tail: flush initial output before possibly blocking

* src/tail.c (main): Flush any output from tail_file,
before calling tail_forever_inotify, which can block.
* tests/tail-2/flush-initial: New file.  Test for the bug.
* tests/Makefile.am (TESTS): Add tail-2/flush-initial.
* 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

@@ -27,6 +27,11 @@ GNU coreutils NEWS                                    -*- outline -*-
   because ls must stat every file in order to obtain a guaranteed-valid
   inode number.  [bug introduced in coreutils-6.0]
 
+  tail -f (inotify-enabled) now flushes any initial output before blocking.
+  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.
+
 ** New features
 
   cp --reflink accepts a new "auto" parameter which falls back to

src/tail.c

@@ -1399,7 +1399,6 @@ tail_forever_inotify (int wd, struct File_spec *f, size_t n_files,
       if (fflush (stdout) != 0)
         error (EXIT_FAILURE, errno, _("write error"));
     }
-
 }
 #endif
 
@@ -1990,6 +1989,12 @@ main (int argc, char **argv)
             error (0, errno, _("inotify cannot be used, reverting to polling"));
           else
             {
+              /* Flush any output from tail_file, now, since
+                 tail_forever_inotify flushes only after writing,
+                 not before reading.  */
+              if (fflush (stdout) != 0)
+                error (EXIT_FAILURE, errno, _("write error"));
+
               tail_forever_inotify (wd, F, n_files, sleep_interval);
 
               /* The only way the above returns is upon failure.  */

tests/Makefile.am

@@ -427,6 +427,7 @@ TESTS =						\
   rmdir/t-slash					\
   tail-2/assert-2				\
   tail-2/big-4gb				\
+  tail-2/flush-initial				\
   tail-2/proc-ksyms				\
   tail-2/start-middle				\
   touch/dangling-symlink			\

tests/tail-2/flush-initial

@@ -0,0 +1,41 @@
+#!/bin/sh
+# inotify-based tail -f didn't flush its initial output before blocking
+
+# 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
+
+. $srcdir/test-lib.sh
+
+fail=0
+echo line > in || fail=1
+stdbuf --output=1K tail -f in > out &
+tail_pid=$!
+
+# Wait for the backgrounded `tail' to start.
+while :; do
+  env kill -0 $tail_pid && break
+  sleep .1
+done
+
+test -s out || fail=1
+
+kill $tail_pid
+
+Exit $fail