Re: [blink-dev] Intent to Deprecate: Deprecate unload event

[Fergal Daly]

[CC: bfcache-dev, BCC: blink-dev and others we can discuss this and get back with a summary, let me know if you want a CC ]

On Tue, 11 Jul 2023 at 16:44, Robert Knight <robert...@gmail.com> wrote:

Hello,

Hypothesis (https://web.hypothes.is, a web page/PDF/ebook annotation tool) uses the "unload" event to signal to one end of a message channel when the other end is in a frame that is about to go away. This is a workaround for the lack of a "close" event in the Channel Messaging API (https://github.com/whatwg/html/issues/1766). If unload events are going to be removed from the web platform, it would be useful to have a proper solution for detecting when a MessagePort becomes disconnected ("disentangled").

I've spent some time digging into that "close" event. It does seem like a clear hole in the platform but there are a lot of details to get right. Fixing it may be an option but I don't know how quickly that could be done.

I would also point out that `unload` is already pretty broken on mobile (and desktop Safari). If the page can enter BFCache it will not fire unload. This is the currently specced behaviour and the current behaviour on everything except Chrome Desktop and Mozilla desktop (and derivatives). So even if we were not deprecating unload, I think you are in need of a work-around (but we are definitely making that need more intense).

I see that some people are using web locks as an alternative. Does that work for you? It only works if communication is same-origin. Channels are intended to work cross-origin, so it's a poor work-around.

Could you give us some more detail on the structure of this communication? How do you end up with a document that has a different life-cycle to the document in question? Is it a long-lived hypothesis window that reacts to sites appearing/disappearing that support Hypothesis?

F

 

Kind Regards,

Robert Knight

On Monday, 10 July 2023 at 08:13:49 UTC+1 Yoav Weiss wrote:

Thanks for chiming in, Brandon!

I'm glad to hear that the Enterprise constituency is comfortable with the plan.

I'm concerned that there may be a couple other constituencies that may not be:

• Third party widgets that currently use unload to send a single "end of page" beacon. fetchLater() is aiming to be that replacement, but it's not ready just yet.
• Enterprise SAAS providers that don't have direct and immediate control over their customers' application configuration, nor on their users' Enterprise Policy.

I think that a short-lived 3P deprecation trial may address these constituencies as well. Would you consider adding that to your plans?

On Sat, Jul 8, 2023 at 12:55 AM 'Brandon Heenan' via blink-dev <blin...@chromium.org> wrote:

Hello, I'm chiming in to provide some thoughts from the enterprise perspective.

Our goal is to not block forward progress to the web, but to improve the web in an enterprise-friendly way. You shouldn't ever hear me say "you can't do X because it's scary to the enterprise team." You should instead hear "We expect X to be risky, but here are the things we know we can do to make it much less risky."

In this case, yes, this is risky for enterprises. We can say this with confidence because we've seen escalations before when we've made changes to unload events (crbug.com/933153,  crbug.com/953228).

Kenji and Daisuke have been working with us, and my understanding of the plan is to:

• Allow developers to opt-in early to the new behavior (unload event ignored) with a permission policy

• Communicate the change on chromestatus and the enterprise release notes (already happening). We will provide a bug link for customers for feedback in a future release.

• Reach out to enterprises and developers we expect to be affected

• Introduce an enterprise policy to allow an IT admin to control unload event behavior

• Introduce a flag in chrome://flags/deprecated to allow end users to control unload event behavior

• As early as M117, change the default for the policy so that unload events will be ignored. This is the breaking change, and there's likely to be friction here. The two escalations mentioned above both resulted in respins the first time they reached this point. However, this time around, IT admins will be able to fix their environment immediately with the enterprise policy, end users will be able to fix themselves with the deprecation flag, and developers will be able to fix their app with the permission policy. With those mitigations in place, the risk of requiring a respin (or Finch rollback) due to enterprise impact is dramatically reduced, and this is how we eventually successfully shipped both of those above escalations.

• We expect a long transition period after that. By default, the unload event is ignored, but different stakeholders are able to revert to legacy behavior. Within enterprise, we expect the enterprise policy to be the most useful mitigation, and the deprecation flag is the backup for BYOD or unmanaged devices. For the above escalations, this migration period was over a year, and I'm expecting something similar this time.

