md5sum, sha*sum, sum: line-buffer the printed checksums

* src/md5sum.c (main): Set stdout to line buffered mode
to ensure parallel running instances don't intersperse
their output.  This adds 5% to the run time in the worst case
of many zero length files, or 2% with standard file sizes.
* src/sum.c (main): Likewise.
* tests/misc/md5sum-parallel: New test for atomic output.
* tests/Makefile.am: Reference it.
* NEWS: Mention the fix

1 files changed, 4 insertions, 0 deletions

diff --git a/src/sum.c b/src/sum.c
index 91d7f34ba..f0e0cc09a 100644
--- a/src/sum.c
+++ b/src/sum.c
@@ -233,6 +233,10 @@ 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);
+
   have_read_stdin = false;
 
   while ((optc = getopt_long (argc, argv, "rs", longopts, NULL)) != -1)