Re: [whatwg/streams] Support transferable streams (postMessage) (#1053)

@MattiasBuelens commented on this pull request.



> +    1. Set |backpressurePromise| to undefined.
+ 1. Add a handler for |port|'s {{MessagePort/messageerror}} event with the following steps:
+  1. Let |error| be a new "{{DataCloneError}}" {{DOMException}}.
+  1. Perform ! [$CrossRealmTransformSendError$](|port|, |error|).
+  1. Perform ! [$WritableStreamDefaultControllerError$](|controller|, |error|).
+  1. Disentangle |port|.
+ 1. Enable |port|'s [=port message queue=].
+ 1. Let |startAlgorithm| be an algorithm that returns undefined.
+ 1. Let |writeAlgorithm| be the following steps, taking a |chunk| argument:
+  1. If |backpressurePromise| is undefined, set |backpressurePromise| to
+     [=a promise resolved with=] undefined.
+  1. Return the result of [=reacting=] to |backpressurePromise| with the following
+     fulfillment steps:
+   1. Set |backpressurePromise| to [=a new promise=].
+   1. Let |promise| be ! [$PackAndPostMessageHandlingError$](|port|, "`chunk`", |chunk|).
+   1. If |promise| is a rejected promise, disentangle |port|.

Perhaps it was important that the port is disentangled synchronously? Not sure.

That said, I think a better way to solve this is to make `PackAndPostMessageHandlingError` throw an error instead of returning a rejected promise. That does mean we need to change all of the call sites to turn the abrupt completion into a rejected promise, but I think it's worth it. (There's only three of them.)

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