Add "../lib/unbuffer_output.c" and use it in gdb.base/interrupt.c

In some scenarios, GDB or GDBserver can be spawned with input _not_
connected to a tty, and then tests that rely on stdio fail with
timeouts, because the inferior's stdout and stderr streams end up
fully buffered.

See discussion here:
  https://sourceware.org/ml/gdb-patches/2015-02/msg00809.html

We have a hack in place that works around this for Windows testing,
that forces every test program to link with an .o file that does
(lib/set_unbuffered_mode.c):

 static int __gdb_set_unbuffered_output (void) __attribute__ ((constructor));
 static int
 __gdb_set_unbuffered_output (void)
 {
   setvbuf (stdout, NULL, _IONBF, BUFSIZ);
   setvbuf (stderr, NULL, _IONBF, BUFSIZ);
 }

That's a bit hacky; it ends up done for _all_ tests.

This patch adds a way to do this unbuffering explicitly from the test
code itself, so it is done only when necessary, and for all
targets/hosts.  For starters, it adjusts gdb.base/interrupt.c to use
it.

Tested on x86_64 Fedora 20, native, and against a remote gdbserver
board file that connects to the target with ssh, with and without -t
(create pty).

gdb/testsuite/
2015-02-27  Pedro Alves  <palves@redhat.com>

	* lib/unbuffer_output.c: New file.
	* gdb.base/interrupt.c: Include "../lib/unbuffer_output.c".
	(main): Call gdb_unbuffer_output.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2015-02-27  Pedro Alves  <palves@redhat.com>
+
+	* lib/unbuffer_output.c: New file.
+	* gdb.base/interrupt.c: Include "../lib/unbuffer_output.c".
+	(main): Call gdb_unbuffer_output.
+
 2015-02-27  Yao Qi  <yao.qi@linaro.org>
 
 	* gdb.base/catch-syscall.exp: Don't skip it on hppa*-hp-hpux*

gdb/testsuite/gdb.base/interrupt.c

@@ -3,6 +3,8 @@
 #include <unistd.h>
 #include <stdlib.h>
 
+#include "../lib/unbuffer_output.c"
+
 #ifdef SIGNALS
 #include <signal.h>
 
@@ -17,6 +19,9 @@ main ()
 {
   char x;
   int nbytes;
+
+  gdb_unbuffer_output ();
+
 #ifdef SIGNALS
   signal (SIGINT, sigint_handler);
 #endif

gdb/testsuite/lib/unbuffer_output.c

@@ -0,0 +1,39 @@
+/* Copyright (C) 2008-2015 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Force output to unbuffered mode if not connected to a terminal.  */
+
+#include <stdio.h>
+#ifndef __MINGW32__
+#include <unistd.h>
+#endif
+
+static int
+gdb_unbuffer_output (void)
+{
+  /* Always force this for Windows testing.  To a native Windows
+     program running under a Cygwin shell/ssh, stdin is really a
+     Windows pipe, thus not a tty and its outputs ends up fully
+     buffered.  */
+#ifndef __MINGW32__
+  if (!isatty (fileno (stdin)))
+#endif
+    {
+      setvbuf (stdout, NULL, _IONBF, BUFSIZ);
+      setvbuf (stderr, NULL, _IONBF, BUFSIZ);
+    }
+}