Add new locking modes to file handle primitives

[nathanmemmott]

Add new locking modes to file handle primitives

Writable file streams and sync access handles can now be created with an optional "mode" parameter.

For sync access handles, "mode" can be "readwrite", "readonly", or "readwrite-unsafe". "readwrite" is the current behavior and the default if "mode" is not specified. Only one sync access handle can be open on a file handle in this mode. The two new modes, "readonly" and "readwrite-unsafe", are shared modes which are added to support multiple readers and writers.

For writable file streams, "mode" can be "exclusive" or "siloed". "siloed" is the current behavior and the default if "mode" is not specified. This mode allows multiple writers to clobber each other. The new "exclusive" mode is added to prevent this clobbering.

For a more detailed explanation, see Multiple Readers and Writers explainer: https://github.com/whatwg/fs/blob/main/proposals/MultipleReadersWriters.md.

• [x] At least two implementers are interested (and none opposed):
  • Chrome
  • Firefox
• [x] Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/master/fs/FileSystemFileHandle-sync-access-handle-lock-modes.https.tentative.worker.js
  • https://github.com/web-platform-tests/wpt/blob/master/fs/FileSystemFileHandle-writable-file-stream-lock-modes.https.tentative.worker.js
  • https://github.com/web-platform-tests/wpt/blob/master/fs/FileSystemFileHandle-cross-primitive-locking.https.tentative.worker.js
  • PR to make non-tentative: https://github.com/web-platform-tests/wpt/pull/43134
• [ ] Implementation bugs are filed:
  • Chromium: https://crbug.com/1382215
  • Gecko: …
  • WebKit: …
• [x] MDN issue is filed: https://github.com/mdn/mdn/issues/479
• [x] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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[nathanmemmott]

cc @jesup @szewai @annevk Can you take a look at this?