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Add and improve operations on BufferSources #987

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merged 10 commits into from
Jun 25, 2021
Merged

Add and improve operations on BufferSources #987

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590b3bb
Add some operations bridging BufferSources and byte sequences
domenic Jun 2, 2021
701e3df
Review comments
domenic Jun 3, 2021
fce17fd
Expand and fix up
domenic Jun 3, 2021
fb7c29e
Add "byte length"
domenic Jun 3, 2021
1190c5d
Fix in response to review
domenic Jun 4, 2021
f935bf4
Fix linking error
domenic Jun 22, 2021
be4f854
Conversions and other fixes
domenic Jun 24, 2021
145a4cd
Add BufferSource/is detached
domenic Jun 24, 2021
1c98981
Review comments and Bikeshed warnings
domenic Jun 25, 2021
d48fe0b
Add "underlying buffer"
domenic Jun 25, 2021
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247 changes: 176 additions & 71 deletions index.bs
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Original file line number Diff line number Diff line change
Expand Up @@ -86,6 +86,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
text: Set; url: sec-set-objects
text: SharedArrayBuffer; url: sec-sharedarraybuffer-objects
text: %AsyncIteratorPrototype%; url: sec-asynciteratorprototype
text: %ArrayBuffer%; url: sec-arraybuffer-constructor
text: %Array.prototype%; url: sec-properties-of-the-array-prototype-object
text: %Error.prototype%; url: sec-properties-of-the-error-prototype-object
text: %Function.prototype%; url: sec-properties-of-the-function-prototype-object
Expand Down Expand Up @@ -116,6 +117,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
type: argument
text: NewTarget; url: sec-built-in-function-objects
type: abstract-op
text: AllocateArrayBuffer; url: sec-allocatearraybuffer
text: ArrayCreate; url: sec-arraycreate
text: Call; url: sec-call
text: CanonicalNumericIndexString; url: sec-canonicalnumericindexstring
Expand All @@ -140,6 +142,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
text: GetFunctionRealm; url: sec-getfunctionrealm
text: GetIterator; url: sec-getiterator
text: GetMethod; url: sec-getmethod
text: GetValueFromBuffer; url: sec-getvaluefrombuffer
text: IfAbruptRejectPromise; url: sec-ifabruptrejectpromise
text: IsArray; url: sec-isarray
text: IsAccessorDescriptor; url: sec-isaccessordescriptor
Expand All @@ -155,9 +158,10 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
text: NewModuleEnvironment; url: sec-newmoduleenvironment
text: NewPromiseCapability; url: sec-newpromisecapability
text: NormalCompletion; url: sec-normalcompletion
text: OrdinaryObjectCreate; url: sec-ordinaryobjectcreate
text: OrdinaryCreateFromConstructor; sec: sec-ordinarycreatefromconstructor
text: OrdinaryDefineOwnProperty; url: sec-ordinarydefineownproperty
text: OrdinaryGetOwnProperty; url: sec-ordinarygetownproperty
text: OrdinaryObjectCreate; url: sec-ordinaryobjectcreate
text: OrdinaryPreventExtensions; url: sec-ordinarypreventextensions
text: OrdinarySetWithOwnDescriptor; url: sec-ordinarysetwithowndescriptor
text: PerformPromiseThen; url: sec-performpromisethen
Expand All @@ -167,6 +171,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
text: SetFunctionName; url: sec-setfunctionname
text: SetImmutablePrototype; url: sec-set-immutable-prototype
text: SetIntegrityLevel; url: sec-setintegritylevel
text: SetValueInBuffer; url: sec-setvalueinbuffer
text: ToBigInt; url: #sec-tobigint
text: ToBoolean; url: sec-toboolean
text: ToInt32; url: sec-toint32
Expand Down Expand Up @@ -237,6 +242,7 @@ urlPrefix: https://tc39.github.io/ecma262/; spec: ECMA-262
text: realm; url: realm
text: ResolvedBinding Record; url: resolvedbinding-record
text: running execution context; url: running-execution-context
text: element size; url: table-the-typedarray-constructors
</pre>

<style>
Expand Down Expand Up @@ -6774,51 +6780,9 @@ There is no way to represent a constant value of any of these types in IDL.
The [=type name=] of all
of these types is the name of the type itself.

At the specification prose level, IDL [=buffer source types=]
are simply references to objects. To inspect or manipulate the bytes inside the buffer,
specification prose must first either
<dfn id="dfn-get-buffer-source-reference" export lt="get a reference to the buffer source">get a reference to the bytes held by the buffer source</dfn>
or <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</dfn>.
With a reference to the buffer source’s bytes, specification prose can get or set individual
byte values using that reference.

