[systemd-devel] odd seek_tail behaviour

[Christian Hesse]

Christian Hesse <list at eworm.de> on Mon, 2014/10/13 23:56:
> Christian Hesse <list at eworm.de> on Mon, 2014/10/13 20:22:
> > Daurnimator <quae at daurnimator.com> on Mon, 2014/10/13 01:27:
> > > Hi All,
> > > 
> > > I was trying to write a program that tailed the journal, but found that
> > > sd_journal_seek_tail() didn't work as expected.
> > > That is: that it would seek to the last/most recent thing in the
> > > journal, and I could tail things from there.
> > > 
> > > I whipped up a quick demonstration program, that shows that messages I
> > > 'next' through, are before the 'cutoff':
> > > 
> > > [code and output]
> > > 
> > > Is this behaviour expected? I'm using systemd 216.
> > 
> > I do see a similar problem in my code [0]. I do call sd_journal_previous()
> > after sd_journal_seek_tail(), but I still do see some older message come
> > up.
> > 
> > sd_journal_next() is the first I call in while loop. Perhaps even this is
> > a problem?
> > 
> > [0]
> > https://github.com/eworm-de/journal-notify/blob/master/journal-notify.c
> 
> Looks like I was right. For any reason sd_journal_next() can jump to old
> journal entries (even if sd_journal_previous() has been called before). That
> happens before sd_journal_wait() is called the first time.
> 
> Sadly I do not know how to reproduce. It happens very seldom and I could not
> find the culprit so far.

Actually the problem happens before. I have an old entry selected after
calling sd_journal_previous().

Mhh, does adding matches (sd_journal_add_match(),
sd_journal_add_conjunction() and sd_journal_add_disjunction()) have an effect
to sd_journal_seek_tail() and sd_journal_previous()?
Looks like the problem goes away if I seek to tail first, then add my matches.
-- 
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