Additional source to guess the local timezone ID on linux

[David Holmes]

Hi Omair,

I have reservations about this but will let others comment.

Looking at the code, readEntireFile doesn't close the file if it returns 
-ENOMEM. It also doesn't free the allocated memory if it returns -ENOMEM 
or via the goto fail path. (I'm not a fan of this goto fail style either.)

Thanks,
David

On 26/08/2013 1:18 PM, Omair Majid wrote:
> 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
>