Background execution + pagination aborts readline/gdb

If pagination occurs as result of output sent as response to a target
event while the target is executing in the background, subsequent
input aborts readline/gdb:

 $ gdb program
 ...
 (gdb) continue&
 Continuing.
 (gdb)
 ---Type <return> to continue, or q <return> to quit---
 *return*
 ---Type <return> to continue, or q <return> to quit---
 Breakpoint 2, after_sleep () at paginate-bg-execution.c:21
 ---Type <return> to continue, or q <return> to quit---
 21        return; /* after sleep */
 p 1
 readline: readline_callback_read_char() called with no handler!
 *abort/SIGABRT*
 $

gdb_readline_wrapper_line removes the handler after a line is
processed.  Usually, we'll end up re-displaying the prompt, and that
reinstalls the handler.  But if the output is coming out of handling
a stop event, we don't re-display the prompt, and nothing restores the
handler.  So the next input wakes up the event loop and calls into
readline, which aborts.

We should do better with the prompt handling while the target is
running (I think we should coordinate with readline, and
hide/redisplay it around output), but that's a more invasive change
better done post 7.8, so this patch is conservative and just
reinstalls the handler as soon as we're out of the readline line
callback.

gdb/
2014-07-14  Pedro Alves  <palves@redhat.com>

	PR gdb/17072
	* top.c (gdb_readline_wrapper_line): Tweak comment.
	(gdb_readline_wrapper_cleanup): If readline is enabled, reinstall
	the input handler callback.

gdb/testsuite/
2014-07-14  Pedro Alves  <palves@redhat.com>

	PR gdb/17072
	* gdb.base/paginate-bg-execution.c: New file.
	* gdb.base/paginate-bg-execution.exp: New file.

1 files changed, 32 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.base/paginate-bg-execution.c b/gdb/testsuite/gdb.base/paginate-bg-execution.c
new file mode 100644
index 00000000000..e741785e72d
--- /dev/null
+++ b/gdb/testsuite/gdb.base/paginate-bg-execution.c
@@ -0,0 +1,32 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2014 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>
+
+static void
+after_sleep (void)
+{
+  return; /* after sleep */
+}
+
+int
+main (void)
+{
+  sleep (3);
+  after_sleep ();
+  return 0;
+}