improperly teeing streams in .clone()

[KhafraDev]

#rs-tee (webstreams) (this is how ReadableStream.prototype.tee is implemented):

Return ? ReadableStreamTee(this, false).

#concept-body-clone (fetch):

Let « out1, out2 » be the result of teeing body’s stream.

"teeing" links to #readablestream-tee (webstreams) (how .clone() tees the streams):

return ? ReadableStreamTee(stream, true).

... which has the note:

Because we pass true as the second argument to ReadableStreamTee, the second branch returned will have its chunks cloned ... from those of the first branch. This prevents consumption of one of the branches from interfering with the other.

Originally posted by @KhafraDev in #5318 (comment)

We need access to ReadableStreamTee from node core. https://github.com/nodejs/node/blob/4219808375c022da1defb4e402fb785f91c6ed50/lib/internal/webstreams/readablestream.js#L1696

const c = new AbortController();

const r = await fetch(
  new URL("/", globalThis.location?.href ?? "https://example.com/"),
  { signal: c.signal },
);

const r2 = r.clone(); // Clone it.

c.abort();

console.log((await r2.text()).slice(0, 100)); // This shouldn't throw

[mcollina]

Can you articulate a bit better how you'd expose it? Maybe open an issue or PR from core?

[KhafraDev]

I have no idea how I would honestly

In this standard ReadableStreamTee is always called with cloneForBranch2 set to false; other specifications pass true via the tee wrapper algorithm.

[mcollina]

I don't even understand the issue.

[KhafraDev]

The code in the OP throws where it shouldn't because ReadableStream.prototype.tee does not clone the chunks, meaning that by closing/erroring the first branch, the second branch is also closed/errored. In the spec, teeing in .clone() calls ReadableStreamTee(stream, true), but the way we are cloning invokes ReadableStreamTee(stream, false).

Because we pass true as the second argument to ReadableStreamTee, the second branch returned will have its chunks cloned (using HTML’s serializable objects framework) from those of the first branch. This prevents consumption of one of the branches from interfering with the other.