• At some point in the future, we expect to remove those mitigations and remove support for the unload event completely. We don't have any specific dates for that yet; we will be responsive to the needs of web stakeholders, enterprise and otherwise.

The two escalations I mentioned above were successfully resolved and the changes to not allow popups on page unload and to not allow synchronous XHRs on page unload were shipped. Both of those changes followed essentially the same plan I just laid out above, and so I think it's reasonable to do the same thing here.

On Thursday, June 29, 2023 at 7:02:06 AM UTC-7 Rick Byers wrote:

On Thu, Jun 29, 2023 at 1:47 AM Kenji Baheux <kenji...@google.com> wrote:

On Thu, Jun 29, 2023 at 1:48 PM Fergal Daly <fer...@google.com> wrote:

On Thu, 29 Jun 2023 at 01:16, Rick Byers <rby...@chromium.org> wrote:

Hi Fergal,

Thanks for pushing through this contentious and challenging deprecation. We discussed this in the API owners meeting today and were worried that this plan seemed likely to be seriously problematic for enterprises (policy opt-out is helpful, but far from a silver bullet unfortunately). To what extent have you engaged with them and worked to follow the enterprise breaking change policy? Our hunch is that at 1% or 5% we'd get escalations forcing us to abandon this plan. Of course, if the enterprise team is OK with it, we could always try anyway and see if our hunch is right. It's possible I'm over-indexing on past experiences like deprecating sync XHR in unload handlers and that the enterprise world is different now, but I doubt it :-).

In addition to Daisuke's response... are you concerned about enterprises that are not using fleet management and so cannot use the opt-out? If you think an enterprise policy will not be sufficient, a mitigation for those enterprises would be for us to publish an extension that allows anyone to re-enable unload (for all sites or for specific sites) by injecting the PP:unload header. Are the escalations that can't be resolved by either a policy or extension?

One extra comment on the extension option (great for desktop).

If you wonder about the mobile BYOD scenarios, where extensions don't exist, then we are a bit lucky here because unload is already unreliable on mobile. So, it seems extremely unlikely that we'd see mobile enterprise/edu products that rely on unload on mobile.

Rick: are there specific scenarios / environments that we haven't covered?

I'm glad to see the conversation with the enterprise team is further along than I had realized. Having skip unload events in the release notes since M113 is a significant mitigation, sorry I wasn't caught up on the latest. And yes some sort of user opt-out for BYOD (extension or chrome::/flags, etc.) seems like an essential mitigation. I defer to the enterprise team's judgement here, so if they're OK with proceeding then we shouldn't let my enterprise fears block us. I expect we do need some easy way for an application to signal that it really does need unload handlers. Setting a permission policy is likely orders of magnitude easier than converting essential unload handlers to pagehide and ensuring they're safe to invoke multiple times.

The other major constituency potentially impacted are ad networks. Perhaps the next step should be a 1% finch trial where we can measure various ad-related metrics? I'd defer to the judgment of the Chrome Ads team (@Josh Karlin).

Anyway, I'm personally OK with 1% stable experiments (and whatever else on dev/beta). But I think we should discussing learnings from such 1% experiments here publicly before approving a plan to go beyond that.

In general Yoav and I disagree with the WebKit and Gecko feedback here and suspect that your original PP default-on proposal is far more likely to be a successful deprecation path for Chrome (and, should they choose to follow, Edge). I can understand why Firefox and WebKit don't have the same constraints around enterprises and so would choose differently for themselves. Yoav and I are happy to help in the standards discussions. I'm about to go on vacation for 2 weeks but Yoav said he'd follow up with you privately to brainstorm next steps. Sound good?

I would love to get moving on PP:unload ASAP no matter what. It's been through OT and is sitting behind a flag with some sites eager to use it. I'm happy to send an I2S for that while we discuss the harder problem. We hope that getting that out there can clear out a large chunk of the 1st- and 3rd-party unload usage,

+1, I'd suggest doing that regardless.

