Re: [HACKERS] Cutting initdb's runtime (Perl question embedded)

Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> writes:
> Tom Lane wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>>> Who says we need a portable way? If we had something that worked on
>>> Linux and macOS, it would cover most developer environments. I wonder
>>> if readlink("/etc/localtime", buf, sz) might be a viable approach.

>> I wondered about that, but I'm afraid it's often a hardlink not a
>> symlink. Still, we could try it.

> In Debian systems, it's a symlink. Apparently in RHEL6 and older it's a
> copy or hardlink, and the file /etc/sysconfig/clock contains a ZONE
> variable that points to the right zone.

Yeah, on my RHEL6 box,

$ ls -l /etc/localtime
-rw-r--r--. 1 root root 3519 May 4 2010 /etc/localtime

The mod date's a little astonishing, considering this system was built
in 2013. It is *not* a hardlink, or even an exact match, to anything
under /usr/share/zoneinfo, though perhaps it was originally. Also:

$ cat /etc/sysconfig/clock
# The time zone of the system is defined by the contents of /etc/localtime.
# This file is only for evaluation by system-config-date, do not rely on its
# contents elsewhere.
ZONE="America/New York"

I'm inclined to respect the comment, especially since I see they are not
spelling the zone name canonically anyway (note space not underscore);
so looking at this wouldn't work without some ill-defined heuristics.

However, more modern Red Hat systems seem to have /etc/localtime as
a symlink, so checking it would work there, and also macOS seems to
do it that way for as far back as I can check.

> This comment is insightful:
> https://github.com/HowardHinnant/date/issues/269#issuecomment-353792132
> It's talking about this code:
> https://github.com/HowardHinnant/date/blob/master/src/tz.cpp#L3652

Interesting. Not sure if we want to try all those files. But I'll
take a look at whipping up something that checks /etc/localtime.

regards, tom lane