Intent to Ship: Multiple Readers and Writers in File System Access API
Nathan Memmott
Contact emails
ds...@google.com, mem...@google.com
Explainer
https://github.com/whatwg/fs/blob/main/proposals/MultipleReadersWriters.md
Specification
https://github.com/whatwg/fs/pull/151
Summary
Currently, only one FileSystemSyncAccessHandle may be open at a time per file, preventing an origin from reading and writing to the same file from multiple tabs easily. Conversely, multiple FileSystemWritableFileStreams can be simultaneously open, letting multiple writers clobber each other.
Introducing new locking modes for FileSystemSyncAccessHandle and FileSystemWritableFileStream allows opening either multiple readers/writers or an exclusive writer to a file entry, depending on the application's use case.
```
handle.createSyncAccessHandle({ mode: 'read-only' });
handle.createSyncAccessHandle({ mode: 'readwrite-unsafe' });
handle.createWritable({ mode: 'exclusive' });
```
Blink component
TAG review
https://github.com/w3ctag/design-reviews/issues/845
TAG review status
Issues addressed
Origin Trial documentation link
https://developer.chrome.com/blog/new-dev-trial-for-multiple-readers-and-writers/
Risks
Interoperability and Compatibility
Gecko: Positive (https://github.com/mozilla/standards-positions/issues/861)
WebKit: No Signal (https://github.com/WebKit/standards-positions/issues/238)
Web developers: Positive (https://github.com/whatwg/fs/issues/34)
Other signals: Emscripten plans to utilize multiple readers and writers for performance improvement, and sites using Emscripten will be able to take advantage of this integration.
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?
None
Debuggability
Feature is web exposed through WebIDL changes. No additional implementation effort is required.
Will this feature be supported on all six Blink platforms (Windows, Mac, Linux, Chrome OS, Android, and Android WebView)?
Yes
Is this feature fully tested by web-platform-tests?
WPTs have been added that fully test the behavior of the new locking modes. They additionally test the interactions between combinations of primitives, their different modes, and file handle operations.
Flag name on chrome://flags
file-system-access-locking-scheme
Finch feature name
FileSystemAccessLockingScheme
Requires code in //chrome?
False
Tracking bug
https://bugs.chromium.org/p/chromium/issues/detail?id=1382215
Launch bug
https://launch.corp.google.com/launch/4261043
Estimated milestones
Anticipated spec changes
Discussion: https://github.com/whatwg/fs/issues/34
No open issues that introduce web compat/interop risk.
Link to entry on the Chrome Platform Status
https://chromestatus.com/feature/5172892632875008
Links to previous Intent discussions
Intent to prototype: https://groups.google.com/a/chromium.org/d/msgid/blink-dev/CAPscNz5Oi-z6-%2BvTVyf9BNgvVnLZeQo5nfzyRhEZ_F400tTP3w%40mail.gmail.com
Yoav Weiss
Ajay Rahatekar
Mike Taylor
Seems like a useful addition to the API. LGTM1
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Rick Byers
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Yoav Weiss
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