Re: detect timezone changes by monitoring /etc/localtime (like glibc)

["A. Wilcox" <awilfox@adelielinux.org>]

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On 09/06/17 07:15, Rich Felker wrote:
> On Fri, Jun 09, 2017 at 05:15:40PM +1000, Patrick Oppenlander
> wrote:
>> During some recent testing I came across a bug when adjusting
>> timezones on an embedded system by changing /etc/localtime. The
>> cause ended up being a behavioural difference between glibc and
>> musl.
> 
> This difference is intentional; I believe there are past
> discussions in the list archives.
> 
> Aside from the glibc behavior giving abysmal performance (syscalls
> on every time operation), there's a more fundamental issue of
> usability of the results. A common important idiom with time
> functions is to perform several operations together in succession
> to get a result -- for example, obtain the current time as time_t,
> format it with localtime[_r], make some adjustment e.g. for a
> relative time, then call mktime to convert back to time_t. Such
> operations rely on the time zone being consistent for each
> suboperation, which is true as long as the application does not
> modify its own environment, but only assuming libc doesn't impose
> asynchronous changes on the timezone.


Fun and true facts: this has actually revealed itself as a horrible
behaviour in Pidgin.  When I moved from the east coast to the central
US, I woke up my laptop and changed the TZ.  It was running a glibc
Linux at that point.  Pidgin wouldn't make noise or show notifications
until an hour went by; some internal counter had a local time_t and it
thought it was receiving old notifications (since the timezone shifted
backwards an hour).

So this isn't a theoretical problem.  It's a very real one.  (And one
I have yet to experience running musl Linux on my laptop now.)

- --arw

- -- 
A. Wilcox (awilfox)
Project Lead, Adélie Linux
http://adelielinux.org
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