GNU bug report logs - #24495
tail -F does not terminate when running out of names to watch

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Julian Büning <julian.buening <at> rwth-aachen.de>
Subject: bug#24495: closed (Re: bug#24495: tail -F does not terminate when
 running out of names to watch)
Date: Wed, 28 Sep 2016 22:44:02 +0000

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Your bug report

#24495: tail -F does not terminate when running out of names to watch

which was filed against the coreutils package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 24495 <at> debbugs.gnu.org.

-- 
24495: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=24495
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Pádraig Brady <P <at> draigBrady.com>
To: Bernhard Voelker <mail <at> bernhard-voelker.de>,
 Julian Büning <julian.buening <at> rwth-aachen.de>,
 24495-done <at> debbugs.gnu.org
Cc: Oscar Soria Dustmann <Oscar.SoriaDustmann <at> comsys.rwth-aachen.de>,
 Daniel Schemmel <daniel.schemmel <at> comsys.rwth-aachen.de>
Subject: Re: bug#24495: tail -F does not terminate when running out of names
 to watch
Date: Wed, 28 Sep 2016 23:43:26 +0100

On 28/09/16 22:37, Bernhard Voelker wrote:
> On 09/28/2016 03:27 PM, Pádraig Brady wrote:
>> The attached patch how has the preferred behavior option 2
>> both with and without inotify.
> 
> Great, this looks good to me.
> +1

Thanks for the reviews.
Pushed at:
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=v8.25-73-gf04daf5

Marking this as done

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From: Julian Büning <julian.buening <at> rwth-aachen.de>
To: <bug-coreutils <at> gnu.org>
Cc: Oscar Soria Dustmann <Oscar.SoriaDustmann <at> comsys.rwth-aachen.de>,
 Daniel Schemmel <daniel.schemmel <at> comsys.rwth-aachen.de>
Subject: tail -F does not terminate when running out of names to watch
Date: Wed, 21 Sep 2016 16:22:07 +0200

observed behavior:

$ mkdir foo
$ tail -F foo &
[1] 1000
tail: error reading 'foo': Is a directory
tail: foo: cannot follow end of this type of file; giving up on this name
$ rmdir foo ; echo moo > foo
$ jobs
[1]+  Running                 tail -F foo &

expected behavior option 1:

$ mkdir foo
$ tail -F foo &
tail: error reading 'foo': Is a directory
tail: foo: cannot follow end of this type of file; giving up on this name
$ rmdir foo ; echo moo > foo
[1]+  Done                 tail -F foo &

expected behavior option 2:

$ mkdir foo
$ tail -F foo &
[1] 1000
tail: error reading 'foo': Is a directory
tail: foo: cannot follow end of this type of file
$ rmdir foo ; echo moo > foo
tail: 'foo' has appeared;  following new file
moo
$ jobs
[1]+  Running                 tail -F foo &

tail does neither terminate nor display any contents of the file foo.
We would assume that either tail terminates after displaying the error
message (because there is no other file left) or that tail would
infinitely retry to read a file with the same name and if such a file
is created.

We could reproduce this behavior with versions 8.25 on ArchLinux and
8.25 and 8.25.71-1db94 on Fedora (package and compiled from source)
with and without ---disable-inotify. At least in version 8.25,
tail_forever_inotify is not executed because any_non_remote_file
returns false, which disables inotify in line 2361 (tail.c).

We can trace the cause of this behavior for the non-inotify version of
tail_forever:
For a directory, the ignore flag in the corresponding struct File_spec
is set to true. Thus, tail_forever skips this file in line 1130 (tail.c)
and does not modify this flag during any iteration of the enclosing
while(1). Since no check is being done if any valid target remains,
this leads to the observed non-terminating behavior.

This behavior was found using Symbolic Execution techniques developed in
the course of the SYMBIOSYS research project at COMSYS, RWTH Aachen
University.

Regards,
Julian Büning

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