Assertion failure in NudgeToCalendarUnit with duration rounding calculations adjacent to 24-hour forward UTC shift

[ptomato]

Discovered thanks to Firefox fuzzing: https://bugzilla.mozilla.org/show_bug.cgi?id=2029455

Reproducer:

const relativeTo = Temporal.ZonedDateTime.from('2012-01-01T12:00:00[Pacific/Apia]');
const d = Temporal.Duration.from({ days: -1 });
d.round({ smallestUnit: 'days', relativeTo })
d.total({ unit: 'days', relativeTo });

const end = relativeTo.add(d);
relativeTo.until(end, { smallestUnit: 'days' })

All of these method calls (round, total, until) fail the assertion in NudgeToCalendarUnit, step 13: "Assert: startEpochNs ≠ endEpochNs." without which there would be a division by zero in step 14.

As we are well aware, there's a 24-hour UTC forward shift in the time zone Pacific/Apia, from 2011-12-29T23:59:99.999999 to 2011-12-31T00:00. The issue is that when determining the nudge window for rounding, even though both ends of the duration are outside of the time zone shift, the far bound of the nudge window (i.e. counting backwards 1 day from 2011-12-31T12:00) is inside it. So the far bound gets moved forward 1 day, putting it at the same instant as the near bound. The assertion assumes a zero-length nudge window should not be possible.

I'm not 100% sure what the results of the above calls should actually be. We should figure that out first, and then add test262 tests and adjust the assertion accordingly, since it is invalid.

[ptomato]

cc @arshaw, you might have some opinions about what to do here

[ptomato]

Looking at it more closely than I did last week, I'm pretty sure that "-1 day" is the only sensible answer to the three calls above.

[ptomato]

Test262: tc39/test262#5044

I created test262 tests with what I believe would be the sensible output to these calls. However, we may decide to turn them into errors, I guess?

Results with current Firefox: -2 days for round/until/since, 0 for total (in a release build, that is, where the assertion isn't hit)
Results with current V8: throws Error: Temporal error: Internal error: . for everything

[ptomato]

Found a related but different assertion failure, in step 6.a.ii of NudgeToCalendarUnit, by running this relativeTo through snapshot tests:

const relativeTo = Temporal.ZonedDateTime.from('2012-01-01T12:00:00[Pacific/Apia]');
const d2 = new Temporal.Duration(0, 0, 0, 0, -25);
d2.round({ smallestUnit: "days", relativeTo })
  // assertion failure, should be -1 day
d2.total({ unit: "days", relativeTo })
  // assertion failure, should be -1 and 1/48???

(and since/until with similar results to round)

The fix that I had in mind for the original issue, does not fix this second issue.

Results with current Firefox: -2 days, and -2.041666... (-2 and 1/24) respectively
Results with current V8: throws Error: Temporal error: Internal error: . for everything

[justingrant]

Could the solultion be to extend the nudge window by one day until it's nonzero length?

[ptomato]

I've been playing around with things in that direction, but I think it's probably more complicated than that.