bug#19765: tail -F stops watching when read permissions are removed

[Stephane Chazelas]

When watchnig a file by name with "tail -F", if read permissions
are removed, tail stops watching even though it has a file
descriptor open on the file.

With inotify:

$ : > file
$ tail -F file &
[1] 20796
$ exec 3>> file
$ echo 1 >&3
1
$ chmod 0 file
tail: %                                                                         
                                                                               
cannot watch ‘file’: Permission denied
tail: ‘file’ has become inaccessible: Permission denied
$ echo 2 >&3  ## not detected at this point
$ chmod +r file
tail: ‘file’ has become accessible
# new content not displayed yet.
$ echo 3 >&3
1   ## all lines displayed
2
3

Without inotify:

$ : > file
$ tail -F ---disable-inotify file &
[1] 20903
$ exec 3>> file
$ echo 1 >&3
1
$ chmod 0 file
$ echo 2 >&3
2  # not detected yet
$ tail: ‘file’ has become inaccessible: Permission denied

$ echo 3 >&3
$ chmod +r file
$ tail: ‘file’ has become accessible
1
2
3

(same except there's a delay before tail detects the file is no longer
readable).

Note that the file in that case is still accessible, one can
still do a stat() on it to check that the file is still the same
one. That's different from when one of the directory components
becomes unreadable/unsearchable, in which case tail can't tell
if it's still reading the right file as in:

   tail -F foo/bar &
   chmod 0 foo

There, tail still has an open file descriptor to foo/bar, but
can't tell if it still points to the "foo/bar" file, so it's
acceptable for it to stop watching in that case.

With inotify though, it doesn't unless the file attributes are
changed (chmod...) or the file is renamed. I think I'll raise a
separate bug report for that and directory components being
renamed.

[tested with git head]

-- 
Stephane