Python for Windows reports wrong local time when run under Cygwin on Europe/Moscow TZ

Ken Brown kbrown@cornell.edu
Thu Jun 10 19:50:12 GMT 2021


On 6/10/2021 2:31 PM, Brian Inglis wrote:
> On 2021-06-10 08:57, Ken Brown via Cygwin wrote:
>> On 6/9/2021 10:36 PM, Brian Inglis wrote:
>>> On 2021-06-09 16:31, Keith Thompson via Cygwin wrote:
>>>> [Sorry if the threading is messed up.  I don't subscribe, so I'm
>>>> constructing this message from the web interface.  It should at least
>>>> show up under the correct subject.]
>>>>
>>>> Brian Inglis wrote:
>>>>> On 2021-06-08 14:03, Mike Kaganski via Cygwin wrote:
>>>>>> On 08.06.2021 16:04, L A Walsh wrote:
>>>>>>> You might ask on a python list if anyone else has experienced
>>>>>>> something similar with python or any other program.  I'm fairly sure
>>>>>>> that neither MS nor cygwin design their OS with python in mind and
>>>>>>> that it is python that is interacting funny when running under some
>>>>>>> merge of both.  Have you asked the python people about this problem?
>>>>>>> What did they suggest?
>>>>>>
>>>>>> FTR: filed https://bugs.python.org/issue44352.
>>>>>
>>>>> See Keith Thompson subthread and my reply with suggested fix:
>>>>>
>>>>> https://cygwin.com/pipermail/cygwin/2021-June/248692.html
>>>>>
>>>>> Windows does not recognize zoneinfo time zone identifiers in TZ only
>>>>> base format POSIX TZ strings with three alphabetic character identifiers:
>>>>>
>>>>> https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/tzset?view=msvc-160 
>>>>>
>>>>>
>>>>> That assumes US switch date "rules": for all years up to current, or
>>>>> just DST, and whether pre- or post-2007 is unstated!
>>>>>
>>>>> Otherwise it defaults to regional settings, used by Cygwin to map to
>>>>> zoneinfo time zone identifiers, so if Python for Windows could clear TZ
>>>>> before it is read by MSVCRT, it should DTRT.
>>>>>
>>>>> Windows does not recognize expanded POSIX TZ format strings with <>
>>>>> quoted alphanumeric characters, "-", "+", and start and end dates/times:
>>>>>
>>>>> https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#bottom 
>>>>>
>>>>>
>>>>> which make them usable outside of the US.
>>>>
>>>> Summary: IMHO Cygwin should adapt its default TZ setting to work
>>>> with Windows.
>>>>
>>>> The suggestion is to modify Python for Windows so it can deal with
>>>> the TZ format used by Cygwin.  I haven't used Python for Windows, but
>>>> as far as I know it's unrelated to Cygwin; rather it, like Cygwin, is
>>>> intended to work on top of Windows.  I'm not convinced it's appropriate
>>>> to ask Python for Windows to make a change purely for the sake of
>>>> interoperating with Cygwin, which many PfW users presumably aren't
>>>> even using.
>>>>
>>>> I've run into another application that has problems with Cygwin's
>>>> settings of $TZ.  It was a internal test application that isn't
>>>> going to change its timezone handling just for this problem.
>>>>
>>>> The ideal solution would be for Windows to recognize TZ values like
>>>> "America/Los_Angeles", but that's not likely to happen any time soon.
>>>>
>>>> My suggestion, since Cygwin is supposed to interoperate with Windows,
>>>> is one of the following:
>>>>
>>>> - Cygwin should avoid setting TZ to a value that Windows doesn't recognize
>>>>    (if I set TZ=PST8PDT, everything seems to work correctly); OR
>>>>
>>>> - Cygwin shouldn't set TZ at all by default.  (I've updated my
>>>>    $HOME/.bash_profile on Cygwin to unset TZ, and Cygwin commands seem
>>>>    to work correctly with TZ unset); OR
>>>>
>>>> - Cygwin, when invoking a non-Cygwin executable, should first either
>>>>    unset TZ or translate it to a format that Windows will recognize.
>>>>    I have no idea how difficult that would be.
>>>
>>> Impossible to set Windows TZ usefully as it obeys unstated US DST rules (like 
>>> posixrules, perhaps 2007+?), and may have limits on hour offset magnitudes.
>>> MS libraries are stuck at POSIX 1996 and C 99 subset compatibility, but 
>>> non-standard-conformant including which headers contain definitions:
>>>
>>>      https://docs.microsoft.com/en-us/cpp/c-runtime-library/compatibility?view=msvc-160 
>>>
>>>
>>> It may be possible to unset TZ when running non-Cygwin programs (possibly 
>>> behind a CYGWIN env var setting e.g. winnotz) by adding TZ= to conv_envvars, 
>>> and writing new helper functions env_tz_to_posix to call tzset and 
>>> env_tz_to_win32 to remove TZ in:
>>>
>>>      https://sourceware.org/git/?p=newlib-cygwin.git;f=winsup/cygwin/environ.cc;a=blob 
>>>
>>>
>>> What is the opinion on this from both Windows users and Cygwin patchers?
>>
>> I'm not convinced it's worth the trouble.  I haven't seen anyone argue that 
>> it's useful for Cygwin to set TZ, and I have seen an argument that it's harmful:
>>
>>    https://cygwin.com/pipermail/cygwin/2017-May/232675.html .
>>
>> So I prefer Keith's second suggestion:
>>
>>  >> - Cygwin shouldn't set TZ at all by default.
> 
> It does so in default startup scripts

Right, and I'm agreeing with Bruno (in the message cited above) that the default 
startup scripts should stop doing that.

> to get the correct behaviour from Cygwin 
> DLL and programs,

Can you be more specific?  What goes wrong if TZ is not set?  I haven't seen any 
POSIX or Linux documentation that says it should be set, and I've just checked 
on two different Linux distros that it's not set by default.

Ken


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