[FFmpeg-devel] [PATCH] lavf/flvdec: normalize exporting date metadata

[Anton Khirnov]

Quoting Marton Balint (2021-05-10 19:36:59)
> 
> 
> On Mon, 10 May 2021, Anton Khirnov wrote:
> 
> > Export them in UTC, not the local timezone. This way the output is
> > the same everywhere. The timezone information stored in the file is
> > still ignored, since there seems to be no simple way to export it
> > correctly.
> >
> > Format them according to ISO 8601, which we generally use for exporting
> > dates.
> > ---
> > If anyone has practical suggestions for exporting the timezone, I would
> > love to hear them. Don't see anything in the standard library for
> > "express this UTC timestamp in a given timezone". Just adding the offset
> > to the timestamp would AFAIU not be correct wrt leap seconds (and maybe
> > something else? dates are hard).
> >
> > Also, the conversion of double to time_t is potentially UB if the source
> > value is out of range, but there seems to be no standard way to check,
> > since the range of time_t is not standard either. Anyone got any ideas?
> > ---
> > libavformat/flvdec.c     | 7 +++++--
> > tests/ref/fate/flv-demux | 2 +-
> > 2 files changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> > index e6c2877a74..ddaceaafe4 100644
> > --- a/libavformat/flvdec.c
> > +++ b/libavformat/flvdec.c
> > @@ -686,8 +686,11 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream,
> >             struct tm t;
> >             char datestr[128];
> >             time =  date.milliseconds / 1000; // to seconds
> > -            localtime_r(&time, &t);
> > -            strftime(datestr, sizeof(datestr), "%a, %d %b %Y %H:%M:%S %z", &t);
> > +            gmtime_r(&time, &t);
> > +
> > +            // timezone is ignored, since there is no easy way to offset the UTC
> > +            // timestamp into the specified timezone
> > +            strftime(datestr, sizeof(datestr), "%Y-%m-%dT%H:%M:%SZ", &t);
> >
> >             av_dict_set(&s->metadata, key, datestr, 0);
> 
> Maybe avpriv_dict_set_timestamp() should be used instead. That way 
> milisecond precision can also be respected.

Huh, I did not know about that function. But since we are converting
from double, I am somewhat concerned about the result remaining the same
on all platforms (which is the point of this patch).

-- 
Anton Khirnov