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Fix BufferSource algorithms for shared and resizable buffers #1529

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Fix BufferSource algorithms for shared and resizable buffers #1529

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5511191
Fix buffer source's byte length
MattiasBuelens Oct 8, 2025
77d7cad
Add byte offset to ArrayBufferView
MattiasBuelens Oct 8, 2025
6ff3146
Fix writing bytes into an ArrayBufferView backed by a SharedArrayBuffer
MattiasBuelens Oct 8, 2025
ab8a53a
Simplify writing into an ArrayBufferView
MattiasBuelens Oct 8, 2025
358a25b
Move assert before conversion
MattiasBuelens Oct 8, 2025
bf273a5
Fix detached check
MattiasBuelens Oct 8, 2025
23de343
Use ArrayBufferCopyAndDetach to transfer an ArrayBuffer
MattiasBuelens Oct 8, 2025
8b7b647
Rework "get a copy of the bytes held"
MattiasBuelens Oct 8, 2025
9f5e36d
Use IsArrayBufferViewOutOfBounds instead
MattiasBuelens Oct 8, 2025
565a02f
Use SEQ-CST ordering to match ECMAScript
MattiasBuelens Oct 8, 2025
4794b1f
Use "then" only for checks with a single step
MattiasBuelens Nov 24, 2025
bff8cca
Auto-link "assert"
MattiasBuelens Nov 24, 2025
5a63453
Check IsTypedArrayOutOfBounds
MattiasBuelens Nov 25, 2025
f6ffbe6
Use ArrayBufferByteLength for both ArrayBuffer and SharedArrayBuffer
MattiasBuelens Nov 25, 2025
ad297de
Add a "then"
MattiasBuelens Nov 25, 2025
0a6dd99
Make byte length throw if DataView is out of bounds
MattiasBuelens Jan 9, 2026
c4abf28
Check if detached before reading byte offset
MattiasBuelens Jan 9, 2026
9607f36
Return immediately if length is 0
MattiasBuelens Jan 9, 2026
a528e68
Reorder steps
MattiasBuelens Jan 9, 2026
70fc250
Add "out of bounds" definition for ArrayBufferViews
MattiasBuelens Jan 11, 2026
aec52c2
Read byte offset before checking length
MattiasBuelens Jan 11, 2026
749ac3e
Add notes for steps that mirror ECMAScript
MattiasBuelens Jan 11, 2026
cf28af9
Remove TODO
MattiasBuelens Jan 11, 2026
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115 changes: 77 additions & 38 deletions index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -9294,39 +9294,84 @@ a reference to the same object that the IDL value represents.
To <dfn id="dfn-get-buffer-source-copy" export lt="get a copy of the buffer source|get a copy of the bytes held by the buffer source">get a copy of the bytes held by the buffer source</dfn>
given a [=buffer source type=] instance |bufferSource|:

1. Let |jsBufferSource| be the result of [=converted to a JavaScript value|converting=]
|bufferSource| to a JavaScript value.
1. Let |jsArrayBuffer| be |jsBufferSource|.
1. If |bufferSource| is [=BufferSource/detached=], then return the empty [=byte sequence=].
1. Let |length| be |bufferSource|'s [=BufferSource/byte length=].
1. Let |arrayBuffer| be |bufferSource|.
1. Let |offset| be 0.
1. Let |length| be 0.
1. If |jsBufferSource| has a \[[ViewedArrayBuffer]] [=/internal slot=], then:
1. Set |jsArrayBuffer| to |jsBufferSource|.\[[ViewedArrayBuffer]].
1. Set |offset| to |jsBufferSource|.\[[ByteOffset]].
1. Set |length| to |jsBufferSource|.\[[ByteLength]].
1. Otherwise:
1. Assert: |jsBufferSource| is an {{ArrayBuffer}} or
{{SharedArrayBuffer}} object.
1. Set |length| to |jsBufferSource|.\[[ArrayBufferByteLength]].
1. If [$IsDetachedBuffer$](|jsArrayBuffer|) is true, then return the empty
[=byte sequence=].
1. If |bufferSource| is a [=buffer view type=] instance:
1. Set |arrayBuffer| to |bufferSource|'s [=underlying buffer=].
1. Set |offset| to |bufferSource|'s [=ArrayBufferView/byte offset=].
<p class="note">This will throw an exception if |bufferSource| is
[=ArrayBufferView/out of bounds=].
1. If |length| is 0, then return the empty [=byte sequence=].
1. [=/Assert=]: |arrayBuffer| is an {{ArrayBuffer}} or {{SharedArrayBuffer}} object.
1. Let |jsArrayBuffer| be the result of [=converted to a JavaScript value|converting=]
|arrayBuffer| to a JavaScript value.
1. [=/Assert=]: [$IsDetachedBuffer$](|jsArrayBuffer|) is false.
1. Let |bytes| be a new [=byte sequence=] of [=byte sequence/length=] equal to |length|.
1. For |i| in [=the range=] |offset| to |offset| + |length| &minus; 1, inclusive, set
|bytes|[|i| &minus; |offset|] to [$GetValueFromBuffer$](|jsArrayBuffer|, |i|, Uint8,
true, Unordered).
1. Return |bytes|.
</div>