<div class="advisement">

Extreme care must be taken when writing specification text that gets a reference
to the bytes held by a buffer source, as the underlying data can easily be changed
by the script author or other APIs at unpredictable times. If you are using a buffer source type
as an operation argument to obtain a chunk of binary data that will not be modified,
it is strongly recommended to get a copy of the buffer source’s bytes at the beginning
of the prose defining the operation.

Requiring prose to explicitly get a reference to or copy of the bytes is intended to
help specification reviewers look for problematic uses of these buffer source types.

</div>

<div class="note">

When designing APIs that take a buffer, it is recommended to use the
{{BufferSource}} typedef rather than {{ArrayBuffer}}
or any of the view types.

When designing APIs that create and return a buffer, it is recommended
to use the {{ArrayBuffer}} type rather than
{{Uint8Array}}.

</div>

Attempting to [=get a reference to the buffer source|get a reference to=] or
[=get a copy of the buffer source|get a copy of the bytes held by a buffer source=]
when the {{ArrayBuffer}} has been [=ArrayBuffer/detached=]
will fail in a language binding-specific manner.

Note: See [[#es-buffer-source-types]] below for
how interacting with buffer source types works in the ECMAScript language binding.

<p class="issue">
We should include an example of specification text that uses these types and terms.
</p>
At the specification prose level, IDL [=buffer source types=] are simply references to objects. To
inspect or manipulate the bytes inside the buffer, specification prose needs to use the algorithms
in [[#es-buffer-source-types]].

<pre class="grammar" id="prod-BufferRelatedType">
BufferRelatedType :
Expand Down Expand Up @@ -8994,37 +8958,178 @@ an IDL value of any [=buffer source type=]
to an ECMAScript value is the Object value that represents
a reference to the same object that the IDL value represents.

<div algorithm="get a reference to a buffer source">

When [=get a reference to the buffer source|getting a reference to=]
or [=get a copy of the buffer source|getting a copy of the bytes held by a buffer source=]
that is an ECMAScript {{ECMAScript/ArrayBuffer}}, {{ECMAScript/DataView}}
or typed array object, these steps must be followed:

1. Let |O| be the ECMAScript object that is the buffer source.
1. Initialize |arrayBuffer| to |O|.
1. Initialize |offset| to 0.
1. Initialize |length| to 0.
1. If |O| has a \[[ViewedArrayBuffer]] [=internal slot=], then:
1. Set |arrayBuffer| to the value of |O|’s \[[ViewedArrayBuffer]] [=internal slot=].
1. Set |offset| to the value of |O|’s \[[ByteOffset]] [=internal slot=].
1. Set |length| to the value of |O|’s \[[ByteLength]] [=internal slot=].
1. Otherwise, set |length| to the value of |O|’s \[[ArrayBufferByteLength]] [=internal slot=].
1. If <a abstract-op>IsDetachedBuffer</a>(|arrayBuffer|) is <emu-val>true</emu-val>, then
return the empty byte sequence.
1. Let |data| be the value of |O|’s \[[ArrayBufferData]] [=internal slot=].
1. Return a reference to or copy of (as required) the |length| bytes in |data|
starting at byte offset |offset|.
<hr>

<div algorithm>
To <dfn export for="ArrayBuffer">create</dfn> an {{ArrayBuffer}} from a [=byte sequence=]
|bytes| in a [=Realm=] |realm|:

1. Let |esArrayBuffer| be [=?=]
[$AllocateArrayBuffer$](|realm|.\[[Intrinsics]].[[{{%ArrayBuffer%}}]], |bytes|'s
[=byte sequence/length=]).
1. Let |arrayBuffer| be the result of [=converted to an IDL value|converting=] |esArrayBuffer|
to an IDL value of type {{ArrayBuffer}}.
1. [=ArrayBuffer/Write=] |bytes| into |arrayBuffer|.
1. Return |arrayBuffer|.
</div>

<div algorithm>
To <dfn export for="ArrayBufferView">create</dfn> one of the {{ArrayBufferView}} types from a
[=byte sequence=] |bytes| in a [=Realm=] |realm|:

1. Assert: if the type is not {{DataView}}, then |bytes|'s [=byte sequence/length=] [=modulo=]
the [=element size=] of that type is 0.
1. Let |arrayBuffer| be the result of [=ArrayBuffer/creating=] an {{ArrayBuffer}} from |bytes|
in |realm|.
1. Let |esArrayBuffer| be the result of [=converted to an ECMAScript value|converting=]
|arrayBuffer| to an ECMAScript value.
1. Let |constructor| be the appropriate constructor from |realm|.\[[Intrinsics]] for the type
of {{ArrayBufferView}} being created.
1. Let |esView| be [=!=] [$Construct$](|constructor|, « |esArrayBuffer| »).
1. Return the result of [=converted to an IDL value|converting=] |esView| into the given type.
</div>

