Passing date into subsequent delay operators seem to accumulate delay

[maxmumford]

Bug Report

Current Behavior
When using the delay operator multiple times, and each time passing a date object into it, subsequent delay operators seem to accumulate the date differential from previous delay operators. See reproduction url for a code example.

Reproduction

• REPL or Repo link:
  https://stackblitz.com/edit/typescript-rx-playground-4fssrc?file=index.ts

import { map, delay, tap, take } from 'rxjs/operators';
import { Observable, of, interval } from 'rxjs';

// console log every 1 second
interval(1000).pipe(
  take(17)
).subscribe(num => {
  if (num === 10) console.log(num + `: Expecting to see 'innitTimePlus10s' here...`);
  else console.log(num);
});

// save time on file init and then create date for 5 seconds and 10 seconds from init
const initTime = new Date();
const initTimePlus5s = new Date(initTime.getTime() + 5000);
const initTimePlus10s = new Date(initTime.getTime() + 10000);

// console log after 5 seconds after init, and again after 10 seconds _after init_
of(null).pipe(
  delay(initTimePlus5s),
  tap(() => console.log('initTimePlus5s')),
  delay(initTimePlus10s),
  tap(() => console.log('initTimePlus10s')),
).subscribe();

Expected behavior

/**
 * Expected result:
 * 
 * 0
 * 1
 * 2
 * 3
 * 4
 * 5
 * initTimePlus5s
 * 6
 * 7
 * 8
 * 9
 * 10
 * initTimePlus10s
 */

Environment

• Runtime: Chrome 79.0.3945.117
• RxJS version: 6.5.2

[cartant]

Yeah. This is a bug. You can see here that when the delay operator is used, a relative delay is calculated using the absolute time:

rxjs/src/internal/operators/delay.ts

Lines 60 to 65 in 56e7ceb

	export function delay<T>(delay: number|Date,
	scheduler: SchedulerLike = async): MonoTypeOperatorFunction<T> {
	const absoluteDelay = isDate(delay);
	const delayFor = absoluteDelay ? (+delay - scheduler.now()) : Math.abs(<number>delay);
	return (source: Observable<T>) => source.lift(new DelayOperator(delayFor, scheduler));
	}

However, that calculation doesn't - and cannot easily - account for other delay operators in the composed observable chain that are also passed a Date.

[maxmumford]

Yep as you say, not an easy fix - probably a complete rewrite of the operator to not use date differentials.

For anybody else following this, the workaround is pretty simple:

of(null).pipe(
  delay(initTimePlus5s),
  tap(() => console.log('initTimePlus5s')),
  delay(initTimePlus10s.getTime() - initTimePlus5s.getTime()),
  tap(() => console.log('initTimePlus10s')),
).subscribe();

Unfortunately it's very easy to be caught out by this bug.

Thanks for your help.

[ehaskins]

It also isn't clear from the documentation what the expected behavior of the delay to date feature is. Should the notifications be delayed until that time and then emitted ASAP, or should the timing between them be maintained?