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

[Alexander Strasser]

Hi Anton!

On 2021-05-14 10:09 +0200, Anton Khirnov wrote:
> Quoting Alexander Strasser (2021-05-12 01:04:28)
> >
> > If the timezone data of an AMF 0 date type is non-zero, include that
> > information as UTC offset in hours and minutes.
> > ---
> >  libavformat/flvdec.c | 18 +++++++++++++++---
> >  1 file changed, 15 insertions(+), 3 deletions(-)
> >
> > diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> > index ddaceaafe4..c941e62e23 100644
> > --- a/libavformat/flvdec.c
> > +++ b/libavformat/flvdec.c
> > @@ -688,9 +688,21 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream,
> >              time =  date.milliseconds / 1000; // to seconds
> >              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);
> > +            strftime(datestr, sizeof(datestr), "%Y-%m-%dT%H:%M:%S", &t);
> > +
> > +            if (date.timezone) {
> > +                int off_tz = date.timezone; // offset in minutes
> > +                char ch_sign = '+';
> > +                if (off_tz < 0) {
> > +                    off_tz = -off_tz;
> > +                    ch_sign = '-';
> > +                }
> > +                if (off_tz > 99*60 + 59)
> > +                    off_tz = 99*60 + 59;
> > +
> > +                av_strlcatf(datestr, sizeof(datestr), "%c%02d%02d", ch_sign, off_tz / 60, off_tz % 60);
>
> I still believe this is wrong, since the spec says the timestamp is in
> UTC. The code you quoted seems to conform to that.

Not to my understanding and testing.

This Ruby program

    t1 = Time.now()
    t2 = Time.now.utc()
    print t1,   " - ", t1.to_f, "\n"
    print t2, "   - ", t2.to_f, "\n"

yields for example:

    2021-05-15 20:05:19 +0200 - 1621101919.509961
    2021-05-15 18:05:19 UTC   - 1621101919.509966


Returning to the code I quoted before now and stating my
understanding of if now.

    def write__AMF_date(time)
      write__UI8 11
      write [(time.to_f * 1000.0)].pack('G')
      write__SI16( (Time.now.gmtoff / 60).to_i )
    end

This writes the time in micro seconds without offset as double.
The GMT offset in minutes is written afterwards as signed 16 bit
integer.


    def read__AMF_date
      utc_time = Time.at((read__AMF_double / 1000).to_i)
      utc_time + (read__SI16 * 60) - Time.now.gmtoff
    end

This first reads the double and converts it into a Time object.
In the following line it reads and adds the stored offset and
subtracts the current offset to get rid of its influence.


  Alexander