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rt: fix *_closed false positives #5231

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merged 11 commits into from Dec 5, 2022
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satakuma
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Motivation

Readiness futures inconsistently return current readiness of a I/O resource, if it is immediately available, or all readiness relevant for the given Interest, if a future needs to wait. In particular, it always returns read_closed for Interest::READABLE and write_closed for Interest::WRITABLE, what often is not true. Tokio should not tolerate false positives for *_closed events because they are considered final states and are not cleared internally.

In case of a io_resource.ready(Interest::READABLE | Interest::WRITABLE) call, this behavior may also lead to false positives of other events.

Solution

If a Readiness future is woken up by a new event, store the relevant readiness and return it later.
It pumps the Waiter and enclosing structs a bit (8 bytes on x86_64). If it's an issue, readiness can be packed into something smaller.

Closes: #5098

@github-actions github-actions bot added the R-loom Run loom tests on this PR label Nov 24, 2022
@Darksonn Darksonn added A-tokio Area: The main tokio crate M-io Module: tokio/io labels Nov 25, 2022
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Looks reasonable to me, but I would also like a review from Carl.

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@carllerche carllerche left a comment

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Thanks for doing this. You are correct the current implementation is buggy. Most of this looks good. I left a comment inline.


// Add more readiness which might have appeared in the meantime.
let curr_ready = Ready::from_usize(READINESS.unpack(curr));
let ready = w.ready | (curr_ready.intersection(w.interest));
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If a readiness bit is not included in curr_ready then something else already cleared it. Including w.ready here would result in additional false positives & unnecessary I/O ops.

Based on discord, it sounds like you want to avoid empty readiness events, but that seems better to me than false positives. The caller would loop and wait again.

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@carllerche I applied your suggestion to return the current readiness.

I also updated the docs of every ready method to mention false-positives and empty events. My motivation is that empty Ready events were not really possible before this change. As far as I can tell from the docs, there is no explicit guarantee about no empty return values, but I can still imagine a program logic which would assume that at least one of the requested state is set. For example, code which would deduce that if !io.ready(Interest::READABLE).await?.is_readable(), then the read must be closed. For that reason I think an explicit mention in the docs will be helpful.

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LGTM thanks 👍

carllerche added a commit that referenced this pull request Dec 5, 2022
 - net: fix Windows named pipe connect ([#5208])
 - io: support vectored writes for `ChildStdin` ([#5216])
 - io: fix `async fn ready()` false positive for OS-specific events ([#5231])

 ### Changed
 - runtime: `yield_now` defers task until after driver poll ([#5223])
 - runtime: reduce amount of codegen needed per spawned task ([#5213])
 - windows: replace `winapi` dependency with `windows-sys` ([#5204])

 [#5208]: #5208
 [#5216]: #5216
 [#5213]: #5213
 [#5204]: #5204
 [#5223]: #5223
 [#5231]: #5231
@carllerche carllerche merged commit 644cb82 into tokio-rs:master Dec 5, 2022
carllerche added a commit that referenced this pull request Dec 5, 2022
### Fixed
 - net: fix Windows named pipe connect ([#5208])
 - io: support vectored writes for `ChildStdin` ([#5216])
 - io: fix `async fn ready()` false positive for OS-specific events ([#5231])

 ### Changed
 - runtime: `yield_now` defers task until after driver poll ([#5223])
 - runtime: reduce amount of codegen needed per spawned task ([#5213])
 - windows: replace `winapi` dependency with `windows-sys` ([#5204])

 [#5208]: #5208
 [#5216]: #5216
 [#5213]: #5213
 [#5204]: #5204
 [#5223]: #5223
 [#5231]: #5231
crapStone pushed a commit to Calciumdibromid/CaBr2 that referenced this pull request Dec 23, 2022
This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dependencies | minor | `1.22.0` -> `1.23.0` |
| [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dev-dependencies | minor | `1.22.0` -> `1.23.0` |

---

### Release Notes

<details>
<summary>tokio-rs/tokio</summary>

### [`v1.23.0`](https://github.com/tokio-rs/tokio/releases/tag/tokio-1.23.0): Tokio v1.23.0

[Compare Source](tokio-rs/tokio@tokio-1.22.0...tokio-1.23.0)

##### Fixed

-   net: fix Windows named pipe connect ([#&#8203;5208])
-   io: support vectored writes for `ChildStdin` ([#&#8203;5216])
-   io: fix `async fn ready()` false positive for OS-specific events ([#&#8203;5231])

##### Changed

-   runtime: `yield_now` defers task until after driver poll ([#&#8203;5223])
-   runtime: reduce amount of codegen needed per spawned task ([#&#8203;5213])
-   windows: replace `winapi` dependency with `windows-sys` ([#&#8203;5204])

[#&#8203;5208]: tokio-rs/tokio#5208

[#&#8203;5216]: tokio-rs/tokio#5216

[#&#8203;5213]: tokio-rs/tokio#5213

[#&#8203;5204]: tokio-rs/tokio#5204

[#&#8203;5223]: tokio-rs/tokio#5223

[#&#8203;5231]: tokio-rs/tokio#5231

</details>

---

### Configuration

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🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about these updates again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiIzNC43MC4wIiwidXBkYXRlZEluVmVyIjoiMzQuNzAuMCJ9-->

Co-authored-by: cabr2-bot <cabr2.help@gmail.com>
Reviewed-on: https://codeberg.org/Calciumdibromid/CaBr2/pulls/1687
Reviewed-by: crapStone <crapstone@noreply.codeberg.org>
Co-authored-by: Calciumdibromid Bot <cabr2_bot@noreply.codeberg.org>
Co-committed-by: Calciumdibromid Bot <cabr2_bot@noreply.codeberg.org>
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