1504471 - Support tee streams with cloneForBranch2 (structured cloning)

Status: NEW

(Core :: JavaScript Engine, enhancement, P3)

Description

[Jason Orendorff [:jorendorff]]

The standard contains this cryptic note:

> Note: In this standard ReadableStreamTee is always called with
> `cloneForBranch2` set to **false**; other specifications pass **true**.

I don't know what "other specifications" uses this feature, so I'm not sure if it should be a priority or not. Seems easy to implement--mostly testing.

Comment 1

[Jason Orendorff [:jorendorff]]

I filed this because our code contains this cryptic note:

    // TODO: add StructuredClone() intrinsic.
    MOZ_ASSERT(!teeState->cloneForBranch2(), "tee(cloneForBranch2=true) should not be exposed");

https://searchfox.org/mozilla-central/rev/50ba1dd30cf013bddce1ae756f1b3c95b26f0628/js/src/builtin/Stream.cpp#1216-1217

The deal is, it is in fact impossible (currently) for the `cloneForBranch2` argument to `ReadableStreamTee` to be `true`, so this assertion is fine, and the not-very-helpful comment can be changed to link to this bug.