Re: [musl] Time zone has not updated after call tzset()

[Rich Felker <dalias@libc.org>]

On Sat, Feb 11, 2023 at 03:10:40PM +0100, alice wrote:
> On Sat Feb 11, 2023 at 7:53 AM CET, zhoujingqiang (A) wrote:
> > Hello,
> >
> > Normally, /etc/localtime is a soft link to a file that stores time zone
> > information.
> >
> > Without setting the TZ environment variable, I change the time zone by
> > changing the file linked to /etc/localtime. After calling tzset(), I find that
> > the time zone does not change. The test code is as follows:
> 
> musl does not support changing the timezone while running.
> see https://www.openwall.com/lists/musl/2017/06/09/9 ,
> for a response to an identical bug report
> 
> tl;dr without semantics: you have to restart a running process to get a new
> timezone.

This is not quite accurate. It does, but only via application intent,
in the form of changing its value of TZ. It does not re-scan files if
the application doesn't do that, for two important reasons:

1. Without explicit action by the application, there is no way to
   synchronize changes to the timezone with application consumption of
   the timezone. Operations like localtime, [some adjustment], mktime
   will give erroneous results under a race where the zone changes
   between the calls. (One might suggest using calls to tzset to
   synchronize, but POSIX specifies that all the functions which use
   time zone behave as if they implicitly called tzset, so this seems
   to be forbidden, and it wouldn't be thread-safe anyway.)

2. Re-scanning the file on disk every time an operation using the time
   zone is performed results in abysmal performance. This is a
   bottleneck for lots of programs that do any kind of logging, and if
   I recall it was the topic of a longstanding, user-infuriating bug
   report against glibc.

Rich