tail -f: handle "-"/stdin once again

* src/tail.c (main) [HAVE_INOTIFY]: When stdin (i.e., "-", or no args,
but not /dev/stdin) is specified on the command line, don't use inotify.
Reported by Bill Brelsford in <http://bugs.debian.org/545422>.
* tests/tail-2/follow-stdin: New file.  Test for this.
* tests/Makefile.am (TESTS): Add the test.
* NEWS (Bug fixes): Mention it.
This bug was introduced in coreutils-7.5 via commit ae494d4b,
2009-06-02, "tail: use inotify if it is available".

NEWS

@@ -31,6 +31,15 @@ GNU coreutils NEWS                                    -*- outline -*-
   Before, this would print nothing and wait: stdbuf -o 4K tail -f /etc/passwd
   Note that this bug affects tail -f only when its standard output is buffered,
   which is relatively unusual.
+  [bug introduced in coreutils-7.5]
+
+  tail -f once again works with standard input.  inotify-enabled tail -f
+  would fail when operating on a nameless stdin.  I.e., tail -f < /etc/passwd
+  would say "tail: cannot watch `-': No such file or directory", yet the
+  relatively baroque tail -f /dev/stdin < /etc/passwd would work.  Now, the
+  offending usage causes tail to revert to its conventional sleep-based
+  (i.e., not inotify-based) implementation.
+  [bug introduced in coreutils-7.5]
 
 ** New features
 

THANKS

@@ -78,6 +78,7 @@ Bernhard Rosenkraenzer              bero@redhat.de
 Bernhard Voelker                    bernhard.voelker@siemens-enterprise.com
 Bert Deknuydt                       Bert.Deknuydt@esat.kuleuven.ac.be
 Bert Wesarg                         bert.wesarg@googlemail.com
+Bill Brelsford                      wb@k2di.net
 Bill Peters                         peters@gaffel.as.arizona.edu
 Bjorn Helgaas                       helgaas@rsn.hp.com
 Bob McCracken                       kerouac@ravenet.com

src/tail.c

@@ -1982,7 +1982,19 @@ main (int argc, char **argv)
   if (forever)
     {
 #if HAVE_INOTIFY
-      if (!disable_inotify)
+      /* If the user specifies stdin via a command line argument of "-",
+         or implicitly by providing no arguments, we won't use inotify.
+         Technically, on systems with a working /dev/stdin, we *could*,
+         but would it be worth it?  Verifying that it's a real device
+         and hooked up to stdin is not trivial, while reverting to
+         non-inotify-based tail_forever is easy and portable.  */
+      bool stdin_cmdline_arg = false;
+
+      for (i = 0; i < n_files; i++)
+        if (STREQ (file[i], "-"))
+          stdin_cmdline_arg = true;
+
+      if (!disable_inotify && !stdin_cmdline_arg)
         {
           int wd = inotify_init ();
           if (wd < 0)

tests/Makefile.am

@@ -428,6 +428,7 @@ TESTS =						\
   tail-2/assert-2				\
   tail-2/big-4gb				\
   tail-2/flush-initial				\
+  tail-2/follow-stdin				\
   tail-2/proc-ksyms				\
   tail-2/start-middle				\
   touch/dangling-symlink			\

tests/tail-2/follow-stdin

@@ -0,0 +1,41 @@
+#!/bin/sh
+# tail -f - would fail with the initial inotify implementation
+
+# Copyright (C) 2009 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/>.
+
+if test "$VERBOSE" = yes; then
+  set -x
+  tail --version
+fi
+
+. $srcdir/test-lib.sh
+
+echo line > exp || framework_failure
+echo line > in || framework_failure
+
+fail=0
+timeout 1 tail -f < in > out 2> err
+
+# tail from coreutils-7.5 would fail
+test $? = 124 || fail=1
+
+# Ensure there was no error output.
+test -s err && fail=1
+
+# Ensure there was
+compare out exp || fail=1
+
+Exit $fail