time: fix wake-up with interval on `Ready` #5553

sgasse · 2023-03-17T10:07:40Z

Motivation

When tokio::time::Interval::poll_tick() returns Poll::Pending, it schedules itself for being woken up again through the waker of the passed context, which is correct behavior. However when Poll::Ready(_) is returned, the interval timer should be reset but not scheduled to be woken up again as this is up to the caller.

Currently, resetting the timer always involves reregistering the last waker which polled the internal Sleep to be woken up on completion.

Solution

This PR fixes the bug by introducing a reset_without_reregister method on TimerEntry which is called by Intervall::poll_tick(cx) in case the delay poll returns Poll::Ready(_).

Closes: #5551

Darksonn

Thank you so much for the PR!

The implementation looks ok to me, but I've asked @carllerche to take a look at whether there's anything special to think about when skipping reregister.

I have some comments regarding your test.

Darksonn · 2023-03-17T12:01:22Z

tokio/tests/time_interval.rs

@@ -209,3 +209,76 @@ fn poll_next(interval: &mut task::Spawn<time::Interval>) -> Poll<Instant> {
 fn ms(n: u64) -> Duration {
    Duration::from_millis(n)
 }
+
+mod tmp_tests {


Why the module?

Initially I intended to keep the specific helper struct with the Stream implementation and the test together, but I guess it's not needed. I removed it.

Darksonn · 2023-03-17T12:02:07Z

tokio/tests/time_interval.rs

+    use std::{
+        pin::Pin,
+        task::{Context, Poll},
+        time::Instant,
+    };


We don't use this import style in Tokio.

Suggested change

use std::{

pin::Pin,

task::{Context, Poll},

time::Instant,

};

use std::pin::Pin;

use std::task::{Context, Poll};

use std::time::Instant;

Yes, we don't currently use it.

As an aside, I wish we did and would take a PR that did a big pass-through of Tokio to update all imports... though others should chime in before anyone does the work.

Ah OK thanks for letting me know - updated.

Darksonn · 2023-03-17T12:02:17Z

tokio/tests/time_interval.rs

+            timer: crate::time::interval(std::time::Duration::from_millis(10)),
+        };
+
+        pin_mut!(stream);


Suggested change

pin_mut!(stream);

tokio::pin!(stream);

Darksonn · 2023-03-17T12:03:29Z

tokio/tests/time_interval.rs

+        }
+    }
+
+    #[tokio::test]


This should make the test execute much faster by having it instantly skip forward to the next timer whenever a sleep happens.

Suggested change

#[tokio::test]

#[tokio::test(start_paused = true)]

OK nice! With this, do I need to manually use any tokio::time::advance, or will it automatically skip forward?

Darksonn · 2023-03-17T12:04:39Z

tokio/tests/time_interval.rs

+                        // Schedule this task for wake-up
+                        cx.waker().wake_by_ref();
+                        Poll::Pending


As far as I can tell, you're not actually testing whether it emits a wakeup or not. This test would pass either way.

Yes, the test was not complete but more a manual thing until I hear back on the approach. Sorry that I did not explain that better. I added two tests now which tease out the difference.

carllerche · 2023-03-17T15:32:16Z

tokio/src/runtime/time/entry.rs

@@ -543,6 +543,17 @@ impl TimerEntry {
        }
    }

+    pub(crate) fn reset_without_reregister(mut self: Pin<&mut Self>, new_time: Instant) {


This function is very similar to reset. Maybe reset should take an argument reregister: bool. I'm not 100% sure, but it is worth considering given the unsafe in the fn.

Ah thanks, good point. I checked and there were not too many usages of TimerEntry::reset to update, so I added the boolean flag and removed reset_without_reregister.

carllerche · 2023-03-17T15:39:28Z

tokio/src/time/sleep.rs

+    /// To call this method, you will usually combine the call with
+    /// [`Pin::as_mut`], which lets you call the method without consuming the
+    /// `Sleep` itself.
+    pub fn reset_without_reregister(self: Pin<&mut Self>, deadline: Instant) {


Adding a new public function should be a high bar. Either the existing behavior is incorrect, and we should fix it or accept it the way it is and evaluate adding a new API.

