`sync::watch`: Add `has_changed` flag and method to `Ref` #4758

uklotzde · 2022-06-08T11:50:19Z

Extend sync::watch::Receiver to cover another use case.

Motivation

As a sync::watch::Receiver I would like to know, if the value obtained from borrow_and_update() has actually changed and marked as seen or not. Subsequent calculations could be skipped without actually inspecting the value if no changes occurred.

Solution

Add an ~~updated~~ has_changed flag and method to Ref.

Discarded solution (informational, just for the record )

Add a new function borrow_and_update_if_changed() that returns a tuple, both the reference and a flag indicating if the value has been detected as changed and marked as seen, i.e. updated. Using a boolean flag is consistent with the return value of has_changed() on success.

This function could otherwise not be implemented in terms of the public API, because invoking has_changed() and borrow_and_update() independently is not thread-safe.

As temporary a workaround you could invoke has_changed() twice, i.e. before and after invoking borrow_and_update(). This will cause some false positives and is less efficient. This doesn't work correctly if the Sender has been dropped, because then has_changed() always returns with an error. Doesn't work as Receiver is already mutably borrowed after borrow_and_update().

This function could otherwise not be implemented in terms of the public API, because invoking has_changed() and borrow_and_update() independently is not thread-safe.

hawkw · 2022-06-08T20:40:22Z

tokio/src/sync/watch.rs

+    /// </details>
+    ///
+    /// [`changed`]: Receiver::changed
+    pub fn borrow_and_update_if_changed(&mut self) -> (Ref<'_, T>, bool) {


here's a minor API design suggestion. what if, instead of a tuple of Ref and bool, we added the bool to ref along with a Ref::changed() method. that way, the meaning of the boolean value would be clearer at the callsite, as opposed to making the user responsible for naming the tuple field. it would also negate the need to have two separate methods.

That seems like a good way to go around it.

Good point! I was already experimenting with such a design in my external wrapper crate. Just wanted to avoid introducing new, named types.

I didn't consider the fact that this solutions enables us to extend the current API without any breaking changes, which makes it even more appealing :) Thank you for the hint!

I was blind 🙈 Your proposal turned out as the perfect solution that only affects borrow_and_update() when focusing on the updated aspect and not considering the more general changed aspect: 63c9987

I noticed that commits will be squashed before merging, so I added the changes as a new commit.

The flag is only set by Receiver::borrow_and_update(). Otherwise it is initialized with `false`.

Darksonn · 2022-06-09T05:45:11Z

tokio/src/sync/watch.rs

+    /// Indicates if the shared value has been detected as _changed_
+    /// **and** marked as seen.
+    ///
+    /// The result is `true` only if this reference has been obtained
+    /// from [`Receiver::borrow_and_update()`] in case changes have been
+    /// detected and marked as seen. In all other cases the result will
+    /// be `false`.
+    pub fn updated(&self) -> bool {
+        self.updated
+    }


It seems more useful to have this function return whether the value was marked as seen before the call. Then the value is also useful for the borrow method.

This would mean to

add an invocation of load() to borrow() and

rename the function to has_changed().

I was unsure if the extra load() in borrow() is desired. It probably doesn't have any noticeable effect and a more consistent interface might be preferable.

Done: ~~d081f83~~ (fixed typo) b4fef98

I also added a comprehensive example.

tokio/src/sync/watch.rs

Co-authored-by: Alice Ryhl <aliceryhl@google.com>

…_update_if_changed'

Darksonn

Thanks. This looks good to me.

