Add immutable array buffer awareness to structuredClone by gibson042 · Pull Request #11033 · whatwg/html

gibson042 · 2025-02-15T19:10:43Z

Ref tc39/proposal-immutable-arraybuffer#30

Immutable ArrayBuffers is currently at Stage 2, but seeking advancement in next week's Ecma TC39 meeting. Because immutable ArrayBuffers cannot be detached, structuredClone should perform the relevant test.

At least two implementers are interested (and none opposed):
- …
- …
Tests are written and can be reviewed and commented upon at:
- Add tests for structured cloning of immutable ArrayBuffers web-platform-tests/wpt#59803
Implementation bugs are filed:
- Chromium: …
- Gecko: …
- WebKit: …
- Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
- Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
~~Corresponding HTML AAM & ARIA in HTML issues & PRs:~~
~~MDN issue is filed:~~
The top of this comment includes a clear commit message to use.

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

/infrastructure.html ( diff )
/structured-data.html ( diff )

domenic · 2025-02-16T03:20:20Z

Can you work on getting the build passing?

Also, it would be good to update the PR title and commit message to specify that this is about preventing transferring of such immutable array buffers; structured cloning them is still allowed.

bakkot · 2025-02-16T06:30:35Z


     <li><p>If <var>transferable</var> has an [[ArrayBufferData]] internal slot and
-     <span>IsSharedArrayBuffer</span>(<var>transferable</var>) is true, then throw a
+     <span>IsSharedArrayBuffer</span>(<var>transferable</var>) is true or


Nit:

Suggested change

IsSharedArrayBuffer(<var>transferable</var>) is true or

either IsSharedArrayBuffer(<var>transferable</var>) is true or

since otherwise a valid reading at least at first glance is "(transferable has an [[ArrayBufferData]] internal slot and IsSharedArrayBuffer(transferable) is true) or IsImmutableBuffer(transferable) is true)".

bakkot · 2025-02-16T06:30:49Z

Cloning should preserve immutability (tc39/proposal-immutable-arraybuffer#19 (comment)), the same way it currently preserves resizability.

…e not detachable) Ref tc39/proposal-immutable-arraybuffer#30

gibson042 · 2025-02-17T20:18:26Z

Can you work on getting the build passing?

Done.

Also, it would be good to update the PR title and commit message to specify that this is about preventing transferring of such immutable array buffers; structured cloning them is still allowed.

Done.

Cloning should preserve immutability (tc39/proposal-immutable-arraybuffer#19 (comment)), the same way it currently preserves resizability.

Done: 2f4d3cb

bakkot

I'm not an HTML reviewer, but LGTM

andreubotella · 2025-02-17T22:31:59Z

+       <li><p>Set <var>serialized</var> to { [[Type]]: "ImmutableArrayBuffer", [[ArrayBufferData]]:
+       <var>value</var>.[[ArrayBufferData]], [[ArrayBufferByteLength]]:
+       <var>value</var>.[[ArrayBufferByteLength]] }.</p></li>


Is value.[[ArrayBufferData]] supposed to be able to be kept alive in the serialization? Unlike for SABs, serializing immutable ArrayBuffers doesn't throw if forStorage is true, meaning that the serialization could be deserialized at some future time in a different process that doesn't share any memory with the current one.

At the very least we should have a note saying that implementations should make sure that a forStorage serialization actually serializes the buffer's contents, but a non-for-storage one might serialize a pointer to the buffer data instead.

Is value.[[ArrayBufferData]] supposed to be able to be kept alive in the serialization? Unlike for SABs, serializing immutable ArrayBuffers doesn't throw if forStorage is true, meaning that the serialization could be deserialized at some future time in a different process that doesn't share any memory with the current one.

That's a good question. I'm not very well versed in the particulars here, but my instinct is that I've got it correct—an immutable array buffer should behave like any other non-shared array buffer when forStorage is true, such that it could be deserialized into a new immutable array buffer with identical contents by a completely independent process at some point in the future.

I like your idea of a note, and have attempted to update accordingly.

…ith forStorage true vs. false

domenic · 2025-02-18T02:17:23Z

+     [[ArrayBufferData]] internal slot value is <var>serialized</var>.[[ArrayBufferData]], whose
+     [[ArrayBufferByteLength]] internal slot value is
+     <var>serialized</var>.[[ArrayBufferByteLength]], and whose [[ArrayBufferIsImmutable]] internal
+     slot is present.</p></li>


Is it possible for a slot to not have a value in this way?

