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bug#31225: undocumented change in tail in 8.28 (pipe output monitoring)

From: Dr. Thomas Orgis
Subject: bug#31225: undocumented change in tail in 8.28 (pipe output monitoring)
Date: Fri, 20 Apr 2018 16:56:23 +0200

Hi all,

during debugging of a background process issue where I have tail -f
feeding into a pipe, I was surprised by a change in tail introduced in
release 8.28 of GNU coreutils: The monitoring of the output pipe and
silent exit of tail e.g. in the case of

        tail -f /dev/null | true

. This differs from earlier versions and also from BSD, for example. I
see some subtle portability bugs here. One one platform, killing the
end of a pipe fed by tail will cleanly end the tail process, too,
without tail attempting to write new data and failing _then_. On the
other platform, the pipe stays there until new data arrives to cause
tail to get the error trying to write to the pipe.

Regardless of what behaviour is sensible, it causes scripts that
take GNU tail's behaviour for granted to have different behaviour with
other versions of this very traditional UNIX tool.

So at the very least, this change should be documented as a possible
implementation difference in the manual, and for sure with the version
that changed behaviour, shouldn't it?


Thomas Orgis

Dr. Thomas Orgis
Universität Hamburg
RRZ / Basis-Infrastruktur / HPC
Schlüterstr. 70
20146 Hamburg
Tel.: 040/42838 8826
Fax: 040/428 38 6270

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