Unhandled rejection event in web streams when stream error is piped

Version

21.1.0

Platform

Linux kunigunde 6.5.9-arch2-1 #1 SMP PREEMPT_DYNAMIC Thu, 26 Oct 2023 00:52:20 +0000 x86_64 GNU/Linux

Subsystem

streams/web

What steps will reproduce the bug?

Run the following code:

import { ReadableStream } from "stream/web";

process.on('unhandledRejection', (err) => {
	console.log(`Unhandled rejection: ${err.message}`);
});

const stream1 = new ReadableStream({
	start(controller) {
		controller.error(new Error('Test'));
	},
});

const stream2 = new TransformStream();

stream1.pipeTo(stream2.writable);

try {
	await stream2.readable.getReader().read();
} catch (err) {
	console.log(`Caught error: ${err.message}`);
}

How often does it reproduce? Is there a required condition?

With the given code example, it happens every time.

It does not happen in the following situations:

• Reading from stream1 directly rather than piping it to stream2
• Reading from stream2 but not catching the error but letting the app crash instead

What is the expected behavior? Why is that the expected behavior?

No unhandledRejection event should be triggered, since the exception is actually handled.

What do you see instead?

An unhandledRejection event is triggered. The output of the sample script is:

Caught error: Test
Unhandled rejection: Test

Workaround

I’m not sure if this has any unintended side effects, but catching the rejection by the promise returned by pipeTo() seems to solve the problem:

stream1.pipeTo(stream2.writable).catch(() => {});

Start by running the supplied reproduction and inspect the streams/web pipeTo path, focusing on how its rejection is propagated when the destination readable is consumed. Done means the caught stream error no longer triggers an unhandledRejection event, while the existing caught-error behavior remains intact.