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

[Pádraig Brady]

On 20/04/18 07:56, Dr. Thomas Orgis wrote:
> 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?
> 
> 
> Regards,
> 
> Thomas Orgis
> 
Well we did document it in NEWS as seen in:
https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=v8.27-42-gce0415f

I still regard tail being more responsive to closed output being an
improvement, and the more general solution, however it was a bit terse
and broached as only an improvement.

I pushed the following change to expand on the info:
https://git.sv.gnu.org/cgit/coreutils.git/commit/?id=22424dd

Marking this as done.

thanks,
Pádraig