wc: line-buffer the printed counts

* src/wc.c (main): Set stdout to line buffered mode
to ensure parallel running instances don't intersperse
their output.  This adds 6.5% to the run time in the worst case
of many zero length files, but has neglible impact for
standard sized files.
* tests/misc/wc-parallel: New test for atomic output.
* tests/Makefile.am: Reference it.
* NEWS: Mention the fix
This is similar to commit 710fe413, 20-10-2009,
"md5sum, sha*sum, sum: line-buffer the printed checksums"

4 files changed, 46 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index ac5bd0742..cad425407 100644
--- a/NEWS
+++ b/NEWS
@@ -17,6 +17,10 @@ GNU coreutils NEWS                                    -*- outline -*-
   adjusted, working around a bug in current Linux kernels.
   [bug introduced in coreutils-8.1]
 
+  wc now prints counts atomically so that concurrent
+  processes will not intersperse their output.
+  [the issue dates back to the initial implementation]
+
 
 * Noteworthy changes in release 8.2 (2009-12-11) [stable]
 
diff --git a/src/wc.c b/src/wc.c
index 52e899e97..48b5a4e39 100644
--- a/src/wc.c
+++ b/src/wc.c
@@ -598,6 +598,10 @@ main (int argc, char **argv)
 
   atexit (close_stdout);
 
+  /* Line buffer stdout to ensure lines are written atomically and immediately
+     so that processes running in parallel do not intersperse their output.  */
+  setvbuf (stdout, NULL, _IOLBF, 0);
+
   print_lines = print_words = print_chars = print_bytes = false;
   print_linelength = false;
   total_lines = total_words = total_chars = total_bytes = max_line_length = 0;
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 5e44202ba..93d4275cd 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -157,6 +157,7 @@ TESTS =						\
   misc/wc					\
   misc/wc-files0-from				\
   misc/wc-files0				\
+  misc/wc-parallel				\
   misc/cat-proc					\
   misc/cat-buf					\
   misc/base64					\
diff --git a/tests/misc/wc-parallel b/tests/misc/wc-parallel
new file mode 100755
index 000000000..7c74c5518
--- /dev/null
+++ b/tests/misc/wc-parallel
@@ -0,0 +1,37 @@
+#!/bin/sh
+# Ensure that wc prints counts atomically
+# so that concurrent processes don't intersperse their output
+
+# 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/>.
+
+. $srcdir/test-lib.sh
+
+if test "$VERBOSE" = yes; then
+  set -x
+  md5sum --version
+fi
+
+
+(mkdir tmp && cd tmp && seq 2000 | xargs touch)
+
+# This will output at least 16KiB per process
+# and start 3 processes, with 2 running concurrently,
+# which triggers often on Fedora 11 at least.
+(find tmp tmp tmp -type f | xargs -n2000 -P2 wc) |
+sed -n '/0 0 0 /!p' |
+grep . > /dev/null && fail=1
+
+Exit $fail