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

@domenic commented on this pull request.



> +          if (forBranch2 === true) {
+            if (canceled1 === false) {
+              const clonedChunk = CloneArrayBufferView(chunk);
+              ReadableByteStreamControllerEnqueue(branch1._controller, clonedChunk);
+            }
+            if (canceled2 === true) {
+              chunk = new Uint8Array(chunk.buffer, chunk.byteOffset, 0);
+            }
+            ReadableByteStreamControllerRespondWithNewView(branch2._controller, chunk);
+          } else {
+            if (canceled2 === false) {
+              const clonedChunk = CloneArrayBufferView(chunk);
+              ReadableByteStreamControllerEnqueue(branch2._controller, clonedChunk);
+            }
+            if (canceled1 === true) {
+              chunk = new Uint8Array(chunk.buffer, chunk.byteOffset, 0);

Thanks so much for the analysis and walking me through it!

> This also means that every underlying byte source implementation must do something like this in its cancel algorithm:

This seems like a significant footgun, especially given that none of the [example byte sources in the spec](https://streams.spec.whatwg.org/#example-rbs-push) do so.

Can we make this easier somehow? E.g.:

- Automatically introspect `byobRequest.view` and if it's transferred, call (the equivalent of) `respondWithNewView(constructedView)`, and if it's not transferred, call `respond(0)`?

- Relax the requirement that the last BYOB read should resolve with `value: emptyChunk`?

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