My concern with reset_without_reregister is that it makes the caller decide. Which variant should they use? Choosing one requires knowing about "waking," which isn't required for using Tokio as a beginner.

Hm that is a good point 🤔 I do think that users of tokio::time::Sleep would always want the re-registering variant. The non-re-registering is only needed for the specific use case in tokio::time::Interval. I changed the visibility to pub(crate). Would that be OK?

carllerche · 2023-03-17T15:48:50Z

tokio/tests/time_interval.rs

+    use std::{
+        pin::Pin,
+        task::{Context, Poll},
+        time::Instant,
+    };


Yes, we don't currently use it.

As an aside, I wish we did and would take a PR that did a big pass-through of Tokio to update all imports... though others should chime in before anyone does the work.

When `tokio::time::Interval::poll_tick()` returns `Poll::Pending`, it schedules itself for being woken up again through the waker of the passed context, which is correct behavior. However when `Poll::Ready(_)` is returned, the interval timer should be reset but not scheduled to be woken up again as this is up to the caller. This commit fixes the bug by introducing a `reset_without_reregister` method on `TimerEntry` which is called by `Intervall::poll_tick(cx)` in case the delay poll returns `Poll::Ready(_)`.

sgasse · 2023-03-27T15:03:29Z

@Darksonn or @carllerche would you have time to check the updated version? 🙂

Darksonn

Thanks.

This PR contains the following updates: | Package | Type | Update | Change | |---|---|---|---| | [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dependencies | minor | `1.26.0` -> `1.27.0` | | [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dev-dependencies | minor | `1.26.0` -> `1.27.0` | --- ### Release Notes <details> <summary>tokio-rs/tokio</summary> ### [`v1.27.0`](https://github.com/tokio-rs/tokio/releases/tag/tokio-1.27.0): Tokio v1.27.0 [Compare Source](tokio-rs/tokio@tokio-1.26.0...tokio-1.27.0) ##### 1.27.0 (March 27th, 2023) This release bumps the MSRV of Tokio to 1.56. ([#5559]) ##### Added - io: add `async_io` helper method to sockets ([#5512]) - io: add implementations of `AsFd`/`AsHandle`/`AsSocket` ([#5514], [#5540]) - net: add `UdpSocket::peek_sender()` ([#5520]) - sync: add `RwLockWriteGuard::{downgrade_map, try_downgrade_map}` ([#5527]) - task: add `JoinHandle::abort_handle` ([#5543]) ##### Changed - io: use `memchr` from `libc` ([#5558]) - macros: accept path as crate rename in `#[tokio::main]` ([#5557]) - macros: update to syn 2.0.0 ([#5572]) - time: don't register for a wakeup when `Interval` returns `Ready` ([#5553]) ##### Fixed - fs: fuse std iterator in `ReadDir` ([#5555]) - tracing: fix `spawn_blocking` location fields ([#5573]) - time: clean up redundant check in `Wheel::poll()` ([#5574]) ##### Documented - macros: define cancellation safety ([#5525]) - io: add details to docs of `tokio::io::copy[_buf]` ([#5575]) - io: refer to `ReaderStream` and `StreamReader` in module docs ([#5576]) [#5512]: tokio-rs/tokio#5512 [#5514]: tokio-rs/tokio#5514 [#5520]: tokio-rs/tokio#5520 [#5525]: tokio-rs/tokio#5525 [#5527]: tokio-rs/tokio#5527 [#5540]: tokio-rs/tokio#5540 [#5543]: tokio-rs/tokio#5543 [#5553]: tokio-rs/tokio#5553 [#5555]: tokio-rs/tokio#5555 [#5557]: tokio-rs/tokio#5557 [#5558]: tokio-rs/tokio#5558 [#5559]: tokio-rs/tokio#5559 [#5572]: tokio-rs/tokio#5572 [#5573]: tokio-rs/tokio#5573 [#5574]: tokio-rs/tokio#5574 [#5575]: tokio-rs/tokio#5575 [#5576]: tokio-rs/tokio#5576 </details> --- ### Configuration 📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined). 🚦 **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. --- - [ ] If you want to rebase/retry this PR, check this box --- This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).  Co-authored-by: cabr2-bot <cabr2.help@gmail.com> Reviewed-on: https://codeberg.org/Calciumdibromid/CaBr2/pulls/1838 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>

