AsyncSeek recommends calling poll_complete before start_seek.
// We don't have to guarantee that the value returned by
// poll_complete called without start_seek is correct,
// so we'll return 0.
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We don't have to guarantee that the value returned by poll_complete called without start_seek is correct,

@carllerche @Darksonn Is this assumption correct? If not, are there rules I need to follow?

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Returning Ok(0) if poll_complete is called before start_seek seems reasonable to me.

taiki-e
taiki-e commented Feb 6, 2021
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tokio/src/io/util/buf_reader.rs
Comment on lines +178 to +183
// We needs to call seek operation multiple times.
// And we should always call both start_seek and poll_complete,
// as start_seek alone cannot guarantee that the operation will be completed.
// poll_complete receives a Context and returns a Poll, so it cannot be called
// inside start_seek.
*self.project().seek_state = SeekState::Start(pos);
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This is another reason why this implementation is more complicated than the futures-rs one.

@taiki-e taiki-e changed the title impl AsyncSeek for BufReader/BufWriter/BufStream io: impl AsyncSeek for BufReader/BufWriter/BufStream Feb 6, 2021
@taiki-e taiki-e marked this pull request as ready for review February 6, 2021 15:19
@rafibaum
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rafibaum commented May 4, 2021

I'm getting some errors from this implementation in the writer when a File is used, where poll_complete isn't getting called before a new start_seek call is queued.

thread 'tokio-runtime-worker' panicked at 'must wait for poll_complete before calling start_seek', /home/rafi/.cargo/git/checkouts/tokio-f564add6bba9226f/fa793aa/tokio/src/fs/file.rs:561:28
stack backtrace:
   0: std::panicking::begin_panic
   1: <tokio::fs::file::File as tokio::io::async_seek::AsyncSeek>::start_seek
   2: <&mut T as tokio::io::async_seek::AsyncSeek>::poll_complete
   3: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
   4: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
   5: tokio::runtime::task::harness::poll_future
   6: tokio::runtime::task::harness::Harness<T,S>::poll
   7: std::thread::local::LocalKey<T>::with
   8: tokio::runtime::thread_pool::worker::Context::run_task
   9: tokio::runtime::thread_pool::worker::Context::run
  10: tokio::macros::scoped_tls::ScopedKey<T>::set
  11: tokio::runtime::thread_pool::worker::run
  12: tokio::loom::std::unsafe_cell::UnsafeCell<T>::with_mut
  13: <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once
  14: tokio::runtime::task::harness::Harness<T,S>::poll
  15: tokio::runtime::blocking::pool::Inner::run
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

I fixed this on a personal fork by having the BufWriter implementation call poll_complete if the seek state is Init and also calling ready!(poll_complete)? before calling start_seek.

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taiki-e commented May 5, 2021

@rafibaum Thanks for finding that problem. I've merged your patch into this PR. (cbd6a5b)

Darksonn
Darksonn approved these changes May 14, 2021
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This seems fine to me.

@taiki-e taiki-e merged commit 8c395df into tokio-rs:master May 14, 2021
@taiki-e taiki-e deleted the io-buf-rw-seek branch May 14, 2021 10:32
@Darksonn Darksonn mentioned this pull request May 14, 2021
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@petrosagg petrosagg mentioned this pull request May 25, 2021
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taiki-e commented May 25, 2021

For some reason, I seemed to have forgotten about BufStream... 😅

@taiki-e taiki-e changed the title io: impl AsyncSeek for BufReader/BufWriter/BufStream io: impl AsyncSeek for BufReader/BufWriter May 25, 2021
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tokio-io: No AsyncSeek implementation for BufReader and BufWriter
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@taiki-e @rafibaum @Darksonn