Re: yesno module consumes too much input

[Bruno Haible]

Eric Blake wrote:
> the current implementation is correct even when stdin is still in the
> initial state.  However, if you want to optimize it further, such patches
> are worth considering.

Here is such a proposed patch.

2007-08-19  Bruno Haible  <address@hidden>

        * modules/closein (Depends-on): Add freadahead.
        * lib/closein.c: Include freadahead.h.
        (close_stdin): Skip the fseeko and fflush calls if freadahead (stdin)
        is zero.

--- modules/closein     12 Apr 2007 16:11:40 -0000      1.1
+++ modules/closein     19 Aug 2007 17:48:44 -0000
@@ -8,6 +8,7 @@
 
 Depends-on:
 closeout
+freadahead
 fflush
 stdbool
 
--- lib/closein.c       27 Apr 2007 17:14:40 -0000      1.3
+++ lib/closein.c       19 Aug 2007 17:48:44 -0000
@@ -32,6 +32,7 @@
 #include "closeout.h"
 #include "error.h"
 #include "exitfail.h"
+#include "freadahead.h"
 #include "quotearg.h"
 
 static const char *file_name;
@@ -80,10 +81,16 @@
 {
   bool fail = false;
 
-  /* Only attempt flush if stdin is seekable, as fflush is entitled to
-     fail on non-seekable streams.  */
-  if (fseeko (stdin, 0, SEEK_CUR) == 0 && fflush (stdin) != 0)
-    fail = true;
+  /* There is no need to flush stdin if we can determine quickly that stdin's
+     input buffer is empty; in this case we know that if stdin is seekable,
+     fseeko (stdin, 0, SEEK_CUR) == lseek (0, 0, SEEK_CUR).  */
+  if (freadahead (stdin) > 0)
+    {
+      /* Only attempt flush if stdin is seekable, as fflush is entitled to
+        fail on non-seekable streams.  */
+      if (fseeko (stdin, 0, SEEK_CUR) == 0 && fflush (stdin) != 0)
+       fail = true;
+    }
   if (close_stream (stdin) != 0)
     fail = true;
   if (fail)