<div algorithm>
The <dfn export for="ArrayBufferView">byte offset</dfn> of an {{ArrayBufferView}}
|view| is the value returned by the following steps:

1. If |view| is [=ArrayBufferView/out of bounds=], then [=JavaScript/throw=] a
<l spec=ecmascript>{{TypeError}}</l>.
1. Let |jsView| be the result of [=converted to a JavaScript value|converting=] |view| to
a JavaScript value.
1. Return |jsView|.\[[ByteOffset]].

<p class="note">These steps are equivalent to
{{%TypedArray%/byteOffset|%TypedArray%.prototype.byteOffset}} (for a
[=typed array type=] instance) or {{DataView/byteOffset|DataView.prototype.byteOffset}}
(for a {{DataView}}).
</div>

<div algorithm>
The <dfn export for="BufferSource">byte length</dfn> of a [=buffer source type=] instance
|bufferSource| is the value returned by the following steps:

1. Let |jsBufferSource| be the result of [=converted to a JavaScript value|converting=]
|bufferSource| to a JavaScript value.
1. If |jsBufferSource| has a \[[ViewedArrayBuffer]] internal slot, then return
|jsBufferSource|.\[[ByteLength]].
1. Return |jsBufferSource|.\[[ArrayBufferByteLength]].
1. If |jsBufferSource| has a \[[ViewedArrayBuffer]] [=/internal slot=]:
1. If |jsBufferSource| has a \[[TypedArrayName]] [=/internal slot=]:
1. [=/Assert=]: |jsBufferSource| is a [=typed array type=] instance.
1. Let |taRecord| be [$MakeTypedArrayWithBufferWitnessRecord$](|jsBufferSource|,
Seq-Cst).
1. If [$IsTypedArrayOutOfBounds$](|taRecord|) is true, then return 0.
1. Return [$TypedArrayByteLength$](|taRecord|).
<p class="note">The above substeps are equivalent to
{{%TypedArray%/byteLength|%TypedArray%.prototype.byteLength}}.
1. Otherwise:
1. [=/Assert=]: |jsBufferSource| is a {{DataView}}.
1. Let |viewRecord| be [$MakeDataViewWithBufferWitnessRecord$](|jsBufferSource|,
Seq-Cst).
1. If [$IsViewOutOfBounds$](|viewRecord|) is true, then [=JavaScript/throw=] a
<l spec=ecmascript>{{TypeError}}</l>.
1. Return [$GetViewByteLength$](|viewRecord|).
<p class="note">The above substeps are equivalent to
{{DataView/byteLength|DataView.prototype.byteLength}}.
1. Otherwise:
1. [=/Assert=]: |jsBufferSource| is an {{ArrayBuffer}} or a {{SharedArrayBuffer}}.
1. If [$IsDetachedBuffer$](|jsBufferSource|) is true, then return 0.
1. Return [$ArrayBufferByteLength$](|jsBufferSource|, Seq-Cst).
<p class="note">The above substeps are equivalent to
{{ArrayBuffer/byteLength|ArrayBuffer.prototype.byteLength}} or
{{SharedArrayBuffer/byteLength|SharedArrayBuffer.prototype.byteLength}}.
</div>

<div algorithm>
An {{ArrayBufferView}} |view| is <dfn export for="ArrayBufferView">out of bounds</dfn>
if the following steps return true:

1. Let |jsView| be the result of [=converted to a JavaScript value|converting=]
|view| to a JavaScript value.
1. Return [$IsArrayBufferViewOutOfBounds$](|jsView|).
</div>
<!-- TODO: should this be stricter around not being detached? -->

<div algorithm>
The <dfn export for="BufferSource">underlying buffer</dfn> of a [=buffer source type=] instance
Expand All @@ -9348,10 +9393,10 @@ a reference to the same object that the IDL value represents.
<dfn id=arraybuffer-write-startingoffset export for="ArrayBuffer/write,SharedArrayBuffer/write">|startingOffset|</dfn>
(default 0):