There are a few large sites that have done some legwork on unload handlers (theirs and third party partners), and are interested in pushing the remaining unload handlers out with PP:unload. Having allies in the ecosystem (i.e. extra incentives to migrate), will be helpful going forward :)

Yep I think this was Yoav and my primary concern. For chrome to have a pragmatic and reasonable deprecation path given our user base, we really need sites adopting such an API. If we're not going to actually ship such an API then I think we'd have to give up on deprecating unload. I'd support shipping this API despite the lack of support from WebKit and Gecko. 

 

F

 

Rick

On Wed, Jun 28, 2023 at 4:07 AM Fergal Daly <fer...@google.com> wrote:

Hi API-owners,

I am now asking for permission to go ahead with the following concrete unload deprecation plan below.

• Tools and outreach

  • M115 Enable `Permission-Policy: unload` (PP:unload) with the default being enabled. This allows sites to opt-in to unload deprecation.

  • Outreach to 1st/3rd parties, to migrate away from using unload and to enforce this with PP:unload.

• Deprecation

  • M117 change the default for PP:unload so that unload handlers are skipped by default for 1% of page loads

  • M118 increase to 5% of page loads

  • M119 (last of 2023) increase to 10% of page loads

  • Evaluate progress on reduction of the use of unload

  • M120-128 increase +10% gradually to 100% of page loads

Enterprise policy would allow opt-out entirely.

Obviously, the deprecation timeline is contingent on unload usage coming down in response to the earlier steps.

We expect that 10% of page loads will provide a noticeable signal to sites that use unload. Also, if we were to just follow the current spec and not run unload when we can BFCache (as happens on Clank/Firefox mobile and all WebKit) we expect that we would skip 30-40% of unload handlers when the main frame navigates.

Decisions:

• Timeline

• All navigations vs main-frame navigations only

Standardising

We have some new data and have had some further discussions with browser vendors. There's no consensus. TL;DR WebKit are opposed to any Permissions-Policy but support removing unload eventually. Mozilla are still discussing.

Both Mozilla and WebKit were opposed to standardising `Permissions-Policy: unload` (defaulting to on) because they worried that a containing frame might selectively disable unload handlers in a child frame for malicious purposes (no specific cases were discussed).

So we flipped to the idea of having PP:unload with the default being disabled. We cannot suddenly do that. We need to roll it out gradually. WebKit folks are opposed to this and have suggested we do a reverse origin trial instead. If our plan works out, eventually we would ROT as the final nail but ROT starting now has downsides for users and sites and no upside for the implementer.

Mozilla has so far not been negative on the Permissions-Policy off-by-default approach but they are still discussing. They are concerned that disabling unloads when subframes are navigating could be a problem. We found that about 1/4 of subframe navigations involve an `unload` handler (most seem to involve handlers in cross-site and same-site site frames). We don't have examples of sites that rely on `unload` handlers in this way, although they probably do exist. Migrating to `pageshow` or using PP:unload for these sites should be trivial.

We have the option to say that PP:unload only applies to main frame navigations. This would mean these sites would be completely unaffected however that has some downsides. It is harder to explain and does not end with full removal of `unload`. We would prefer to have this apply to all navigations unless we find a good reason not to. If we were to change part-way, there would be no breakage. We hope that once we drive down usage in 3rd-part iframes with PP:unload that the number of unload handlers running in subframe navigations decreases significantly.

Finally there was some discussion about how Permissions-Policy off-by-default should work. Our current version requires every page to set the header and every parent to set the iframe `allow` attribute. This is maximally conservative. If at some point later on there is agreement to standardise on something less conservative, it will not break pages that have already re-enabled `unload`.

Overall it seems hard to standardise in advance but if we succeed in driving down `unload` usage, other browsers are on-board with removing unload. The worst case scenario would be where we implement PP:unload (which the others do not agree with) but make no noticeable progress on `unload` usage. If that happens we can just go with the currently specced behaviour (don't run `unload` if BFCaching is possible) and maybe revert the PP:unload,

F

On Tue, 9 May 2023 at 16:01, Fergal Daly <fer...@google.com> wrote:

On Mon, 8 May 2023 at 17:51, Rick Byers <rby...@chromium.org> wrote:

Hi Fergal,

It's exciting to see this moving forward! Just to clarify, this is effectively an I2S for the unload permissions-policy, is that right? Or are you also requesting permission to stop firing unload events now too?  The latter is going to require some significant compat analysis, but could be greatly informed by the experience of having some top-level sites opt-out of unload for their frame tree.

