id: avoid unnecessary buffer flushing

* src/groups.c (main):
* src/id.c (main, print_stuff):
Don’t flush stdout before testing for write error.
Do the test only when in a loop, as a one-shot will
test for write error soon anyway.

NEWS

@@ -22,7 +22,7 @@ GNU coreutils NEWS                                    -*- outline -*-
 
 ** Improvements
 
-  'groups' and 'id' will now exit immediately upon receiving a write error,
+  'groups' and 'id' will now exit sooner after a write error,
   which is significant when listing information for many users.
 
   'nl' now supports multi-byte --section-delimiter characters.

src/groups.c

@@ -135,7 +135,7 @@ main (int argc, char **argv)
             ok = false;
           putchar ('\n');
 
-          if (fflush (stdout) < 0)
+          if (ferror (stdout))
             write_error ();
         }
     }

src/id.c

@@ -277,6 +277,8 @@ main (int argc, char **argv)
               ruid = euid = pwd->pw_uid;
               rgid = egid = pwd->pw_gid;
               print_stuff (pw_name);
+              if (ferror (stdout))
+                write_error ();
             }
           free (pw_name);
         }
@@ -458,7 +460,4 @@ print_stuff (char const *pw_name)
     {
       putchar (opt_zero ? '\0' : '\n');
     }
-
-  if (fflush (stdout) < 0)
-    write_error ();
 }