fromFetch throws AbortError

[strout]

Bug Report

Current Behavior
fromFetch seems to fail with AbortError when I would expect it to succeed.

Reproduction

• REPL or Repo link:
  Here's an example using fromFetch(...) and from(fetch(...)) that I would expect to have the same result.

This code throws an AbortError:

fromFetch('https://jsonplaceholder.typicode.com/todos/1')
  .pipe(switchMap(resp => resp.json()))
  .subscribe(json => console.log(json));

Expected behavior
The snippit above should complete and log the response to the console.

Environment

• Runtime: Chrome 74, Firefox 66
• RxJS version: 6.5.1

Possible Solution
In the definition of fromFetch(), don't call controller.abort() if it has already emitted a value. Perhaps set a flag (reuse/rename the unsubscribed flag) before calling subscriber.next.

[felixfbecker]

@cartant how does this happen? Shouldn't the AbortError be ignored if the Observable unsubscribed?

[cartant]

It aborts the promise returned by resp.json(), IIRC. I think I put more info in the related PR.

[felixfbecker]

So in that code sample provided in the issue description, why would that matter?

[cartant]

Try the StackBlitz and you'll see. Sometimes it works and sometimes it fails. Bottom line is that abort should not be called upon the normal unsubscription that follows completion. See also #4744.