Re: [PATCH] Fix multiple minor tzset glitches [BZ #24004]

[Carlos O'Donell <codonell at redhat dot com>]

On 4/10/19 7:57 AM, Florian Weimer wrote:
> * Paul Eggert:
>
> > On 2/18/19 1:37 AM, Florian Weimer wrote:
> > > I thought this polling (for reloading /etc/localtime if it has changed)
> > > was a feature.  I'm not sure if we can remove it.
> >
> > No such polling is documented. On the contrary, the glibc manual says
> > that an unset TZ is treated as if it were set to
> > ':/etc/localtime'. There is no polling when you set
> > TZ=':/etc/localtime' (or to TZ='/etc/localtime', or to any other
> > value). So the code and the documentation disagree, and one or the
> > other needs to be fixed.
> >
> > Neither Solaris nor OpenBSD poll, so portable programs cannot assume
> > that polling occurs. (These are the only two other systems I checked.)
>
> It turns out that polling was implemented in response to this user bug
> report:
>
>    <https://sourceware.org/bugzilla/show_bug.cgi?id=154>
>
> I must say that the use case in the bug report is not unreasonable.

I agree that today we have *millions* of people travelling through timezones
and they may be adjusting localtime on their laptop continuously and the
timezone should be updated and reflected in the logs.

I think this continues to show the following trend in expectation:

* APIs are MT-safe.

* APIs dynamically respond to changing context.
  - Location.
  - Network.
  - Language.

Few if any developers are asking us to make APIs data read-once
and immutable after reading.

Timezone information is updated on average a dozen times a year,
that's a once-a-month restart that's required to get this data
if we don't internally reload that data.

I think there *have* to be ways in which this data can be dynamically
updated without a huge performance penalty.

I find the following arguments weak:
(1) Solaris nor OpenBSD Poll.
(2) Portable programs will have to poll.

The reason I find them weak is because they do not restrict us from
doing the best we can for our developers on GNU/Linux.

We can offer polling by default, and users who only care about GNU/Linux
can write less code, and save on the maintenance of doing polling
correctly (or having to pay the full cost of tzset()).

$0.02.

--
Cheers,
Carlos.