Re: Strange ``tail -f'' behaviour - kqueue-related?

[Paul Goyette <paul%whooppee.com@localhost>]

On Thu, 10 Feb 2022, Paul Goyette wrote:

> I've been seeing some very odd behaviour lately, with ``tail -f''.
>
> I've got a bunch of packages being built (in a chroot sandbox), and
> occassionally I do a ``tail -f'' on the current package's log file
> just to make sure it is still making progress.  So the file is open
> by both the package-building shell and the tail process.
>
> Occassionally the tail process fails to notice any further changes
> to the file's size, and just sits there.  Indeed, it is now more
> than 30 minutes since a particular build completed (and the shell
> has long ago closed the log file), but the tail output is still
> stuck somewhere in the middle of the file.  (I've seen this a few
> times now, and while it happens again and again, it does not seem
> predictable on when it might happen.)
>
> I'm wondering if some of the recent changes to kqueue might be
> responsible.  I don't ever remember seeing this prior to my recent
> update from 9.99.92 to .93

Just to test this assumption, I checked with ``ps -o wchan'' and sure
enough the tail process is waiting on kqueue.

> I can keep the "hung" tail process around for a while in case of
> any diagnostic requests.


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