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

From: Julian Büning <[email-masked]>
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