tail: fix checking of currently unavailable directories

* src/tail.c (tail_forever_inotify): Handle the case where
tail --follow=name with inotify, is not able to add a watch on
a specified directory.  This may happen due to inotify resource
limits or if the directory is currently missing or inaccessible.
In all these cases, revert to polling which will try to reopen
the file later.  Note inotify returns ENOSPC when it runs out
of resources, and instead we report a particular error message,
lest users think one of their file systems is full.
(main): Document another caveat with using inotify, where we
currently don't recheck directories recreated after the
initial watch is setup.
* tests/tail-2/F-vs-rename: Fix the endless loop triggered by
the above issue.
* tests/tail-2/inotify-hash-abuse: Likewise.
* tests/tail-2/wait: Don't fail in the resource exhaustion case.
* tests/tail-2/F-vs-missing: A new test for this failure mode
which was until now just triggered on older buggy linux kernels
which returned ENOSPC constantly from inotify_add_watch().
* NEWS: Mention the fix.

1 files changed, 49 insertions, 0 deletions

diff --git a/tests/tail-2/F-vs-missing b/tests/tail-2/F-vs-missing
new file mode 100755
index 000000000..8c1d01898
--- /dev/null
+++ b/tests/tail-2/F-vs-missing
@@ -0,0 +1,49 @@
+#!/bin/sh
+# demonstrate that tail -F works for currently missing dirs
+# Before coreutils-8.6, tail -F missing/file would not
+# notice any subsequent availability of the missing/file.
+
+# Copyright (C) 2010 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+test "$VERBOSE" = yes && tail --version
+
+debug='---disable-inotify'
+debug=
+tail $debug -F -s.1 missing/file > out 2>&1 & pid=$!
+
+check_tail_output()
+{
+  local delay="$1"
+  grep "$tail_re" out > /dev/null ||
+    { sleep $delay; return 1; }
+}
+
+# Wait up to 6.3s for tail to start with diagnostic:
+# tail: cannot open `missing/file' for reading: No such file or directory
+tail_re='cannot open' retry_delay_ check_tail_output .1 7 || fail=1
+
+mkdir missing || fail=1
+(cd missing && echo x > file)
+
+# Wait up to 6.3s for this to appear in the output:
+# "tail: `...' has appeared;  following end of new file"
+tail_re='has appeared' retry_delay_ check_tail_output .1 7 ||
+  { echo "$0: file: unexpected delay?"; cat out; fail=1; }
+
+kill -HUP $pid
+
+Exit $fail