Additional source to guess the local timezone ID on linux

[Omair Majid]

Hi,

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"

With this, we should be able to identify the exact timezone ID.

Thanks,
Omair
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