[tz] Timezone change detection

[Edward Tomasz Napierała]

On 0903T1340, Paul Eggert wrote:
> On 9/3/21 9:53 AM, Edward Tomasz Napierała via tz wrote:
> > The use case is to make the applications handle
> > moving between timezones, or changing the default system timezone.
> 
> Does the use case include installing new timezone tables when a new tzdb 
> release comes out? Or is it to handle only the case where /etc/timezone 
> changes because the system has moved or has changed its default timezone 
> for other reasons?

The former - it compares both the device/inode numbers, and file
modification/change times.  It also handles a case where there's
a changed symlink in the zone file path.

> Won't the change break applications like 'zdump' that call 'localtime' 
> several times and assume that all the 'localtime' calls return 
> consistent values for a single timezone? Perhaps it would be better to 
> do change detection only when tzset is called explicitly; this would be 
> less likely to break zdump etc.

That's a really good point.  The mechanism is not compiled in by default,
so it is already kind of opt-in, but yeah, I see the problem.  I need
to think about it some more.

> A smaller question: when opening the TZif file for the first time, 
> wouldn't it be more efficient (and avoid a race) to do open+fstat 
> instead of stat+open?

I believe that's how it works right now in FreeBSD version of localtime.c.