util: rework `Either` #637

davidpdrsn · 2022-02-08T13:58:17Z

In practice I've found Either be hard to use since it changes the error type to BoxError. That means if you combine two infallible services you get a service that, to the type system, is fallible. That doesn't work well with axum's error handling model which requires all services to be infallible and thus always return a response. So you end up having to add boilerplate just to please the type system.

Additionally, the fact that Either implements Future also means we cannot fully remove the dependency on pin-project since pin-project-lite doesn't support tuple enum variants, only named fields.

This PR reworks Either to address these:

It now requires the two services to have the same error type so no type information is lost. I did consider doing something like where B::Error: From<A::Error> but I hope this simpler model will lead to better compiler errors.
Changes the response future to be a struct with a private enum using pin-project-lite
Removes the Future impl so we can remove the dependency on pin-project

Goes without saying that this is a breaking change so we have to wait until tower 0.5 to ship this.

cc @jplatte

Fixes #594
Fixes #550

jplatte · 2022-02-08T14:03:42Z

Have you considered implementing Layer and Service for futures_util::future::Either instead and using that type in the relevant places? That approach seemed a little bit cleaner to me when going over the possible solutions last time.

jplatte · 2022-02-08T14:05:19Z

Additionally, IIRC there was some talk about changing A and B to Left and Right, which should maybe be part of the same PR?

davidpdrsn · 2022-02-08T14:11:56Z

Have you considered implementing Layer and Service for futures_util::future::Either instead and using that type in the relevant places? That approach seemed a little bit cleaner to me when going over the possible solutions last time.

That would mean tower gets a public dependency on futures_util which I don't think we have today 🤔

Additionally, IIRC there was some talk about changing A and B to Left and Right, which should maybe be part of the same PR?

Yeah that seems fine to me. I'll do that.

hawkw · 2022-02-08T17:09:52Z

Have you considered implementing Layer and Service for futures_util::future::Either instead and using that type in the relevant places? That approach seemed a little bit cleaner to me when going over the possible solutions last time.

That would mean tower gets a public dependency on futures_util which I don't think we have today

FWIW, tower does depend on futures_util, but it's not exposed in any public APIs as far as I know.

davidpdrsn · 2022-02-08T17:16:13Z

Actually implementing Service and Layer directly for futures_util::future::Either would require putting those impls in tower-service and tower-layer and probably get in the way of making them 1.0, at least until futures_util itself is 1.0. Dunno what the timeline is on that but I doubt it'll happen any time soon.

hawkw · 2022-02-08T17:18:43Z

Actually implementing Service and Layer directly for futures_util::future::Either would require putting those impls in tower-service and tower-layer and probably get in the way of making them 1.0, at least until futures_util itself is 1.0. Dunno what the timeline is on that but I doubt it'll happen any time soon.

Oh, you're right, I didn't think through the orphan rules here. I don't think we'll be able to do that, then.

We could use futures_util::future::Either instead of the custom response future type, but I don't think it's worth making it public API for that purpose. The implementation is trivial enough that we don't need to use it from futures_util, I think the only motivation would be compatibility, and compatibility would require putting it in tower-service, which I don't think we should do.

hawkw

seems good to me (in 0.5, of course)

tower/src/util/either.rs

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

olix0r · 2022-02-08T22:02:53Z

I'm generally +1 on this. we should do 0.5 sooner than later...

The CI failure seems legit, though.

This reverts commit b464abb.

Tower already has `option_layer` for conditionally applying a `Layer` but since it uses `tower::util::Either` it always changes the error type to `BoxError`. That requires using `HandleErrorLayer` which is inconvenient for axum users. That has been changed in tower (tower-rs/tower#637) but still has some issues (tower-rs/tower#665). Its also a breaking change so hasn't been released yet. In the meantime I figure we can provide our own thing in axum-extra, since we already have an `Either` type there.

this commit makes a noöp change to the `SwitchReady<A, B>` machinery provided by our `linkerd-stack` library. this commit is a small refactor that is intended to pave the way for an impending upgrade to tower v0.5, which notably includes breaking changes to the `tower::util::Either<A, B>` service. as of tower v0.5, by way of tower-rs/tower#637, the `Either<A, B>` service is no longer itself a `Future`. so, we can instead use the future provided by `futures`. for more information, see: * linkerd/linkerd2#8733 * #3504 * https://github.com/linkerd/linkerd2-proxy/pull/3504/files#r1988082658 * tower-rs/tower#637 Signed-off-by: katelyn martin <kate@buoyant.io>

in tower-rs#637, breaking changes were made to the `Either<A, B>` service. this commit adds documentation of these breaking changes to the changelog, so that users upgrading from 0.4 to 0.5 have record of what changed when, and why.

