Re: [musl] Broken mktime calculations when crossing DST boundary

[Rich Felker <dalias@libc.org>]

On Mon, Mar 25, 2024 at 06:57:49PM +0000, Alexander Weps wrote:
> I am not sure which one you mean, all latest codes even includes
> headers and main...

https://www.openwall.com/lists/musl/2024/03/25/3

> I have no idea what to tell you.

The first version I found that's actually compilable is:

https://www.openwall.com/lists/musl/2024/03/25/11

It roughly behaves as expected on musl, except possibly not applying
the tm_isdst=0, which is what was making the output confusing on
glibc -- that threw the input back across the rule change cutoff.

With tm_isdst=1 and tm_mday=31, on glibc, I get:

before: 2011-12-31 00:00:00 WSDT 0
after1: 2011-12-31 00:00:00 WSDT 1325239200
after2: 2011-12-30 00:00:00 WSDT -1
after3: 2011-12-31 00:00:00 WSDT 1325239200

The -1 in the after2 line indicates that mktime failed with an error
(and should not have modified tm; that's arguably a bug in glibc). The
partial modification that it made reflects the initial normalization
(type 1 in my notation) but not the rule change normalization (type 2
in my notation) since glibc has failed the operation for an input date
that does not exist on the calendar (it does not do type 2
normalization at all; it just rejects it).

Running this same change on musl, I get:

before: 2011-12-31 00:00:00  0
after1: 2011-12-31 00:00:00 +14 1325239200
after2: 2011-12-29 00:00:00 -10 1325152800
after3: 2011-12-29 00:00:00 -10 1325152800

which again is what I expect. From one side, the move-by-1-day changes
the time to the next calendar day in that direction. From the other
side, it's unable to change it.

I'll look into why the tm_isdst=0 application was not happening.

Rich






> On Monday, March 25th, 2024 at 19:53, Rich Felker <dalias@libc.org> wrote:
> 
> > On Mon, Mar 25, 2024 at 06:28:14PM +0000, Alexander Weps wrote:
> >
> > > On Monday, March 25th, 2024 at 19:02, Rich Felker dalias@libc.org wrote:
> > >
> > > > On Mon, Mar 25, 2024 at 09:42:53AM -0400, Rich Felker wrote:
> > > >
> > > > > On Mon, Mar 25, 2024 at 01:24:57PM +0000, Alexander Weps wrote:
> > > > >
> > > > > > See below.
> > > > > >
> > > > > > AW
> > > > > >
> > > > > > On Monday, March 25th, 2024 at 14:13, Rich Felker dalias@libc.org wrote:
> > > > > >
> > > > > > > On Mon, Mar 25, 2024 at 12:55:28PM +0000, Alexander Weps wrote:
> > > > > > >
> > > > > > > > > If you take your test program and switch it to initialize with
> > > > > > > > > tm_mday=31, then do -=1 instead of +=1, you'll find that it gives
> > > > > > > > > 2011-12-29 01:00:00 -10 as well, only now it seems like the correct,
> > > > > > > > > expected thing to happen. Any change to "fix" the case you're
> > > > > > > > > complaining about would necessarily break this case.
> > > > > > > >
> > > > > > > > So (- day, +day):
> > > > > > > >
> > > > > > > > Musl:
> > > > > > > > 2011-12-31 01:00:00 +14
> > > > > > > > 2011-12-29 01:00:00 -10
> > > > > > > > 2011-12-29 01:00:00 -10
> > > > > > > >
> > > > > > > > Glibc:
> > > > > > > > 2012-01-01 01:00:00 +14
> > > > > > > > 2011-12-31 01:00:00 +14
> > > > > > > > 2012-01-01 01:00:00 +14
> > > > > > > >
> > > > > > > > Seems like musl doesn't even interpret the initial struct tm
> > > > > > > > correctly in that case. It is off by day.
> > > > > > > >
> > > > > > > > Because December only had 30 days, 31s day after normalization is
> > > > > > > > January 1st.
> > > > > > >
> > > > > > > This is nonsense. December has a day 31, which you can clearly see
> > > > > > > from the glibc output. For this particular year in this zone, with the
> > > > > > > zone rule change, there are "only 30 days" in December, but they are
> > > > > > > numbered 1-29 and 31, not 1-30.
> > > > > >
> > > > > > You confuse day of month which is represented in tm_mday with
> > > > > > calendar day that is interpreted by strftime.
> > > > > >
> > > > > > You said to set tm_mday = 31, which would be January 1st after normalization.
> > > > > > December 31s is 30th day of month represented as tm_mday = 30.
> > > > >
> > > > > OK, I meant tm_mday=31-1.
> > > >
> > > > Um, no, where did you get that idea? I just assumed you were right
> > > > because I always forget which tm_* are off-by-1, but tm_mday is
> > > > one-based not zero-based:
> > > >
> > > > int tm_mday; // day of the month -- [1, 31]
> > > >
> > > > (per the standard). So how did you end up getting the wrong thing? Are
> > > > you even running the code you say you are?
> > >
> > > I have to sincerely ask if you are feeling ok?
> > > You seem not able to follow this conversation.
> > >
> > > What idea do you mean?
> > > Also you have the codes. You can like "I don't know" run them yourself?
> > > You question I run those codes without trying to run them yourself? Again?!
> > > What is going on?
> >
> >
> > The first few pieces of code you posted did not work because they
> > depended on other code you did not include, so I stopped trying to run
> > them.
> >
> > > Maybe I reiterate some basic facts for you and that will put you
> > > back on track.
> > >
> > > This was an example from an article provided earlier in this thread (by somebody).
> > > We are in TZ=Pacific/Apia.
> > > The 30th December was skipped in 2011. There was no December 30th.
> > > So, there were only 30 days in December.
> > > 30th day of the month December was December 31st.
> > >
> > > And run those examples yourself. I have no idea why I am being
> > > questioned if they generate the output when you can easily verify it
> > > yourself.
> >
> >
> > Which piece of self-contained, actually-runnable code would you like
> > me to look at that demonstrates something wrong? (i.e. not something I
> > have already said is behaving as expected)