Thanks.

We're not requesting permission to stop firing at this point. It is the far-away end-point. 

 

Any plan to trigger a deprecation warning / report for the installation of unload handlers? It might be tricky to find a good balance of useful warnings without being too spammy.

Permission policy will do this as is with a console warning and Reporting-API if you attempt to install a handler that is disallowed by policy.

 

A couple more questions / comments inline:

On Mon, May 8, 2023 at 7:43 AM Fergal Daly <fer...@chromium.org> wrote:

Contact emails

fer...@chromium.org, kenji...@chromium.org

Explainer

https://github.com/fergald/docs/blob/master/explainers/permissions-policy-deprecate-unload.md

Specification

https://github.com/whatwg/html/pull/7915

This is still marked as draft. Can you get this ready for review? If it's blocked only on having a 2nd implementor show support, then I'd be fine shipping based on a PR. But we should at least do what we can to solicit feedback on the spec change prior to shipping.

Yes. There's nothing in the spec change that isn't in the requests for positions but since neither of those are supportive yet, I have not asked for review of the PR. I'm hopeful that once we have data on use on unload in subframe navigations as discussed here that Mozilla will be supportive. Those metrics are in 113 but based on the data from beta, we need to change how we record them.

Summary

A Permission-Policy for creating unload event listeners will be added. 

Initially, the default policy will be set to allow. From there, Chrome will gradually migrate the default policy to deny (i.e. increasingly disallow the creation of unload event listeners, eventually reaching a state where deny fully becomes the default policy). The ultimate goal is to remove support for unload event.

Blink component

Blink>PermissionsAPI

Motivation

The unload event is extremely unreliable. It is ignored in most cases by all mobile browsers except Firefox on Android. Furthermore, in Safari, the unload event is ignored on both desktop & mobile platforms.

In the current state, unload is a major BFCache blocker (~18 percentage points reduction of hit rate for Chrome).

The change  will unlock a large fraction of that hit-rate while providing an opt-out for those who need more time to migrate. It also sends a clear signal that unload should not be used in new development.

Sidenote: the spec was changed to say that unload should only run if the page cannot enter BFCache, which reflects Safari’s behavior, However neither Chrome nor Mozilla have implemented this behavior. In Chrome's case, we believe that this would suddenly break various sites and would make it hard for developers to know if/when unload may run.

Initial public proposal

None

TAG review

https://github.com/w3ctag/design-reviews/issues/738

TAG review status

Pending

Risks

Interoperability and Compatibility

If no other browsers implement this, there is a risk that devs continue to use unload widely and pages malfunction on chrome. However given that alternatives to unload exist it seems entirely possible for sites that are actively maintained to move off unload.

Gecko: (https://github.com/mozilla/standards-positions/issues/691#issuecomment-1484997320) It's possible that pages are depending on `unload` handlers in subframes for functionality even without any main frame navigation. We should try to understand how common this is before breaking it. It should be possible to measure how often subframe unloads fire when the mainframe is not navigating. This will give us an upper bound on the size of the problem, - Chrome: we have landed code to measure the occurrence of unload in different scenarios. We will report back the findings.

WebKit: https://github.com/WebKit/standards-positions/issues/127

From a quick skim, it sounds like WebKit is already happy with their tradeoff of not firing unload and doesn't see a need for an API that reduces unload further, is that about right? WebKit has mostly shipped heuristics here without trying to spec them first, right? In general I'm not too concerned 

Yes, there's no great upside for them. I believe the situation as specced where unload is unpredictable and likely biased is bad for devs and is probably skewing data collected via WebKit (and Chrome/Mozilla mobile) but nobody is complaining.

I believe there was support expressed offline for the prospect of killing off unload.

 

Web developers: Positive (https://groups.google.com/a/chromium.org/g/bfcache-dev/c/zTIMx7u4uxo/m/-M4IS6LDBgAJ) The web communities we reached out had positive reactions to our proposal and we have not heard about any concrete blockers.

