[Vm-dev] classic VM: timezone correction in sqUnixUtcWithOffset()

[David T. Lewis]

Hi David,

Thanks for the fix and for the clear explanation of the problem. I pushed
your patch to SVN r3801:

  http://squeakvm.org/cgi-bin/viewvc.cgi/squeak/trunk/platforms/unix/vm/sqUnixMain.c?r1=3800&r2=3801

Dave


On Sun, Apr 25, 2021 at 08:09:56PM +0200, stes at PANDORA.BE wrote:
>  
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> 
> Hi,
> 
> Here's another fix for sqUnixMain.c in the SVN (subversion) repository.
> 
> This is for timezone correction in sqUnixUtcWithOffset().
> 
> Basically there is code for the Linux/glibc case which is correct:
> 
>  #if defined(HAVE_TM_GMTOFF)
>    *offset= localtime(&seconds)->tm_gmtoff;
>  #else
> 
> The above code is correct but it depends on tm_gmtoff (glibc) as on Linux.
> 
> The incorrect code is for platforms that use the plain standard C library,
> without the glibc extension (the #else clause).
> 
> The timezone (HAVE_TIMEZONE) is itself also extension which is enabled
> by default for Solaris because it is _XOPEN_SOURCE but not _STRICT_STDC.
> 
> - -  {
> - -    struct tm *local= localtime(&seconds);
> - -    struct tm *gmt= gmtime(&seconds);
> - -    int d= local->tm_yday - gmt->tm_yday;
> - -    int h= ((d < -1 ? 24 : 1 < d ? -24 : d * 24) + local->tm_hour - gmt->tm_hour);
> - -    int m= h * 60 + local->tm_min - gmt->tm_min;
> - -    *offset= m * 60;
> - -  }
> 
> The above is incorrect because localtime() and gmtime() will overwrite the
> same buffer, so the gmtime() call erases the value of localtime().
> 
> I have a patch in attach that uses the same logic as plugins/LocalePlugin/sqLocalePlugin.c and as in convertToSqueakTime() which is doing it right.
> 
> +#ifdef HAVE_TIMEZONE
> +  *offset = ((daylight) * 60 * 60) - timezone;
> +#else
> +#error: cannot determine timezone correction
>  #endif
> +#endif
>    return 0;
>  }
>  
> Please apply the patch in attach to subversion ...
> 
> I think it is safe to apply this patch because (1) it does not change
> the #if HAVE_TM_GMTOFF case (the Linux case is unchanged) and (2) it is
> consistent with plugins/LocalePlugin/sqLocalePlugin.c and convertToSqueakTime() and (3) the code with the difference between localtime() and gmtime() is certainly wrong.
> 
> Thanks,
> David Stes
> 
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