merge() does not propagate iteration end to the children iterators

[dko-slapdash]

Example code:

import delay from "delay";
import merge from "mergeiterator";

async function* iterable(name: string, dt: number) {
  try {
    for (let i = 0; ; i++) {
      console.log(`${name}: ${i}`);
      yield `${name}: ${i}`;
      await delay(dt);
    }
  } finally {
    console.log(`Exited ${name}`);
  }
}

async function* caller() {
  //yield* iterable("A", 900);
  yield* merge(iterable("A", 900));
}

async function main() {
  for await (const message of caller()) {
    if (message.includes("4")) {
      break;
    }

    console.log(`Received ${message}`);
  }

  console.log("Finishing");
  await delay(3000);
}

main().catch((e) => console.log(e));

In this example I do a "dummy" merging of 1 iterable for simplicity (but we can merge multiple, the effect persists). The output is:

A: 0
Received A: 0
A: 1
Received A: 1
A: 2
Received A: 2
A: 3
Received A: 3
A: 4
Finishing

Notice that finally {} block in iterable() function was never executed. But it should: replace the call to merge() with yield* iterable("A", 900); to see the correct output (with "Exited A"):

A: 0
Received A: 0
A: 1
Received A: 1
A: 2
Received A: 2
A: 3
Received A: 3
A: 4
Exited A
Finishing

How it works: both for-await and yield* instructions call the source iterator's return() method once the loop is over, and they propagate that signal further up the stack. I think merge() just doesn't do this.

[vadzim]

@dko-slapdash hi, sorry for delay, will check