Other signals:

WebView application risks

Does this intent deprecate or change behavior of existing APIs, such that it has potentially high risk for Android WebView-based applications?

On WebView, we will introduce the Permissions-Policy but not move the default to "deny". BFCache does not work on WebView, so the benefit is lower. Meanwhile the risk seems higher as we have far less visibility into the HTML being run in WebViews. A roll-out to WebView should be done independently and in consultation with the WebView team.

Sounds like the right strategy to me, thanks! 

Debuggability

None

Is this feature fully tested by web-platform-tests?

Yes

Flag name

None

Please put the new policy behind a RuntimeEnabledFeature. It's effectively a new API so is required to have a finch killswitch. It sounds to me like it should be unlikely that simply adding the new policy could break things, but maybe some scenario is possible where we decide breakage in 3p iframes is bad enough to warrant an emergency fix?

Yes, there will be a flag, maybe more than one. The implementation details of rolling this out gradually have not been worked out. See below.

 

Requires code in //chrome?

False

Estimated milestones

M115 for availability of Permissions-Policy

M115 is the earliest we would start to disable unload, however

Is this a typo? Or are you considering disabling the event in the same release we first make the permissions policy available?

The plan is to make the PP available with a default of enabled and then gradually flip the default to disabled. The details are here. It's not particularly nice. We have the option to just stop 100% but that seems fairly disruptive,

F

 

Link to entry on the Chrome Platform Status

https://chromestatus.com/feature/5579556305502208

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[Robert Knight]

Hi Fergal,

In the general case the frames involved in communication may be cross origin, though I think we could find workarounds for the most common use cases, possibly using Web Locks.

> Could you give us some more detail on the structure of this communication? How do you end up with a document that has a different life-cycle to the document in question?

> Is it a long-lived hypothesis window that reacts to sites appearing/disappearing that support Hypothesis?

Essentially, yes. The environment consists of a "host" frame where Hypothesis is initially loaded, a long-lived "sidebar" frame that displays the user's annotations, and shorter-lived "guest" frames that contain content that can be annotated. In the simple case, there is one guest frame which is also the host frame. The sidebar is always cross-origin. The host and guest frames may be cross origin, though they are currently same origin in the use cases we care about most.

The typical use case would be an ebook reader that has a container frame (the host) with book navigation controls, and a child frame (the guest) that contains the content for the current chapter, which gets swapped out each time the user navigates from one chapter to another. 

Kind Regards,

Robert.

[Robert Knight]

> Robert, in your case could you use the permission policy to re-enable unload events on the frame until we come up with a better fix? Or are there scenarios where you lack the ability to add "allow=" attributes to the iframe elements?

If I understand correctly, this attribute has to be set before the frames load?

[Fergal Daly]

[BCC blink-dev, we can add them back when we have something concrete]

On Fri, 28 Jul 2023 at 16:42, Robert Knight <robert...@gmail.com> wrote:

> Robert, in your case could you use the permission policy to re-enable unload events on the frame until we come up with a better fix? Or are there scenarios where you lack the ability to add "allow=" attributes to the iframe elements?

If I understand correctly, this attribute has to be set before the frames load?

Yes, the allow attribute needs to be on the frame from the start.

You will need to set the Permission-Policy on the HTTP response for your frame and its ancestors and you will need to set the allow attribute on all of the iframes up to the top-level frame. That is probably not easy.

I have had some discussions with people familiar with MessagePort in Chrome. My next step is to see if we can get some movement on that github issue to get a more general solution. It does seem to me that MessageChannel has a fundamental conflict between resource cleanup and privacy that cannot ever be resolve but it should be possible to solve it for the case where the recipient of the port wants to co-operate.

Can you give a bit more detail on your specific setup?

- Is there a general hypothesis site that you can put any guest into or do you expect sites to supply the outer frame and the guest frame but running your script in both?

- Is there another case where hypothesis runs in a separate window or is it always a single window with guest frames?

F

[Robert Knight]

- Is there a general hypothesis site that you can put any guest into or do you expect sites to supply the outer frame and the guest frame but running your script in both?

The website will supply both the outer ("host") frame (eg. the book navigation UI) and the guest frame (eg. book chapter content) and our code will be loaded into both.