With tokio 1.27.0, the Interval type changed in backwards incompatible ways, now necessitating a call to Interval::reset to schedule another wake up [0]. This change makes sure that we follow the new procedure. [0]: tokio-rs/tokio#5553

d-e-s-o · 2023-06-23T15:42:17Z

For what it's worth, I would say that this change is most certainly backwards compatibility breaking, as now an additional call is necessary to get the previous behavior when using the Interval API (behavior which had been there since tokio 1.0, I suppose).
I can see why it's been done, but such a change in behavior can cause tricky production issues. I am not sure how to reconcile this with any semantic versioning guarantees that I understand tokio provides, to be honest :-(

d-e-s-o · 2023-06-23T15:53:13Z

The more appropriate course of action, from my perspective, would have been to introduce a new method with the new behavior, deprecate the existing poll_tick, and then after a couple of minor releases or so, replace the old poll_tick with the new one (or remove the old one), for example (I would guess you have a policy in place for deprecations). I understand that this is overhead, but that's the price to pay with 1.0 crates, in my opinion (not that it's specific to them, just that the barrier for 2.0 is probably higher than for incompatible changes before 1.0).

Darksonn · 2023-06-26T07:29:27Z

@d-e-s-o As far as I understand, your code was already broken. The interval would not update the waker when it returns Poll::Ready, so if you change which waker the advance method uses, then it would wake the wrong waker. Similarly, if the next tick is immediately ready, then I don't think you would ever get a wakeup, even if its the same waker (whether that can happen depends on which missed tick behavior you're using).

To add to that, your fix is incorrect. The reset method is not supposed to schedule for wakeup. The correct fix is to call poll_tick again, because poll methods generally only guarantee that you will get a wakeup when they return Poll::Pending. The easiest way to do that is probably to call ctx.waker().wake_by_ref().

To be clear, this is how all poll methods work. It's not something special to Interval. Also, you don't need to bump the Tokio dependency if you use it correctly — the old implementation will still work if you call poll_tick again.

I am not sure how to reconcile this with any semantic versioning guarantees that I understand tokio provides, to be honest :-(

This change is a bug fix. Bug fixes are always breaking changes.

Of course, there is always a trade-off between fixing bugs and keeping broken behavior.

d-e-s-o · 2023-06-26T13:45:25Z

Ah okay. Thanks for the explanation!

github-actions bot added the R-loom Run loom tests on this PR label Mar 17, 2023

sgasse force-pushed the fix/interval_do_not_reregister_on_ready branch from c48f061 to b2d67ef Compare March 17, 2023 10:13

Darksonn changed the title ~~time: fix wake-up with interval on Ready (#5551)~~ time: fix wake-up with interval on Ready Mar 17, 2023

Darksonn added A-tokio Area: The main tokio crate M-time Module: tokio/time labels Mar 17, 2023

Darksonn reviewed Mar 17, 2023

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carllerche reviewed Mar 17, 2023

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sgasse force-pushed the fix/interval_do_not_reregister_on_ready branch 2 times, most recently from 82b9a7f to c2586d6 Compare March 17, 2023 17:13

sgasse force-pushed the fix/interval_do_not_reregister_on_ready branch from c2586d6 to 8dffc8d Compare March 17, 2023 17:18

Darksonn approved these changes Mar 27, 2023

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Darksonn merged commit 68b02db into tokio-rs:master Mar 27, 2023

Darksonn mentioned this pull request Mar 27, 2023

chore: prepare Tokio v1.27.0 #5584

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justanotherdot mentioned this pull request Mar 29, 2023

Bump tokio from 1.26.0 to 1.27.0 justanotherdot/knifey#49

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time: fix wake-up with interval on `Ready` #5553

time: fix wake-up with interval on `Ready` #5553

sgasse commented Mar 17, 2023 •

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time: fix wake-up with interval on Ready #5553

time: fix wake-up with interval on Ready #5553

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sgasse commented Mar 17, 2023 • edited by Darksonn Loading

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time: fix wake-up with interval on `Ready` #5553

time: fix wake-up with interval on `Ready` #5553

sgasse commented Mar 17, 2023 •

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