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Tokio v1.0.0 support. #33

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Tokio v1.0.0 support. #33

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6168e0a
bump up to tokio 1.0.0
Dec 23, 2020
9624ebe
add loop. remove debugging
Dec 26, 2020
86d32a4
remove depn on tokio_stream
Dec 26, 2020
7c2554d
update to tokio 1.0.1 after 1.0.0 was pulled
Dec 26, 2020
ace6436
remove loop
Dec 26, 2020
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16 changes: 7 additions & 9 deletions src/lib.rs
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Expand Up @@ -222,15 +222,13 @@ impl AsyncRead for Serial {
cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
loop {
let mut guard = ready!(self.io.poll_read_ready(cx))?;
match guard.try_io(|_| {
let read = self.io.get_ref().read(buf.initialize_unfilled())?;
return Ok(buf.advance(read));
}) {
Ok(result) => return Poll::Ready(result),
Err(_would_block) => continue,
}
let mut guard = ready!(self.io.poll_read_ready(cx))?;
match guard.try_io(|_| {
let read = self.io.get_ref().read(buf.initialize_unfilled())?;
return Ok(buf.advance(read));
}) {
Ok(result) => return Poll::Ready(result),
Err(_would_block) => return Poll::Pending,
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This breaks as the read() can return would block in which case you go pending without the async fd actually registered with the runtime; The original version with the loop is correct as read() returns would_block then the poll_read_ready() call should return Pending and everything should work nicely :).

fwiw i hit this problem in practise while playing with this branch and a fdti usb dongle in linux

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I'm confused about this: if the read returns EWOULDBLOCK doesn't that imply a bug somewhere else? We can't arrive at line 227 without being signaled by tokio that there are in fact some bytes to read. I haven't been using this branch but I have some code that is morally equiv that I have been testing with ftdi dongles (snap!) and I haven't seen this condition triggered (yet).

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It is possible to get a false positive from poll_read_ready and get the would block error, but the snippet is still incorrect — you have to loop around and call poll_read_ready again if that happens.

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I see what you are saying. Thanks for making that connection. I'd conflated the clearing of the tokio-side readiness state (which is handled by try_io) with the need to poll again to get registered. I also completely misread the sample code for AsyncFd. I guess I'd just gotten (un)lucky with my testing :)

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@mateuszkj mateuszkj Jun 13, 2021 •
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@mnbbrown Please look at PR: mnbbrown#1

We got bug with "hanging" usart and adding loops fixed the problem.

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I can confirm I have been seeing this bug too (when working on tokio-modbus's update to tokio-1.0) and rfael's patch fixed it.

}
}
}
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