1388050 - optimize consuming a ReadableStream with js "bytes" source

Status: NEW

(Core :: DOM: Core & HTML, enhancement, P3)

Description

[Ben Kelly [:bkelly, not reviewing]]

The streams spec allows js to specify that the underlying source of a ReadableStream has the type "bytes".  In this case the stream detaches the ArrayBuffer when it is read by the stream.

We should add an optimization path for native code consuming fetch body streams that have a js byte source.  This would let us essentially "steal" the underlying Uint8Array and just store it in an nsIInputStream internally.  This would avoid the need to copy from the Uint8Array to our nsPipe.

Note, chrome does not implement byte streams yet.  So for right now this is a nice-to-have, but we should not block on it.