This PR contains the following updates: | Package | Type | Update | Change | |---|---|---|---| | [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dependencies | minor | `1.19.2` -> `1.20.0` | | [tokio](https://tokio.rs) ([source](https://github.com/tokio-rs/tokio)) | dev-dependencies | minor | `1.19.2` -> `1.20.0` | --- ### Release Notes <details> <summary>tokio-rs/tokio</summary> ### [`v1.20.0`](https://github.com/tokio-rs/tokio/releases/tag/tokio-1.20.0) [Compare Source](tokio-rs/tokio@tokio-1.19.2...tokio-1.20.0) ##### 1.20.0 (July 12, 2022) ##### Added - tokio: add track_caller to public APIs ([#4772], [#4791], [#4793], [#4806], [#4808]) - sync: Add `has_changed` method to `watch::Ref` ([#4758]) ##### Changed - time: remove `src/time/driver/wheel/stack.rs` ([#4766]) - rt: clean up arguments passed to basic scheduler ([#4767]) - net: be more specific about winapi features ([#4764]) - tokio: use const initialized thread locals where possible ([#4677]) - task: various small improvements to LocalKey ([#4795]) ##### Fixed ##### Documented - fs: warn about performance pitfall ([#4762]) - chore: fix spelling ([#4769]) - sync: document spurious failures in oneshot ([#4777]) - sync: add warning for watch in non-Send futures ([#4741]) - chore: fix typo ([#4798]) ##### Unstable - joinset: rename `join_one` to `join_next` ([#4755]) - rt: unhandled panic config for current thread rt ([#4770]) [#4677]: tokio-rs/tokio#4677 [#4741]: tokio-rs/tokio#4741 [#4755]: tokio-rs/tokio#4755 [#4758]: tokio-rs/tokio#4758 [#4762]: tokio-rs/tokio#4762 [#4764]: tokio-rs/tokio#4764 [#4766]: tokio-rs/tokio#4766 [#4767]: tokio-rs/tokio#4767 [#4769]: tokio-rs/tokio#4769 [#4770]: tokio-rs/tokio#4770 [#4772]: tokio-rs/tokio#4772 [#4777]: tokio-rs/tokio#4777 [#4791]: tokio-rs/tokio#4791 [#4793]: tokio-rs/tokio#4793 [#4795]: tokio-rs/tokio#4795 [#4798]: tokio-rs/tokio#4798 [#4806]: tokio-rs/tokio#4806 [#4808]: tokio-rs/tokio#4808 </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, click this checkbox. --- 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/1458 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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sync::watch: Add Receiver::borrow_and_update_if_changed()

6445c44

This function could otherwise not be implemented in terms of the public API, because invoking has_changed() and borrow_and_update() independently is not thread-safe.

uklotzde force-pushed the sync-watch-receiver-borrow_and_update_if_changed branch from 80298c6 to 6445c44 Compare June 8, 2022 13:36

hawkw reviewed Jun 8, 2022

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Darksonn added A-tokio Area: The main tokio crate M-sync Module: tokio/sync labels Jun 8, 2022

sync::watch: Add an updated flag and method to Ref

63c9987

The flag is only set by Receiver::borrow_and_update(). Otherwise it is initialized with `false`.

uklotzde changed the title ~~sync::watch: Add Receiver::borrow_and_update_if_changed()~~ sync::watch: Add updated flag and method to Ref Jun 8, 2022

uklotzde changed the title ~~sync::watch: Add updated flag and method to Ref~~ sync::watch: Add updated flag and method to Ref Jun 8, 2022

Darksonn reviewed Jun 9, 2022

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sync::watch::Ref: Replace updated() with has_changed()

b4fef98

uklotzde force-pushed the sync-watch-receiver-borrow_and_update_if_changed branch from d081f83 to b4fef98 Compare June 9, 2022 08:58

uklotzde changed the title ~~sync::watch: Add updated flag and method to Ref~~ sync::watch: Add has_changed flag and method to Ref Jun 9, 2022

Darksonn reviewed Jun 28, 2022

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tokio/src/sync/watch.rs Outdated Show resolved Hide resolved

uklotzde and others added 2 commits June 28, 2022 16:36

Reword API docs

67f94a7

Co-authored-by: Alice Ryhl <aliceryhl@google.com>

Merge 'upstream/master' into 'uklotzde/sync-watch-receiver-borrow_and…

f173977

…_update_if_changed'

Darksonn approved these changes Jun 29, 2022

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Darksonn enabled auto-merge (squash) June 29, 2022 08:57

Darksonn merged commit 7ed2737 into tokio-rs:master Jun 29, 2022

Darksonn mentioned this pull request Jul 12, 2022

chore: prepare tokio v1.20.0 #4830

Merged

uklotzde deleted the sync-watch-receiver-borrow_and_update_if_changed branch September 19, 2023 16:27

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`sync::watch`: Add `has_changed` flag and method to `Ref` #4758

`sync::watch`: Add `has_changed` flag and method to `Ref` #4758

uklotzde commented Jun 8, 2022 •

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sync::watch: Add has_changed flag and method to Ref #4758

sync::watch: Add has_changed flag and method to Ref #4758

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uklotzde commented Jun 8, 2022 • edited Loading

Motivation

Solution

Discarded solution (informational, just for the record )

hawkw Jun 8, 2022

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`sync::watch`: Add `has_changed` flag and method to `Ref` #4758

`sync::watch`: Add `has_changed` flag and method to `Ref` #4758

uklotzde commented Jun 8, 2022 •

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uklotzde Jun 8, 2022 •

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uklotzde Jun 9, 2022 •

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