- Is there another case where hypothesis runs in a separate window or is it always a single window with guest frames?

It is always a single window, and the frames communicating with each other have either a direct parent-child or sibling relationship.

Kind regards,

Robert.

[Fergal Daly]

On Thu, 10 Aug 2023 at 21:35, Robert Knight <robert...@gmail.com> wrote:

- Is there a general hypothesis site that you can put any guest into or do you expect sites to supply the outer frame and the guest frame but running your script in both?

The website will supply both the outer ("host") frame (eg. the book navigation UI) and the guest frame (eg. book chapter content) and our code will be loaded into both.

So if you really wanted to have a timely signal, you could use pagehide. In a subframe, pagehide == unload because subframes never go into BFCache.

 

- Is there another case where hypothesis runs in a separate window or is it always a single window with guest frames?

It is always a single window, and the frames communicating with each other have either a direct parent-child or sibling relationship.

Some good news. I've played around a bit more with MessageChannel and I think there is a solution.

In this demo, I only hold `WeakRef`s to the objects. I pass the port to a frame then destroy the frame. When GC occurs, the WeakRef is empty.

So while there's no timely signal, you can do resource management by occasionally scanning for WeakRefs that have become empty.

Also given that, I think the privacy argument against onclose is pretty weak. I will ping that github thread as soon as I hear something from the MessageChannel folks,

F

[Fergal Daly]

I have updated the explainer to document a way of using of MessageChannel that works regardless of unload etc.

I also wrote a draft explainer that I hope captures the issues and presents a good argument for why the simple solution (`close` event) would not really expose anything that isn't already exposed.

Robert, please confirm that the strategies given in these docs fulfill your requirements. Obviously, a `close` event is ergonomic but I would like to confirm that `pagehide` is equivalent to `unload` for your use-case.

F

[Fergal Daly]

Robert, do the pagehide and WeakRef solutions below work for your use case? It seems like adding the WeakRef is desirable even while using unload since unload itself is not 100% reliable given crashes and timeouts,

F

[Robert Knight]

> Robert, do the pagehide and WeakRef solutions below work for your use case? It seems like adding the WeakRef is
> desirable even while using unload since unload itself is not 100% reliable given crashes and timeouts,

The MDN docs state that "pagehide" might not be reliably fired on
mobile and it has the same reliability issues as "unload". I guess
that is no worse than our current usage of "unload"? This would be
quite easy to adopt at least.

For WeakRef, I presume we'd have no guarantees over when the GC
occurs, only that it will happen eventually?

For our needs I think we can find a workable solution from the toolbox
of suggestions here. A "close" event for message channels would allow
for cleaner / more straightforward code though, so it would still be
welcome if it can be implemented in future.

Kind regards,
Robert.

[Fergal Daly]

On Mon, 4 Sept 2023 at 20:31, Robert Knight <robert...@gmail.com> wrote:

> Robert, do the pagehide and WeakRef solutions below work for your use case? It seems like adding the WeakRef is
> desirable even while using unload since unload itself is not 100% reliable given crashes and timeouts,

The MDN docs state that "pagehide" might not be reliably fired on
mobile and it has the same reliability issues as "unload". I guess
that is no worse than our current usage of "unload"? This would be
quite easy to adopt at least.

Pagehide will fire more reliably than unload (since on mobile and on all Safari, unload handlers are ignored if the page can be BFCached). However it's kind of irrelevant in the sense that the cases where pagehide doesn't fire are where the renderer(s) or even the whole browser has been killed off by something external to the browser. So there's no need to think about freeing resources.

 

For WeakRef, I presume we'd have no guarantees over when the GC
occurs, only that it will happen eventually?

Yes, although "eventually" hides a lot of detail. I have very little knowledge in this area and it could vary vastly from browser to browser but I can imagine that a page that is just sitting there with no activity may go indefinitely without performing GC.

For our needs I think we can find a workable solution from the toolbox
of suggestions here. A "close" event for message channels would allow
for cleaner / more straightforward code though, so it would still be
welcome if it can be implemented in future.

Thanks.

I will post my explainer to that github issue but I don't know how it will be received. Even if there is agreement, the implementation will probably be low priority,

F