[fpc-devel] Problem with Now() and time changed by ntpd

[Michael Van Canneyt]

On Tue, 1 Nov 2011, zeljko wrote:

> 
> 
> > We'll simply need to store the next moment when the DST correction changes,
> 
> > compare the result of gettimeofday with that and re-base the time
> 
> > calculation. If we decide to add some check for the timestamp of the
> 
> > timezone file - that would make Date(), Time() and Now() VERY expensive
> 
> > operations.
> 
> 
> IMO, better correct and slow than incorrect and fast (especially in this case , since 24/7 apps compiled with fpc aren't 24/7 apps anymore).
> 
> 
> At least this must be documented in fpc until it's fixed or provided some kind of workaround, eg that our 24/7 service (or any other app - doesn't matter
> at all) can trigger rtl from time to time to read tzdata.
> 
> 
> eg. imagine bool FpReadTZDataNextTime: Boolean = False; (global variable).
> 
> when we call fpgettimeofday() we check if FpReadTZDataNextTime is false/true.

It will be something like that, yes. I was thinking of introducing a

CheckRefreshTZInfoInterval: integer;

-1 : never check for refresh (current behaviour)
0  : refresh on each call (like LibC, will be new default)
>0 : Only refresh every N [yourfavouriteinterval].

But of course not in fpgettimeofday, in EpochToLocalTime. I repeat:
fpgettimeofday returns UTC time, it does not need to read timezone info.

It is the step which converts this time to local time that must re-read the timezone files if needed.
(EpochToLocalTime, the equivalent of localtime_r in libc)

However, meanwhile:

1. You can force a re-calculation of DST with the call (unix unit):

   GetLocalTimezone(fptime);


This will not refresh the TZ data from file, but will re-calculate the DST settings.

2. You can force a re-read of the timezone file with (unix unit):

   ReadTimezoneFile(GetTimezoneFile);

Then you must still do the recalculation of DST.
These calls were already present in the RTL.

3. A more simple mechanism is using the (newly added) ReReadTimeZone (unix unit):

   ReReadTimeZone;

Which will re-read TZ and re-calculate DST settings all in one go.
I added this in revision 19569. The idea is that the new 'automation' 
mechanism will call this routine.

Michael.