Technically yes, but it's obviously not great and I see now that things are moving away from it. I've updated accordingly.

But for the record, the document.all HTMLAllCollection seems to be another example—it has an [[IsHTMLDDA]] internal slot for which no value is ever specified AFAICT.

And in ECMAScript, Object Internal Methods and Internal Slots defines the initial value of an internal slot to be undefined.

True although see also tc39/ecma262#3399

…uffer with forStorage true vs. false

…hich are not detachable)

…rialize}

domenic

Thanks for doing this; LGTM. We can merge this whenever there is sufficient implementer interest, but if we want to merge it before TC39 stage 4 then instead of linking to https://tc39.es/ecma262/#sec-isimmutablebuffer we'll need to update the link to something that resolves. (See other examples of how HTML references pre-stage-4 TC39 proposals below the big list of terms.)

I'll mark this "do not merge yet" until we're ready in that regard, either way, but the spec text itself looks great.

…eviewers,sfink Spec PR: <whatwg/html#11033> Differential Revision: https://phabricator.services.mozilla.com/D249411

…eviewers,sfink Spec PR: <whatwg/html#11033> Differential Revision: https://phabricator.services.mozilla.com/D249411 UltraBlame original commit: c3d3c452364f15aa0ba169d968075d5acca6d2ca

…eviewers,sfink Spec PR: <whatwg/html#11033> Differential Revision: https://phabricator.services.mozilla.com/D249411 UltraBlame original commit: da0c4055945e2adeb6d7e5d24bc34e0f114364a7

…eviewers,sfink Spec PR: <whatwg/html#11033> Differential Revision: https://phabricator.services.mozilla.com/D249411 UltraBlame original commit: c3d3c452364f15aa0ba169d968075d5acca6d2ca

…eviewers,sfink Spec PR: <whatwg/html#11033> Differential Revision: https://phabricator.services.mozilla.com/D249411 UltraBlame original commit: da0c4055945e2adeb6d7e5d24bc34e0f114364a7

…eviewers,sfink Spec PR: <whatwg/html#11033> Differential Revision: https://phabricator.services.mozilla.com/D249411 UltraBlame original commit: c3d3c452364f15aa0ba169d968075d5acca6d2ca

…eviewers,sfink Spec PR: <whatwg/html#11033> Differential Revision: https://phabricator.services.mozilla.com/D249411 UltraBlame original commit: da0c4055945e2adeb6d7e5d24bc34e0f114364a7

* The serializer must preserve immutability. * Transferring an immutable array buffer is not allowed. Currently the backing store is always copied. Potentially it could be shared for postMessage. See: whatwg/html#11033 Bug: 450237486 Change-Id: I0be6081a29454c8557b81c364b9f49332d15427c Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/7462754 Auto-Submit: Olivier Flückiger <olivf@chromium.org> Commit-Queue: Olivier Flückiger <olivf@chromium.org> Reviewed-by: Leszek Swirski <leszeks@chromium.org> Cr-Commit-Position: refs/heads/main@{#104746}

bakkot · 2026-04-08T20:39:14Z

Looks like there's implementations in both Spidermonkey and V8. Presumably that counts as two interested implementations, so, time to land?

styfle · 2026-05-08T02:00:49Z

Who has the ability to merge now? Perhaps @annevk?

annevk

This looks good, but someone needs to fill out OP. There's a lot of checkboxes not checked.

annevk · 2026-05-08T13:15:11Z

     </li>

+     <li>
+      <p>Otherwise, if <span>IsImmutableBuffer</span>(<var>value</var>) is true, then:</p>


Suggested change

Otherwise, if IsImmutableBuffer(<var>value</var>) is true, then:

Otherwise, if IsImmutableBuffer(<var>value</var>) is true:

annevk · 2026-05-08T13:15:39Z

   </li>

+   <li>
+    <p>Otherwise, if <var>serialized</var>.[[Type]] is "ImmutableArrayBuffer", then:</p>


Suggested change

Otherwise, if <var>serialized</var>.[[Type]] is "ImmutableArrayBuffer", then:

Otherwise, if <var>serialized</var>.[[Type]] is "ImmutableArrayBuffer":

Why would the pattern used for type "ImmutableArrayBuffer" differ from the pattern used for types "SharedArrayBuffer" and "GrowableSharedArrayBuffer"?

Yeah we need to do a more general pass to remove these.

gibson042 · 2026-05-12T00:16:33Z

This looks good, but someone needs to fill out OP. There's a lot of checkboxes not checked.

