GDBserver: Fix ignored Ctrl-C after reconnection

This fixes the issue reported by Dmitry Antipov <dantipov@nvidia.com>
here:
  https://sourceware.org/ml/gdb/2017-10/msg00048.html

The problem is that GDBserver stops listening to Ctrl-C/interrupt
requests if you disconnect and reconnect back.

Dmitry wrote:

~~~
Currently gdbserver installs SIGIO handler just once, in
initialize_async_io() called from captured_main(), and this handler is
removed when remote_desc is closed in remote_close().  Next, when a
new instance of remote_desc is fetched from accept() and has '\003'
arrived, input_interrupt() is never called because it is not
registered as SIGIO handler.
~~~

The fix here is not remove the SIGIO handler in the first place, thus
going back to the original before-first-connection state.

(I haven't gone back to try it, but I think this was a regression
caused by commit 8b20733984 ("[GDBserver] Block and unblock SIGIO"),
which was what made remote_close remove the signal handler.)

New test included.

gdb/gdbserver/ChangeLog:
2017-11-16  Pedro Alves  <palves@redhat.com>

	* remote-utils.c (remote_close): Block SIGIO signals instead of
	uninstalling the SIGIO handler.

gdb/testsuite/ChangeLog:
2017-11-16  Pedro Alves  <palves@redhat.com>

	* gdb.server/reconnect-ctrl-c.c: New file.
	* gdb.server/reconnect-ctrl-c.exp: New file.

gdb/gdbserver/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-16  Pedro Alves  <palves@redhat.com>
+
+	* remote-utils.c (remote_close): Block SIGIO signals instead of
+	uninstalling the SIGIO handler.
+
 2017-11-16  Alan Hayward  <alan.hayward@arm.com>
 
 	* tdesc.c (tdesc_get_features_xml): Allow null osabi.

gdb/gdbserver/remote-utils.c

@@ -378,10 +378,7 @@ remote_close (void)
 {
   delete_file_handler (remote_desc);
 
-#ifndef USE_WIN32API
-  /* Remove SIGIO handler.  */
-  signal (SIGIO, SIG_IGN);
-#endif
+  disable_async_io ();
 
 #ifdef USE_WIN32API
   closesocket (remote_desc);

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2017-11-16  Pedro Alves  <palves@redhat.com>
+
+	* gdb.server/reconnect-ctrl-c.c: New file.
+	* gdb.server/reconnect-ctrl-c.exp: New file.
+
 2017-11-16  Phil Muldoon  <pmuldoon@redhat.com>
 
 	* gdb.python/py-rbreak.exp: New file.

gdb/testsuite/gdb.server/reconnect-ctrl-c.c

@@ -0,0 +1,28 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2017 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/>.  */
+
+#include <unistd.h>
+
+int
+main ()
+{
+  int secs = 30;
+
+  while (secs--)
+    sleep (1);
+  return 0;
+}

gdb/testsuite/gdb.server/reconnect-ctrl-c.exp

@@ -0,0 +1,72 @@
+# This testcase is part of GDB, the GNU debugger.
+#
+# Copyright 2017 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/>.
+
+# Test that Ctrl-C works after reconnecting.
+
+load_lib gdbserver-support.exp
+
+if { [skip_gdbserver_tests] } {
+    verbose "skipping gdbserver tests"
+    return -1
+}
+
+standard_testfile
+if [prepare_for_testing "failed to prepare" $testfile $srcfile] {
+    return -1
+}
+
+# Make sure we're disconnected, in case we're testing with an
+# extended-remote board, therefore already connected.
+with_test_prefix "preparation" {
+  gdb_test "disconnect" ".*"
+}
+
+# Connect, continue, send Ctrl-C and expect a SIGINT stop.
+
+proc connect_continue_ctrl_c {} {
+    global gdbserver_protocol gdbserver_gdbport
+
+    set res [gdb_target_cmd $gdbserver_protocol $gdbserver_gdbport]
+    if ![gdb_assert {$res == 0} "connect"] {
+	return
+    }
+
+    set test "continue for ctrl-c"
+    gdb_test_multiple "continue" $test {
+	-re "Continuing" {
+	    pass $test
+	}
+    }
+
+    after 1000 {send_gdb "\003"}
+    gdb_test "" "Program received signal SIGINT.*" "stop with control-c"
+}
+
+with_test_prefix "first" {
+    # Start GDBserver.
+    set gdbserver_reconnect_p 1
+    set res [gdbserver_start "" $binfile]
+    set gdbserver_protocol [lindex $res 0]
+    set gdbserver_gdbport [lindex $res 1]
+
+    connect_continue_ctrl_c
+}
+
+with_test_prefix "second" {
+    gdb_test "disconnect" "Ending remote debugging."
+    connect_continue_ctrl_c
+}