bug#55635: `make-decoded-time' incorrectly sets DST to nil, it should be -1 (guess)

[Paul Eggert]

On 5/27/22 03:40, Lars Ingebrigtsen wrote:
> > This looks wrong. Shouldn't it leave the DST flag alone? I.e., just this:
> >
> >    (unless (decoded-time-zone time)
> >      (setf (decoded-time-zone-time) default-zone))
> >
> > That is, if we assume that for the DST component -1 means "unknown"
> > and nil means "standard time", it should be OK for
> > decoded-time-set-defaults to leave the DST component alone, for the
> > same reason that it leaves the DOW component alone.

> Yes, I think so.  But you changed this in a391ffa2f03, and you usually
> have a good reason for changes like this, so I thought there must be
> something subtle going on here I didn't quite get.  😀

Thanks for the compliment, not sure it's deserved here....


> The old code doesn't look quite right, either, I think...
>
> -  ;; When we don't have a time zone and we don't have a DST, then mark
> -  ;; it as unknown.
> -  (when (and (not (decoded-time-zone time))
> -             (not (decoded-time-dst time)))
> -    (setf (decoded-time-dst time) -1))
> -
> -  (when (and (not (decoded-time-zone time))
> -             default-zone)
> -    (setf (decoded-time-zone time) 0))

Yes, that old code was wrong because it incorrectly assumeed that (not 
(decoded-time-dst time)) means the DST flag is unspecified, whereas it 
really means that the DST flag is specifying standard time.

It also looked odd because default-zone was used only as a boolean, even 
though its name suggests that it's the default time zone. This usage 
dates back to commit fa648a59c9818ae284209ac7ae4f3700aebd92c9 which you 
installed in July 2019. The only call using default-zone in Emacs is in 
newsticker--decode-iso8601-date, which passes 0 so that the oddity in 
the implementation makes no difference there.

Part of the confusion here is that nil doesn't mean "no time zone is 
known"; it means "use the Emacs default time zone". In other words, nil 
has the same interpretation problem in time zones that it has in DST 
flags - it doesn't mean "unknown".

To try to lessen the confusion I installed the attached, which fixes the 
reported bug so I'll close the bug report. Please feel free to revert if 
you see a problem with it (I'm just trying to save time here by being bold).

0001-decoded-time-set-defaults-now-leaves-DST-alone.patch
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