<div algorithm>
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 {{BufferSource}} |bufferSource|:

1. Let |esBufferSource| be be the result of [=converted to an ECMAScript value|converting=]
|bufferSource| to an ECMAScript value.
1. Let |esArrayBuffer| be |esBufferSource|.
1. Let |offset| be 0.
1. Let |length| be 0.
1. If |esBufferSource| has a \[[ViewedArrayBuffer]] [=internal slot=], then:
1. Set |esArrayBuffer| to |esBufferSource|.\[[ViewedArrayBuffer]].
1. Set |offset| to |esBufferSource|.\[[ByteOffset]].
1. Set |length| to |esBufferSource|.\[[ByteLength]].
1. Otherwise:
1. Assert: |esBufferSource| is an {{ECMAScript/ArrayBuffer}} or
{{ECMAScript/SharedArrayBuffer}} object.
1. Set |length| to |esBufferSource|.\[[ArrayBufferByteLength]].
1. If [=!=] [$IsDetachedBuffer$](|esArrayBuffer|) is true, then return the empty
[=byte sequence=].
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 [=!=] [$GetValueFromBuffer$](|esArrayBuffer|, |i|, Uint8,
true, Unordered).
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1. Return |bytes|.
</div>

<div algorithm>
The <dfn export for="BufferSource">byte length</dfn> of a {{BufferSource}} |bufferSource| is
the value returned by the following steps:

To <dfn id="dfn-detach" for="ArrayBuffer" export>detach</dfn> an {{ArrayBuffer}}, these steps must be followed:
1. Let |esBufferSource| be be the result of [=converted to an ECMAScript value|converting=]
|bufferSource| to an ECMAScript value.
1. If |esBufferSource| has a \[[ViewedArrayBuffer]] internal slot, then return
|esBufferSource|.\[[ByteLength]].
1. Return |esBufferSource|.\[[ArrayBufferByteLength]].
</div>

<div algorithm>
The <dfn export for="BufferSource">underlying buffer</dfn> of a {{BufferSource}} |bufferSource|
is the value returned by the following steps:

1. If |bufferSource| is an {{ArrayBuffer}}, return |bufferSource|.
1. Let |esBufferSource| be be the result of [=converted to an ECMAScript value|converting=]
|bufferSource| to an ECMAScript value.
1. Return the result of [=converted to an IDL value|converting=]
|esBufferSource|.\[[ViewedArrayBuffer]] to an IDL value of type {{ArrayBuffer}}.
</div>

<div algorithm="ArrayBuffer/write">
To <dfn export for="ArrayBuffer">write</dfn> a [=byte sequence=] |bytes| into an {{ArrayBuffer}}
|arrayBuffer|, optionally given a <dfn export for="ArrayBuffer/write">|startingOffset|</dfn>
(default 0):

1. Let |esArrayBuffer| be be the result of [=converted to an ECMAScript value|converting=]
|arrayBuffer| to an ECMAScript value.
1. Assert: |bytes|'s [=byte sequence/length=] ≤ |esArrayBuffer|.\[[ArrayBufferByteLength]]
&minus; |startingOffset|.
1. For |i| in [=the range=] |startingOffset| to |startingOffset| + |bytes|'s [=byte
sequence/length=] &minus; 1, inclusive, perform [=!=] [$SetValueInBuffer$](|esArrayBuffer|,
|i|, Uint8, |bytes|[|i| - |startingOffset|], true, Unordered).
</div>

<div algorithm="ArrayBufferView/write">
To <dfn export for="ArrayBufferView">write</dfn> a [=byte sequence=] |bytes| into an
{{ArrayBufferView}} |view|, optionally given a
<dfn export for="ArrayBufferView/write">|startingOffset|</dfn> (default 0):

1. Let |esView| be be the result of [=converted to an ECMAScript value|converting=] |view| to
an ECMAScript value.
1. Assert: |bytes|'s [=byte sequence/length=] ≤ |esView|.\[[ByteLength]] &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=]
|esView|.\[[ViewedArrayBuffer]] to an IDL value of type {{ArrayBuffer}}.
1. [=ArrayBuffer/Write=] |bytes| into |arrayBuffer| with
<i>[=ArrayBuffer/write/startingOffset=]</i> set to |esView|.\[[ByteOffset]] +
|startingOffset|.
</div>

<div class="advisement">
Extreme care must be taken when writing specification text that
[=ArrayBuffer/writes=] into an {{ArrayBuffer}} or {{ArrayBufferView}}, as the underlying data
can easily be changed by the script author or other APIs at unpredictable times. This is
especially true if the [{{AllowShared}}] [=extended attribute=] is involved.