in #637, breaking changes were made to the `Either<A, B>` service. this commit adds documentation of these breaking changes to the changelog, so that users upgrading from 0.4 to 0.5 have record of what changed when, and why.

this commit updates our tower dependency from 0.4 to 0.5. note that this commit does not affect the `tower-service` and `tower-layer` crates, reëxported by `tower` itself. the `Service<T>` trait and the closely related `Layer<S>` trait have not been changed. the `tower` crate's utilities have changed in various ways, some of particular note for the linkerd2 proxy. see these items, excerpted from the tower changelog: - **retry**: **Breaking Change** `retry::Policy::retry` now accepts `&mut Req` and `&mut Res` instead of the previous mutable versions. This increases the flexibility of the retry policy. To update, update your method signature to include `mut` for both parameters. ([#584]) - **retry**: **Breaking Change** Change Policy to accept &mut self ([#681]) - **retry**: **Breaking Change** `Budget` is now a trait. This allows end-users to implement their own budget and bucket implementations. ([#703]) - **util**: **Breaking Change** `Either::A` and `Either::B` have been renamed `Either::Left` and `Either::Right`, respectively. ([#637]) - **util**: **Breaking Change** `Either` now requires its two services to have the same error type. ([#637]) - **util**: **Breaking Change** `Either` no longer implemenmts `Future`. ([#637]) - **buffer**: **Breaking Change** `Buffer<S, Request>` is now generic over `Buffer<Request, S::Future>.` ([#654]) see: * <tower-rs/tower#584> * <tower-rs/tower#681> * <tower-rs/tower#703> * <tower-rs/tower#637> * <tower-rs/tower#654> the `Either` trait bounds are particularly impactful for us. because this runs counter to how we treat errors (skewing towards boxed errors, in general), we temporarily vendor a version of `Either` from the 0.4 release, whose variants have been renamed to match the 0.5 interface. updating to box the inner `A` and `B` services' errors, so we satiate the new `A::Error = B::Error` bounds, can be addressed as a follow-on. that's intentionally left as a separate change, due to the net size of our patchset between this branch and #3504. * <tower-rs/tower@v0.4.x...master> * <https://github.com/tower-rs/tower/blob/master/tower/CHANGELOG.md> this work is based upon #3504. for more information, see: * linkerd/linkerd2#8733 * #3504 Signed-off-by: katelyn martin <kate@buoyant.io> X-Ref: tower-rs/tower#815 X-Ref: tower-rs/tower#817 X-Ref: tower-rs/tower#818 X-Ref: tower-rs/tower#819

this commit updates our tower dependency from 0.4 to 0.5. note that this commit does not affect the `tower-service` and `tower-layer` crates, reëxported by `tower` itself. the `Service<T>` trait and the closely related `Layer<S>` trait have not been changed. the `tower` crate's utilities have changed in various ways, some of particular note for the linkerd2 proxy. see these items, excerpted from the tower changelog: - **retry**: **Breaking Change** `retry::Policy::retry` now accepts `&mut Req` and `&mut Res` instead of the previous mutable versions. This increases the flexibility of the retry policy. To update, update your method signature to include `mut` for both parameters. ([tower-rs/tower#584]) - **retry**: **Breaking Change** Change Policy to accept &mut self ([tower-rs/tower#681]) - **retry**: **Breaking Change** `Budget` is now a trait. This allows end-users to implement their own budget and bucket implementations. ([tower-rs/tower#703]) - **util**: **Breaking Change** `Either::A` and `Either::B` have been renamed `Either::Left` and `Either::Right`, respectively. ([tower-rs/tower#637]) - **util**: **Breaking Change** `Either` now requires its two services to have the same error type. ([tower-rs/tower#637]) - **util**: **Breaking Change** `Either` no longer implemenmts `Future`. ([tower-rs/tower#637]) - **buffer**: **Breaking Change** `Buffer<S, Request>` is now generic over `Buffer<Request, S::Future>.` ([tower-rs/tower#654]) see: * <tower-rs/tower#584> * <tower-rs/tower#681> * <tower-rs/tower#703> * <tower-rs/tower#637> * <tower-rs/tower#654> the `Either` trait bounds are particularly impactful for us. because this runs counter to how we treat errors (skewing towards boxed errors, in general), we temporarily vendor a version of `Either` from the 0.4 release, whose variants have been renamed to match the 0.5 interface. updating to box the inner `A` and `B` services' errors, so we satiate the new `A::Error = B::Error` bounds, can be addressed as a follow-on. that's intentionally left as a separate change, due to the net size of our patchset between this branch and #3504. * <tower-rs/tower@v0.4.x...master> * <https://github.com/tower-rs/tower/blob/master/tower/CHANGELOG.md> this work is based upon #3504. for more information, see: * linkerd/linkerd2#8733 * #3504 Signed-off-by: katelyn martin <kate@buoyant.io> X-Ref: tower-rs/tower#815 X-Ref: tower-rs/tower#817 X-Ref: tower-rs/tower#818 X-Ref: tower-rs/tower#819

