Re: fsmonitor on linux is triggered due to unison changes its fingerprint cache

Hello T=C3=B5ivo,

thanks!

On Friday 03 October 2025, T=C3=B5ivo Leedj=C3=A4rv wrote:
> On Mon, 29 Sept 2025 at 21:17, Ulrich Gemkow
> <[email protected]> wrote:
> >
> > - When I exclude the _whole_ directory ~/.unison/homesync with
> >   an ignore-Statement like
> >
> >   ignore =3D Path .../.unison/homesync
> >
> >   then the 1-second open-modify-close cycle does not happen, the
> >   fsmonitor does not wake up and my watch tool shows no events.
>=20
> This is to be expected. The fsmonitor implicitly knows about ignored
> paths because Unison only asks it to monitor non-ignored sub-dirs.
>=20
> > So when ignoring only the files, the problem happens. When excluding
> > the whole directory with the files inside there is no problem. This
> > seems to be a little strange - I really hope that I made no mistake.
> >
> > I tried to find the reason in the source but failed.
> >
> > Has anyone an idea what unison does every second with its fp-File
> > when the directory where it lives is not ignored? Does a bell ring?
>=20
> It's a feedback loop caused by a slight inefficiency in Unison's code.
> The inefficiency being that Unison writes to the fingerprint cache
> file at every scan, even if it has nothing new to write to the cache.
> This causes an inotify event, which causes a new scan, which writes
> the cache file, which causes an inotify event, ...
>=20
> There is no tmp file or rename(2) involved. The issue is that the
> cache file is truncated and a preamble written at every scan. This
> causes the MODIFY inotify event.

Thanks for the explanation, this explains the behavior.

But the question remains why unison performs a scan when it should
only "watch" using the fsmonitor. And why every second.

> I'm considering improving this to skip writing the cache file if there
> is nothing new to add to the cache.

Thanks again and best regards

Ulrich

=2D-=20
Ulrich Gemkow
University of Stuttgart
Institute of Communication Networks and Computer Engineering (IKR)