Re: [whatwg/streams] Support teeing readable byte streams (#1114)

@ricea commented on this pull request.

I've checked the algorithms and cross-referenced against the reference implementation and it all looks good.

I haven't analysed it for re-entrancy risk. I'm a bit worried about that.

> +   1. Perform ! [$ReadableByteStreamControllerError$](|branch2|.[=ReadableStream/[[controller]]=],
+      |r|).
+   1. If |canceled1| is false or |canceled2| is false, [=resolve=] |cancelPromise| with undefined.
+ 1. Let |pullWithDefaultReader| be the following steps:
+  1. If |reader| [=implements=] {{ReadableStreamBYOBReader}},
+   1. Assert: |reader|.[=ReadableStreamBYOBReader/[[readIntoRequests]]=] is [=list/is empty|empty=].
+   1. Perform ! [$ReadableStreamReaderGenericRelease$](|reader|).
+   1. Set |reader| to ! [$AcquireReadableStreamDefaultReader$](|stream|).
+   1. Perform |forwardReaderError|, given |reader|.
+  1. Let |readRequest| be a [=read request=] with the following [=struct/items=]:
+   : [=read request/chunk steps=], given |chunk|
+   ::
+    1. [=Queue a microtask=] to perform the following steps:
+     1. Set |reading| to false.
+     1. Let |chunk1| and |chunk2| be |chunk|.
+     1. If |canceled1| is false and |canceled2| is false, set |chunk2| to ?

@domenic What actually happens if CloneAsUint8Array throws here, given that we're running as a microtask?

> +     1. If |otherCanceled| is false and
+        |otherBranch|.[=ReadableStream/[[controller]]=].[=ReadableByteStreamController/[[pendingPullIntos]]=]
+        is not [=list/is empty|empty=], perform !
+        [$ReadableByteStreamControllerRespond$](|otherBranch|.[=ReadableStream/[[controller]]=], 0).
+    1. If |byobCanceled| is false or |otherCanceled| is false, [=resolve=] |cancelPromise| with undefined.
+
+   : [=read-into request/error steps=]
+   ::
+    1. Set |reading| to false.
+  1. Perform ! [$ReadableStreamBYOBReaderRead$](|reader|, |view|, |readIntoRequest|).
+ 1. Let |pull1Algorithm| be the following steps:
+  1. If |reading| is true, return [=a promise resolved with=] undefined.
+  1. Set |reading| to true.
+  1. Let |byobRequest| be ! [$ReadableByteStreamControllerGetBYOBRequest$](|branch1|.[=ReadableStream/[[controller]]=]).
+  1. If |byobRequest| is null,
+   1. Perform |pullWithDefaultReader|.

Please put the action on the same line as the condition. I think @domenic already said this, but I can't find his comment.

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