Allow resetting an empty inferior-tty

This patch allows the user to set the inferior-tty to "empty", in order
to come back to the default behaviour of using the same tty as gdb is
using.

This is already supported in MI (and tested in gdb.mi/mi-basics.exp).

I added a new test, set-inferior-tty.exp, where I test only the setting
and unsetting of the parameter.  It would be nice to actually test that
the inferior output properly goes to the separate tty, but that will be
for another day.

gdb/ChangeLog:

	* infcmd.c (set_inferior_io_terminal): Set inferior terminal to
	NULL if terminal_name is an empty string.
	(_initialize_infcmd): Make the argument of "set inferior-tty"
	optional, mention it in the help doc.

gdb/doc/ChangeLog:

	* gdb.texinfo (Input/Output): Mention possibility to unset
	inferior-tty.

gdb/testsuite/ChangeLog:

	* gdb.base/set-inferior-tty.exp: New file.
	* gdb.base/set-inferior-tty.c: New file.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2016-08-24  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* infcmd.c (set_inferior_io_terminal): Set inferior terminal to
+	NULL if terminal_name is an empty string.
+	(_initialize_infcmd): Make the argument of "set inferior-tty"
+	optional, mention it in the help doc.
+
 2016-08-24  Carl Love  <cel@us.ibm.com>
 
 	* rs6000-tdep.c (rs6000_gdbarch_init): Remove call

gdb/doc/ChangeLog

@@ -1,3 +1,8 @@
+2016-08-24  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* gdb.texinfo (Input/Output): Mention possibility to unset
+	inferior-tty.
+
 2016-07-25  Tim Wiederhake  <tim.wiederhake@intel.com>
 
 	* gdb.texinfo: Resume btrace on reconnect.

gdb/doc/gdb.texinfo

@@ -2499,9 +2499,11 @@ display the name of the terminal that will be used for future runs of your
 program.
 
 @table @code
-@item set inferior-tty /dev/ttyb
+@item set inferior-tty [ @var{tty} ]
 @kindex set inferior-tty
-Set the tty for the program being debugged to /dev/ttyb.
+Set the tty for the program being debugged to @var{tty}.  Omitting @var{tty}
+restores the default behavior, which is to use the same terminal as
+@value{GDBN}.
 
 @item show inferior-tty
 @kindex show inferior-tty

gdb/infcmd.c

@@ -151,7 +151,11 @@ void
 set_inferior_io_terminal (const char *terminal_name)
 {
   xfree (current_inferior ()->terminal);
-  current_inferior ()->terminal = terminal_name ? xstrdup (terminal_name) : 0;
+
+  if (terminal_name != NULL && *terminal_name != '\0')
+    current_inferior ()->terminal = xstrdup (terminal_name);
+  else
+    current_inferior ()->terminal = NULL;
 }
 
 const char *
@@ -3224,14 +3228,16 @@ _initialize_infcmd (void)
   const char *cmd_name;
 
   /* Add the filename of the terminal connected to inferior I/O.  */
-  add_setshow_filename_cmd ("inferior-tty", class_run,
-			    &inferior_io_terminal_scratch, _("\
+  add_setshow_optional_filename_cmd ("inferior-tty", class_run,
+				     &inferior_io_terminal_scratch, _("\
 Set terminal for future runs of program being debugged."), _("\
 Show terminal for future runs of program being debugged."), _("\
-Usage: set inferior-tty /dev/pts/1"),
-			    set_inferior_tty_command,
-			    show_inferior_tty_command,
-			    &setlist, &showlist);
+Usage: set inferior-tty [TTY]\n\n\
+If TTY is omitted, the default behavior of using the same terminal as GDB\n\
+is restored."),
+				     set_inferior_tty_command,
+				     show_inferior_tty_command,
+				     &setlist, &showlist);
   add_com_alias ("tty", "set inferior-tty", class_alias, 0);
 
   cmd_name = "args";

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2016-08-24  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* gdb.base/set-inferior-tty.exp: New file.
+	* gdb.base/set-inferior-tty.c: New file.
+
 2016-08-23  Pedro Alves  <palves@redhat.com>
 
 	PR gdb/20494

gdb/testsuite/gdb.base/set-inferior-tty.c

@@ -0,0 +1,24 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2016 Free Software Foundation, Inc.
+
+   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/>.
+
+*/
+
+int
+main (void)
+{
+  return 0;
+}

gdb/testsuite/gdb.base/set-inferior-tty.exp

@@ -0,0 +1,40 @@
+# Copyright 2016 Free Software Foundation, Inc.
+
+# 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/>.
+
+standard_testfile
+
+set compile_options "debug"
+if {[build_executable $testfile.exp $testfile ${srcfile} ${compile_options}] == -1} {
+    untested "failed to compile $testfile"
+    return -1
+}
+
+proc test_set_inferior_tty { } {
+    global binfile
+
+    clean_restart ${binfile}
+
+    gdb_test_no_output "set inferior-tty hello" "set inferior-tty to hello"
+    gdb_test "show inferior-tty" \
+	     "Terminal for future runs of program being debugged is \"hello\"." \
+	     "show inferior-tty shows hello"
+
+    gdb_test_no_output "set inferior-tty" "set inferior-tty to empty"
+    gdb_test "show inferior-tty" \
+	     "Terminal for future runs of program being debugged is \"\"." \
+	     "show inferior-tty shows empty"
+}
+
+test_set_inferior_tty