Re: fetch API and filesystem: URLs (a Manifest v3 migration blocker)

[Nigel Tao]

Sending again (this time hopefully to chromium-extensions@).

On Fri, Jul 5, 2024 at 5:14 PM Nigel Tao <nige...@chromium.org> wrote:
>
> Hi third_party/blink/renderer/core/fetch/OWNERS (nidhijaju, ricea),
> CC'ing blink-dev and chromium-extensions. Feel free to CC other people
> or lists if they're relevant.
>
> How hard is it to add filesystem: URL suport to the fetch API? I have
> worked with chromium code for some years now, but haven't really
> touched blink code generally (and fetch code specifically). For
> example, fetch is "a blink thing" and I don't know how problematic
> it'll be that Chromium's storage/browser/file_system code lives
> outside of third_party/blink.
>
> If it's hard, are there any workarounds to get my XMLHttpRequest-using
> extension on to Manifest v3? AFAICT this is a hard blocker for me to
> get off Manifest v2.
>
> ---
>
> Background:
>
> I work on ChromiumOS' "file manager" app, which uses an "image_loader"
> helper extension. This has been MV2 (Manifest v2) for some years now,
> and works great.
> https://source.chromium.org/chromium/chromium/src/+/main:ui/file_manager/image_loader/manifest.json;l=8;drc=c2aae80e18cbdadde0a3565fa8d5635f2ad378ae
>
> I'm trying to migrate it to MV3 (Manifest v3). This apprently requires
> migrating from XHR (XMLHttpRequest) to fetch.
> https://developer.chrome.com/docs/extensions/develop/migrate/to-service-workers#replace-xmlhttprequest
>
> https://crrev.com/c/5679823 attempts to do just that (assuming that
> fetch should work with either MV2 or MV3). But it doesn't.
>
> ChromiumOS' file manager works with real (kernel-visible) files (e.g.
> your local downloads) but also virtual files (e.g. cloud storage like
> Google Drive or Microsoft OneDrive, or "connect your phone to your
> Chromebook via USB cable and see your files over the MTP, uh,
> protocol"). While "filesystem:etc" URLs might have been created 10+
> years ago as a web-facing API, the storage::FileSystemURL class
> currently powers the ChromiumOS file manager too.
> https://source.chromium.org/chromium/chromium/src/+/main:storage/browser/file_system/file_system_url.h;l=83-84;drc=71bd8f1596b301a842247d3488f901d7ae3dfad2
>
> filesystem: URLs work with XHR. They do not work with fetch.
>
> When manually testing the https://crrev.com/c/5679823 change, I get an
> error logged:
>
> "Fetch API cannot load filesystem:chrome-extension://etc/etc. URL
> scheme "filesystem" is not supported."
>
> This comes from here:
> https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/fetch/fetch_manager.cc;l=926-928;drc=6c848b551a600625f8b361d1eaad4f8e94d1e40a
>
> It's triggered by passing "RendererCorsIssueCode::kCorsDisabledScheme"
> here (although I don't think CORS is actually involved; the error code
> is a placeholder):
> https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/fetch/fetch_manager.cc;l=907-910;drc=6c848b551a600625f8b361d1eaad4f8e94d1e40a
>
> In fact, right next to that trigger is "// FIXME: implement other
> protocols". The FIXME dates from 2014:
> https://codereview.chromium.org/399543002/diff/370001/Source/modules/serviceworkers/FetchManager.cpp
>
> I'm guessing that, if it hasn't been fixed in 10 years, it's probably
> not going to get fixed.
>
> If it's not going to get fixed, is there a workaround?

[Oliver Dunk]

Hi Nigel,

This chromium-extensions list is a public one mostly used for announcements and questions from external developers. Unfortunately, it's not the best place to ask implementation questions. I can definitely sync with you offline and share some additional contacts.

Is there a possibility of using the file:// scheme here? I'm not familiar enough with ChromeOS and the filesystem:// scheme to know if that would work, but that scheme is supported in `fetch` and what we would expect to be used most often. For testing, note that you need to request the `file://*` host permission and enable the "Allow access to file URLs" toggle on the extension's details page.

Thanks,

Oliver Dunk | DevRel, Chrome Extensions | https://developer.chrome.com/ | London, GB

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