1. [=/Assert=]: |bytes|'s [=byte sequence/length=] ≤ |arrayBuffer|'s [=byte length=]
&minus; |startingOffset|.
1. Let |jsArrayBuffer| be the result of [=converted to a JavaScript value|converting=]
|arrayBuffer| to a JavaScript value.
1. Assert: |bytes|'s [=byte sequence/length=] ≤ |jsArrayBuffer|.\[[ArrayBufferByteLength]]
&minus; |startingOffset|.
1. For |i| in [=the range=] |startingOffset| to |startingOffset| + |bytes|'s [=byte
sequence/length=] &minus; 1, inclusive, perform [$SetValueInBuffer$](|jsArrayBuffer|,
|i|, Uint8, |bytes|[|i| - |startingOffset|], true, Unordered).
Expand All @@ -9362,16 +9407,12 @@ a reference to the same object that the IDL value represents.
{{ArrayBufferView}} |view|, optionally given a
<dfn export for="ArrayBufferView/write">|startingOffset|</dfn> (default 0):

1. Let |jsView| be the result of [=converted to a JavaScript value|converting=] |view| to
a JavaScript value.
1. Assert: |bytes|'s [=byte sequence/length=] ≤ |jsView|.\[[ByteLength]] &minus;
1. [=/Assert=]: |bytes|'s [=byte sequence/length=] ≤ |view|'s [=byte length=] &minus;
|startingOffset|.
1. Assert: if |view| is not a {{DataView}}, then |bytes|'s [=byte sequence/length=] [=modulo=]
the [=element size=] of |view|'s type is 0.
1. Let |arrayBuffer| be the result of [=converted to an IDL value|converting=]
|jsView|.\[[ViewedArrayBuffer]] to an IDL value of type {{ArrayBuffer}}.
1. [=ArrayBuffer/Write=] |bytes| into |arrayBuffer| with
<i>[=ArrayBuffer/write/startingOffset=]</i> set to |jsView|.\[[ByteOffset]] +
1. [=ArrayBuffer/Write=] |bytes| into |view|'s [=underlying buffer=] with
<i>[=ArrayBuffer/write/startingOffset=]</i> set to |view|'s [=ArrayBufferView/byte offset=] +
|startingOffset|.
</div>

Expand Down Expand Up @@ -9429,22 +9470,20 @@ a reference to the same object that the IDL value represents.
1. Return true.
</div>

<div algorithm>
<div algorithm="ArrayBuffer/transfer">
To <dfn for="ArrayBuffer" export>transfer</dfn> an {{ArrayBuffer}} |arrayBuffer|, optionally
given a [=realm=] |targetRealm|:
given <dfn export for="ArrayBuffer/transfer">|preserveResizability|</dfn> (default false):

1. Let |jsArrayBuffer| be the result of [=converted to a JavaScript value|converting=]
|arrayBuffer| to a JavaScript value.
1. If [$IsDetachedBuffer$](|jsArrayBuffer|) is false, then [=JavaScript/throw=] a
1. If [$IsDetachedBuffer$](|jsArrayBuffer|) is true, then [=JavaScript/throw=] a
<l spec=ecmascript>{{TypeError}}</l>.
1. Let |arrayBufferData| be |jsArrayBuffer|.\[[ArrayBufferData]].
1. Let |arrayBufferByteLength| be |jsArrayBuffer|.\[[ArrayBufferByteLength]].
1. Perform [=?=] [$DetachArrayBuffer$](|jsArrayBuffer|).
1. If |targetRealm| is not given, let |targetRealm| be the [=current realm=].
1. Let |jsTransferred| be [=?=]
[$AllocateArrayBuffer$](|targetRealm|.\[[Intrinsics]].\[[{{%ArrayBuffer%}}]], 0).
1. Set |jsTransferred|.\[[ArrayBufferData]] to |arrayBufferData|.
1. Set |jsTransferred|.\[[ArrayBufferByteLength]] to |arrayBufferByteLength|.
1. If |preserveResizability| is true:
1. Let |jsTransferred| be [=?=] [$ArrayBufferCopyAndDetach$](|jsArrayBuffer|, undefined,
Preserve-Resizability).
1. Otherwise:
1. Let |jsTransferred| be [=?=] [$ArrayBufferCopyAndDetach$](|jsArrayBuffer|, undefined,
Fixed-Length).
1. Return the result of [=converted to an IDL value|converting=] |jsTransferred| to an IDL
value of type {{ArrayBuffer}}.

Expand Down