1. Let |O| be the ECMAScript object that is the {{ArrayBuffer}}.
1. Perform [=!=] <a abstract-op>DetachArrayBuffer</a>(|O|).
For the non-shared cases, a more recommended pattern is to [=ArrayBuffer/transfer=] the
{{ArrayBuffer}} first if possible, to ensure the writes cannot overlap with other modifications,
and then give the new {{ArrayBuffer}} instance to author code as necessary. Alternately, you can
[=get a copy of the bytes held by the buffer source=], modify those bytes, and then use them to
[=ArrayBuffer/create=] a new {{ArrayBuffer}} or {{ArrayBufferView}} to give back to author code.
</div>

<div algorithm>
To <dfn id="dfn-detach" for="ArrayBuffer" export>detach</dfn> an {{ArrayBuffer}} |arrayBuffer|:

1. Let |esArrayBuffer| be be the result of [=converted to an ECMAScript value|converting=]
|arrayBuffer| to an ECMAScript value.
1. Assert: [=!=] [$IsSharedArrayBuffer$](|esArrayBuffer|) is false.
1. Perform [=?=] [$DetachArrayBuffer$](|esArrayBuffer|).

<p class="note">This will throw an exception if |esArrayBuffer| has an \[[ArrayBufferDetachKey]]
that is not undefined, such as is the case with the value of {{Memory|WebAssembly.Memory}}'s
{{Memory/buffer}} attribute. [[WASM-JS-API-1]]

<p class="note">Detaching a buffer that is already [=BufferSource/detached=] is a no-op.
</div>

<div algorithm>
A {{BufferSource}} |bufferSource| is <dfn for="BufferSource" export>detached</dfn> if the
following steps return true:

1. Let |esArrayBuffer| be be the result of [=converted to an ECMAScript value|converting=]
|bufferSource| to an ECMAScript value.
1. If |esArrayBuffer| has a \[[ViewedArrayBuffer]] internal slot, then set |esArrayBuffer| to
|esArrayBuffer|.\[[ViewedArrayBuffer]].
1. Return [=!=] [$IsDetachedBuffer$](|esArrayBuffer|).
</div>

<div algorithm>
To <dfn for="ArrayBuffer" export>transfer</dfn> an {{ArrayBuffer}} |arrayBuffer|, optionally
given a [=Realm=] |targetRealm|:

1. Let |esArrayBuffer| be be the result of [=converted to an ECMAScript value|converting=]
|arrayBuffer| to an ECMAScript value.
1. Assert: [=!=] [$IsSharedArrayBuffer$](|esArrayBuffer|) is false.
1. Assert: [=!=] [$IsDetachedBuffer$](|esArrayBuffer|) is false.
1. Let |arrayBufferData| be |esArrayBuffer|.\[[ArrayBufferData]].
1. Let |arrayBufferByteLength| be |esArrayBuffer|.\[[ArrayBufferByteLength]].
1. Perform [=?=] [$DetachArrayBuffer$](|esArrayBuffer|).
1. If |targetRealm| is not given, let |targetRealm| be the [=current Realm=].
1. Let |esTransferred| be [=!=]
[$AllocateArrayBuffer$](|targetRealm|.\[[Intrinsics]].[[{{%ArrayBuffer%}}]], 0).
1. Set |esTransferred|.\[[ArrayBufferData]] to |arrayBufferData|.
1. Set |esTransferred|.\[[ArrayBufferByteLength]] to |arrayBufferByteLength|.
1. Return the result of [=converted to an IDL value|converting=] |esTransferred| to an IDL
value of type {{ArrayBuffer}}.

<p class="note">This will throw an exception under the same circumstances as
[=ArrayBuffer/detaching=].
</div>

<h4 id="es-frozen-array">Frozen arrays — FrozenArray&lt;|T|&gt;</h4>

Expand Down Expand Up @@ -14285,7 +14390,7 @@ A {{DOMException}} is represented by a

<div algorithm="to create a simple exception or DOMException">

To [=create=] a [=simple exception=] or {{DOMException}} |E|, with a string giving the
To create a [=simple exception=] or {{DOMException}} |E|, with a string giving the
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Unrelated? FWIW, this link is a good one.

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The "create" term is now ambiguous and this didn't feel like it was referencing the existing "create" to me so I deleted it instead of disambiguating.

[=error name=] |N| for the {{DOMException}} case and optionally a string giving a user
agent-defined message |M|:

Expand Down