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

[Florian Weimer <fweimer at redhat dot com>]

* 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.)
>
> Commentary like this:
>
> https://blog.packagecloud.io/eng/2017/02/21/set-environment-variable-save-thousands-of-system-calls/
>
> says that polling is a bad idea, and gives TZ=':/etc/localtime' as a
> workaround for glibc. I'm sympathetic, as the polling makes glibc look
> bad compared to the competition.

On the other hand, for files which we do not poll for changes (such as
/etc/resolv.conf previously, and /etc/gai.conf and /etc/nsswitch.conf
presently), we regularly receive requests to add such polling.  For
nsswitch.conf, this is clearly documented in the manual page, and yet
people still ask for it.

Thanks,
Florian