Submitted tests at web-platform-tests/wpt#59803 , and crossed off AAM/ARIA/MDN checkboxes as no-op. I think the rest need implementer engagement, but am happy to be corrected.

bakkot · 2026-05-12T00:23:50Z

You can find links to V8 and SM implementations in the pingbacks, which I think count as interest?

The others presumably need bugs filed (though it probably doesn't make sense to file bugs on Node or Deno in advance of V8 implementing, so I guess just JSC).

annevk · 2026-05-12T07:30:47Z

What's the Web IDL story for these?

gibson042 · 2026-05-12T13:19:50Z

What's the Web IDL story for these?

whatwg/webidl#1487 and whatwg/webidl#1598

tc39/proposal-import-bytes#28 also covers a lot

annevk · 2026-05-12T14:36:44Z

Thanks for the pointers. So the Web IDL integration seems incomplete. The tentative proposal is that any API that takes a buffer or view today will accept an immutable buffer automatically. This seems like it should work fine for Fetch, but not Encoding's encodeInto(). We'll need something for the latter and tests for a representative sample of all impacted APIs.

This was referenced Feb 15, 2025

Mention proposed integration with "structured cloning" tc39/proposal-immutable-arraybuffer#19

Closed

Review tc39/proposal-immutable-arraybuffer#30

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bakkot reviewed Feb 16, 2025

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gibson042 changed the title ~~Prevent structuredClone of immutable array buffers (which are not detachable)~~ Prevent structuredClone transfer of immutable array buffers (which are not detachable) Feb 17, 2025

gibson042 added 2 commits February 17, 2025 14:19

Prevent structuredClone transfer of immutable array buffers (which ar…

06282ff

…e not detachable) Ref tc39/proposal-immutable-arraybuffer#30

Preserve ArrayBuffer immutability in Structured{Serialize,Deserialize}

2f4d3cb

gibson042 force-pushed the structured-clone-immutable-arraybuffer branch from ef5df7e to 2f4d3cb Compare February 17, 2025 20:17

gibson042 changed the title ~~Prevent structuredClone transfer of immutable array buffers (which are not detachable)~~ Add immutable array buffer awareness to structuredClone Feb 17, 2025

bakkot approved these changes Feb 17, 2025

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andreubotella reviewed Feb 17, 2025

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erights mentioned this pull request Feb 17, 2025

Path to Stage 4! tc39/proposal-immutable-arraybuffer#1

Open

45 tasks

Comment on StructuredSerializeInternal for an immutable ArrayBuffer w…

a48c69a

…ith forStorage true vs. false

domenic reviewed Feb 18, 2025

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gibson042 added 3 commits February 18, 2025 13:14

fixup! Comment on StructuredSerializeInternal for an immutable ArrayB…

1c0cf64

…uffer with forStorage true vs. false

fixup! Prevent structuredClone transfer of immutable array buffers (w…

88b5fdf

…hich are not detachable)

fixup! Preserve ArrayBuffer immutability in Structured{Serialize,Dese…

56c3332

…rialize}

gibson042 mentioned this pull request Feb 18, 2025

Editorial: Assign [[ArrayBufferIsImmutable]] branding slots permanent undefined values tc39/proposal-immutable-arraybuffer#40

Merged

domenic approved these changes Feb 26, 2025

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domenic added do not merge yet Pull request must not be merged per rationale in comment topic: script integration Better coordination across standards needed labels Feb 26, 2025

anba mentioned this pull request May 16, 2025

Add [AllowImmutable] extended attribute for immutable array buffers and views whatwg/webidl#1487

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annevk reviewed May 8, 2026

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gibson042 mentioned this pull request May 12, 2026

Add tests for structured cloning of immutable ArrayBuffers web-platform-tests/wpt#59803

Open

	<span>IsSharedArrayBuffer</span>(<var>transferable</var>) is true or
	either <span>IsSharedArrayBuffer</span>(<var>transferable</var>) is true or

	<p>Otherwise, if <span>IsImmutableBuffer</span>(<var>value</var>) is true, then:</p>
	<p>Otherwise, if <span>IsImmutableBuffer</span>(<var>value</var>) is true:</p>

	<p>Otherwise, if <var>serialized</var>.[[Type]] is "ImmutableArrayBuffer", then:</p>
	<p>Otherwise, if <var>serialized</var>.[[Type]] is "ImmutableArrayBuffer":</p>

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