* chore(deps)!: upgrade to tower 0.5 this commit updates our tower dependency from 0.4 to 0.5. note that this commit does not affect the `tower-service` and `tower-layer` crates, reëxported by `tower` itself. the `Service<T>` trait and the closely related `Layer<S>` trait have not been changed. the `tower` crate's utilities have changed in various ways, some of particular note for the linkerd2 proxy. see these items, excerpted from the tower changelog: - **retry**: **Breaking Change** `retry::Policy::retry` now accepts `&mut Req` and `&mut Res` instead of the previous mutable versions. This increases the flexibility of the retry policy. To update, update your method signature to include `mut` for both parameters. ([tower-rs/tower#584]) - **retry**: **Breaking Change** Change Policy to accept &mut self ([tower-rs/tower#681]) - **retry**: **Breaking Change** `Budget` is now a trait. This allows end-users to implement their own budget and bucket implementations. ([tower-rs/tower#703]) - **util**: **Breaking Change** `Either::A` and `Either::B` have been renamed `Either::Left` and `Either::Right`, respectively. ([tower-rs/tower#637]) - **util**: **Breaking Change** `Either` now requires its two services to have the same error type. ([tower-rs/tower#637]) - **util**: **Breaking Change** `Either` no longer implemenmts `Future`. ([tower-rs/tower#637]) - **buffer**: **Breaking Change** `Buffer<S, Request>` is now generic over `Buffer<Request, S::Future>.` ([tower-rs/tower#654]) see: * <tower-rs/tower#584> * <tower-rs/tower#681> * <tower-rs/tower#703> * <tower-rs/tower#637> * <tower-rs/tower#654> the `Either` trait bounds are particularly impactful for us. because this runs counter to how we treat errors (skewing towards boxed errors, in general), we temporarily vendor a version of `Either` from the 0.4 release, whose variants have been renamed to match the 0.5 interface. updating to box the inner `A` and `B` services' errors, so we satiate the new `A::Error = B::Error` bounds, can be addressed as a follow-on. that's intentionally left as a separate change, due to the net size of our patchset between this branch and #3504. * <tower-rs/tower@v0.4.x...master> * <https://github.com/tower-rs/tower/blob/master/tower/CHANGELOG.md> this work is based upon #3504. for more information, see: * linkerd/linkerd2#8733 * #3504 Signed-off-by: katelyn martin <kate@buoyant.io> X-Ref: tower-rs/tower#815 X-Ref: tower-rs/tower#817 X-Ref: tower-rs/tower#818 X-Ref: tower-rs/tower#819 * fix(stack/loadshed): update test affected by tower-rs/tower#635 this commit updates a test that was affected by breaking changes in tower's `Buffer` middleware. see this excerpt from the description of that change: > I had to change some of the integration tests slightly as part of this > change. This is because the buffer implementation using semaphore > permits is _very subtly_ different from one using a bounded channel. In > the `Semaphore`-based implementation, a semaphore permit is stored in > the `Message` struct sent over the channel. This is so that the capacity > is used as long as the message is in flight. However, when the worker > task is processing a message that's been recieved from the channel, > the permit is still not dropped. Essentially, the one message actively > held by the worker task _also_ occupies one "slot" of capacity, so the > actual channel capacity is one less than the value passed to the > constructor, _once the first request has been sent to the worker_. The > bounded MPSC changed this behavior so that capacity is only occupied > while a request is actually in the channel, which broke some tests > that relied on the old (and technically wrong) behavior. bear particular attention to this: > The bounded MPSC changed this behavior so that capacity is only > occupied while a request is actually in the channel, which broke some > tests that relied on the old (and technically wrong) behavior. that pr adds an additional message to the channel in tests exercising the laod-shedding behavior, on account of the previous (incorrect) behavior. https://github.com/tower-rs/tower/pull/635/files#r797108274 this commit performs the same change for our corresponding test, adding an additional `ready()` call before we hit the buffer's limit. Signed-off-by: katelyn martin <kate@buoyant.io> * review: use vendored `Either` for consistency #3744 (comment) Signed-off-by: katelyn martin <kate@buoyant.io> --------- Signed-off-by: katelyn martin <kate@buoyant.io>

