Clarification on Accept-CH Lifetime about client-hints-infrastructure

[Tanych]

@nicjansma initially client hints only persist on browsing session level, however, starting from M103, we made the client hints persisted even browser restarts. we probably need to clean up some documents to avoid the confusion.

context: https://bugs.chromium.org/p/chromium/issues/detail?id=1296339

I tested the latest chrome canary on Android and mac os:

• 112.0.5579.0 on mac os. has no issue when restart.
• 112.0.5579.0 on Android (pixel 5) has no issue when restart.

I will try to verify any issues in Windows 11 as well.

[Tanych]

I'm also not seeing any unexpected behavior in the above scenarios, another note is that you won't see client hints change in the first request header after you change the accept_ch list in the response header because client is unaware of the server’s most recent preferences. All sub-requests will notice the accept_ch header changes.
You can use critical_ch, If corresponding client hints headers didn't send in the original request, the user agent will retry the request along with the client hints headers rather than render the page.

[arichiv]

It looks like the client hints cache should persist between browser sessions: https://source.chromium.org/chromium/chromium/src/+/main:components/content_settings/core/browser/website_settings_registry.cc;l=179;drc=f775d850d7f5beebd9308f5d02111d6069083b28;bpv=0;bpt=1

However the persistence of the cache is marked as lossy so there may be some chance it's not being written? Although I would assume all lossy caches are at least written on normal browser close.

It seems like that comment that "cache is cleared when session cookies are cleared" is false, as it should persist unless all cookies are cleared manually.

Can you detail how you tested the persistence?

@yoavweiss if I'm missing something

[miketaylr]

It seems like that comment that "cache is cleared when session cookies are cleared" is false, as it should persist unless all cookies are cleared manually.

Yeah, that should just say "cookies and other site data", right?

[nicjansma]

I was just using user-agent-client-hints.glitch.me for testing, i.e. asking for Sec-CH-UA-Platform-Version

Browser Restart Scenario:

1. Nav to https://user-agent-client-hints.glitch.me/headers?uach=Sec-CH-UA-Platform-Version
   • Client requests with default hints, i.e. w/out Sec-CH-UA-Platform-Version
   • Server sends Accept-CH: Sec-CH-UA-Platform-Version
2. Reload of the same page
   • Client requests with Sec-CH-UA-Platform-Version
   • Server sends Accept-CH: Sec-CH-UA-Platform-Version
3. Close browser
4. Open Browser
5. Nav to https://user-agent-client-hints.glitch.me/headers?uach=Sec-CH-UA-Platform-Version
   • Client requests with default hints, i.e. w/out Sec-CH-UA-Platform-Version <-- this shows browse restarts don't persist Accept-CH
   • Server sends Accept-CH: Sec-CH-UA-Platform-Version
6. Reload of the same page
   • Client requests with Sec-CH-UA-Platform-Version
   • Server sends Accept-CH: Sec-CH-UA-Platform-Version

[miketaylr]

Thanks @nicjansma - I filed https://bugs.chromium.org/p/chromium/issues/detail?id=1413216 to investigate.

[arichiv]

I'm not seeing this reproduce locally. If I load and refresh https://user-agent-client-hints.glitch.me/headers?uach=Sec-CH-UA-Platform-Version so that the hint is included it works. If I open a new window or tab and load the page the hint appears without needing a reload. If I close all tabs and windows and re-open the browser and load https://user-agent-client-hints.glitch.me/headers?uach=Sec-CH-UA-Platform-Version it's still there without a refresh.

[nicjansma]

Ok, I've done some additional testing:

• On macOS Monterey / 111.0.5554.0, Accept-CH hints are persisted between restarting Chrome Canary
• On Windows 11 / 112.0.5576.0, Accept-CH hints are not persisted between restarting Chrome Canary

So maybe this is just a Windows bug? I haven't tested Android yet either.

(I'm not using an In Private session or anything to test, just closing and re-opening a vanilla Canary browser).

Back to my main question though: If I'm understanding the comments above correctly, the intent is for Accept-CH hints to persist not only through the browsing session (which I think people generally understand to be until the browser closes), but across browser restarts as well, correct?

If so, when I'm reading documentation explaining hints ([1] [2]) etc, that talk about hint lifetimes persisting "for the browsing session" are accurate, but not complete, and could improve clarity by saying something like "for the browsing session and persisted (through browser restarts) until the site's data is cleared", or something.

i.e. MDN even says:

In other words, the request for a specific set of hints does not expire until the browser is shut down

Happy to file tickets or make these changes for those docsets to clarify, just making sure I understand the intent :)

[Tanych]

The latest canary should fix the persisted client hints issue on Windows.

[nicjansma]

@Tanych that's great! I'll followup with trying to get those other docsets updated. I'll probably open a PR for this repo too to add that clarification in the explainer, if that's OK.