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

2 files changed, 8 insertions, 2 deletions

diff --git a/src/md5sum.c b/src/md5sum.c
index aa2a144a7..b7db03ecd 100644
--- a/src/md5sum.c
+++ b/src/md5sum.c
@@ -513,7 +513,6 @@ digest_check (const char *checkfile_name)
               if (!status_only)
                 {
                   printf (_("%s: FAILED open or read\n"), filename);
-                  fflush (stdout);
                 }
             }
           else
@@ -539,7 +538,6 @@ digest_check (const char *checkfile_name)
                     printf ("%s: %s\n", filename, _("FAILED"));
                   else if (!quiet)
                     printf ("%s: %s\n", filename, _("OK"));
-                  fflush (stdout);
                 }
             }
         }
@@ -619,6 +617,10 @@ 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);
+
   while ((opt = getopt_long (argc, argv, "bctw", long_options, NULL)) != -1)
     switch (opt)
       {
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)