util: rework Either

44bf116

davidpdrsn added the A-util Area: The tower "util" module label Feb 8, 2022

A/B -> Left/Right

35eeb24

jplatte approved these changes Feb 8, 2022

View reviewed changes

olix0r added this to the 0.5 milestone Feb 8, 2022

hawkw approved these changes Feb 8, 2022

View reviewed changes

tower/src/util/either.rs Show resolved Hide resolved

tower/src/util/either.rs Show resolved Hide resolved

tower/src/util/either.rs Show resolved Hide resolved

Update tower/src/util/either.rs

b464abb

Co-authored-by: Eliza Weisman <eliza@buoyant.io>

davidpdrsn and others added 2 commits February 9, 2022 09:16

Revert "Update tower/src/util/either.rs"

0b01824

This reverts commit b464abb.

Merge branch 'master' into rework-either

5217e98

hawkw enabled auto-merge (squash) February 17, 2022 20:04

Merge branch 'master' into rework-either

9a72f6f

hawkw merged commit 522687c into master Feb 17, 2022

hawkw deleted the rework-either branch February 17, 2022 20:09

davidpdrsn mentioned this pull request Jun 12, 2022

Either rework broke option_layer #665

Open

davidpdrsn mentioned this pull request Jan 14, 2023

Add option_layer tokio-rs/axum#1696

Merged

hlbarber mentioned this pull request Jun 19, 2023

Remove our custom Either smithy-lang/smithy-rs#2787

Open

markdingram mentioned this pull request Sep 24, 2024

Upgrades Tower to 0.5.1 kube-rs/kube#1589

Merged

cratelyn mentioned this pull request Mar 11, 2025

chore(deps)!: upgrade to hyper 1.x linkerd/linkerd2-proxy#3504

Merged

cratelyn mentioned this pull request Mar 11, 2025

refactor(stack/switch_ready): use Either<A, B> future linkerd/linkerd2-proxy#3739

Merged

This was referenced Mar 11, 2025

chore: add breaking changes to changelog cratelyn/tower#1

Closed

chore: add breaking changes to changelog #815

Merged

cratelyn mentioned this pull request Mar 14, 2025

chore(deps): upgrade to tower 0.5 linkerd/linkerd2-proxy#3744

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

util: rework `Either` #637

util: rework `Either` #637

Uh oh!

davidpdrsn commented Feb 8, 2022 •

edited

Loading

Uh oh!

jplatte commented Feb 8, 2022

Uh oh!

jplatte commented Feb 8, 2022

Uh oh!

davidpdrsn commented Feb 8, 2022

Uh oh!

hawkw commented Feb 8, 2022

Uh oh!

davidpdrsn commented Feb 8, 2022 •

edited

Loading

Uh oh!

hawkw commented Feb 8, 2022

Uh oh!

hawkw left a comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

olix0r commented Feb 8, 2022

Uh oh!

Uh oh!

util: rework Either #637

util: rework Either #637

Uh oh!

Conversation

davidpdrsn commented Feb 8, 2022 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

jplatte commented Feb 8, 2022

Uh oh!

jplatte commented Feb 8, 2022

Uh oh!

davidpdrsn commented Feb 8, 2022

Uh oh!

hawkw commented Feb 8, 2022

Uh oh!

davidpdrsn commented Feb 8, 2022 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

hawkw commented Feb 8, 2022

Uh oh!

hawkw left a comment

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Uh oh!

Uh oh!

olix0r commented Feb 8, 2022

Uh oh!

Uh oh!

util: rework `Either` #637

util: rework `Either` #637

davidpdrsn commented Feb 8, 2022 •

edited

Loading

davidpdrsn commented Feb 8, 2022 •

edited

Loading