xargs: wait for process before prompting in interactive mode (-p)

In interactive mode, there is a race between xargs prompting
about the next command to be run, vs. the execution of the
echo command:

  $ echo 1 2 | xargs -n 1 -p
  /bin/echo 1 ?...y
  /bin/echo 2 ?...1
  y
  2

This behavior violates POSIX:
(http://pubs.opengroup.org/onlinepubs/9699919799/utilities/xargs.html):

  The xargs utility shall then invoke the constructed command line
  and wait for its completion. This sequence shall be repeated until
  one of the following occurs: [...]

* xargs/xargs.c (xargs_do_exec): Move the code for waiting for forked
processes up, so that the child process terminates before we prompt
for the next command.
* NEWS: Mention the fix.

Reported by jidanni@jidanni.org in
http://lists.gnu.org/archive/html/bug-findutils/2013-07/msg00015.html

1 files changed, 8 insertions, 7 deletions

diff --git a/xargs/xargs.c b/xargs/xargs.c
index d773bde6..5e373f2b 100644
--- a/xargs/xargs.c
+++ b/xargs/xargs.c
@@ -1179,15 +1179,16 @@ xargs_do_exec (struct buildcmd_control *ctl, void *usercontext, int argc, char *
   (void) argc;
   (void) usercontext;
 
+  if (proc_max)
+    {
+      while (procs_executing >= proc_max)
+        {
+          wait_for_proc (false, 1u);
+        }
+    }
+
   if (!query_before_executing || print_args (true))
     {
-      if (proc_max)
-	{
-	  while (procs_executing >= proc_max)
-	    {
-	      wait_for_proc (false, 1u);
-	    }
-	}
       if (!query_before_executing && print_command)
 	print_args (false);