Additional source to guess the local timezone ID on linux

[Omair Majid]

Hi David,

Thanks for looking at the patch!

On 08/25/2013 09:59 PM, David Holmes wrote:
> On 24/08/2013 7:09 AM, Omair Majid wrote:
>> The algorithm that OpenJDK uses to guess the local timezone ID on Linux
>> goes like this:
>>
>> 1. If TZ environment variable is set, use that
>> 2. If /etc/timezone is readable, read the time zone from there
>> 3. If /etc/localtime is a symlink, resolve that, and use the name to
>> guess the time zone.
>> 4. Scan /usr/share/zoneinfo for a file whose contents match the contents
>> of /etc/localtime.
>>
>> Step 4 (if it is ever reached) is probably going to lead to incorrect
>> results since there are a number of timezones that have the same
>> zoneinfo data (such as Europe/London and Europe/Belfast). So it seems
>> sensible to me to try and use additional sources to guess the timezone
>> ID before resorting to file content comparisons.
>>
>> The webrev adds a step between 2 and 3 that reads and parses
>> /etc/sysconfig/clock to extract the timezone:
>>
>> http://cr.openjdk.java.net/~omajid/webrevs/timezone-read-sysconfig-clock/00/
>>
>>
>> This file exists on some Red Hat Enterprise Linux (and derivative)
>> distributions and contains contents that look this:
>>
>>> # 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="Europe/Zurich"
> 
> Surely the implication here is that if this file exists and has this
> data then /etc/localtime should be correctly configured - hence we
> should use that. ie at best this should come after step 3 not before it.

The systems that I was looking at where /etc/sysconfig/clock exists,
/etc/localtime is not a symlink. So step 3 is doesn't actually do
anything. Anyway, I can move this to after step 3 - I have no issues
with that.

> But even then when someone writes "do not rely on its
> contents elsewhere" I'm inclined to do what they suggest!

Yes, I would prefer to not use this file either, but Petr Machata (who
maintains these timezone related packages in Fedora) tells me it's the
closest thing to an authoritative source. In fact, on an update of the
timezone packages, a script already parses /etc/sysconfig/clock and
copies the zoneinfo file over to /etc/localtime [1].

Oh, and I forgot to mention that this is not a problem on newer
systemd-using Linux distributions (including Arch, Fedora and Gentoo)
where /etc/localtime is always a symlink. They do not carry this
/etc/sysconfig/clock file.

Thanks,
Omair

[1] https://bugzilla.redhat.com/show_bug.cgi?id=186742#c8

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