Re: [whatwg/streams] Allow other specifications to create readable byte streams (#1130)

@domenic commented on this pull request.

@yutakahirano, can you take a look at the latest commit? I should also update the Fetch and Web Serial PRs to make sure this works well for them too.

> + 1. If |byobRequest| is null, then return null.
+ 1. Return |byobRequest|.[=ReadableStreamBYOBRequest/[[view]]=].
+</div>
+
+<div algorithm>
+ To <dfn export for="ReadableStream">enqueue bytes</dfn> into a {{ReadableStream}} |stream| given an
+ {{ArrayBufferView}} |view|:
+
+ 1. Assert: |stream|.[=ReadableStream/[[controller]]=] [=implements=]
+    {{ReadableByteStreamController}}.
+ 1. Let |byobRequest| be !
+    [$ReadableByteStreamControllerGetBYOBRequest$](|stream|.[=ReadableStream/[[controller]]=]).
+ 1. If |byobRequest| is non-null, and |view|.\[[ViewedArrayBuffer]].\[[ArrayBufferData]] is the same
+    [=Data Block=] as |byobRequest|.\[[view]].\[[ViewedArrayBuffer]].\[[ArrayBufferData]], then:
+  1. Assert: |view|.\[[ByteOffset]] is |byobRequest|.\[[view]].\[[ByteOffset]].
+  1. Assert: |view|.\[[ByteLength]] ≤ |byobRequest|.\[[view]].\[[ByteLength]].

I made these asserts instead of if tests (compared to https://github.com/w3c/webtransport/pull/297#discussion_r661130619) since it seems like it would be bad if the caller used the view but went past the requested bounds.

> + 1. Assert: |stream|.[=ReadableStream/[[controller]]=] [=implements=]
+    {{ReadableByteStreamController}}.
+ 1. Let |byobRequest| be !
+    [$ReadableByteStreamControllerGetBYOBRequest$](|stream|.[=ReadableStream/[[controller]]=]).
+ 1. If |byobRequest| is non-null, and |view|.\[[ViewedArrayBuffer]].\[[ArrayBufferData]] is the same
+    [=Data Block=] as |byobRequest|.\[[view]].\[[ViewedArrayBuffer]].\[[ArrayBufferData]], then:
+  1. Assert: |view|.\[[ByteOffset]] is |byobRequest|.\[[view]].\[[ByteOffset]].
+  1. Assert: |view|.\[[ByteLength]] ≤ |byobRequest|.\[[view]].\[[ByteLength]].
+     <p class="note">These asserts ensure that the caller does not write outside the requested
+     range in the [=ReadableStream/current BYOB request view=].
+  1. If |view|.\[[ViewedArrayBuffer]] equals |byobRequest|.\[[view]].\[[ViewedArrayBuffer]], then
+     perform ? [$ReadableByteStreamControllerRespond$](|stream|.[=ReadableStream/[[controller]]=],
+     |view|.\[[ByteLength]]).
+  1. Otherwise, perform ?
+     [$ReadableByteStreamControllerRespondWithNewView$](|stream|.[=ReadableStream/[[controller]]=],
+     |view|).

I handled the case where you transfer the backing Data Block here, which goes a bit beyond https://github.com/w3c/webtransport/pull/297#discussion_r661130619 but seems like a good idea.

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