Linux: make target_is_async_p return false when async is off

linux_nat_is_async_p currently always returns true, even when the
target is _not_ async.  That confuses
gdb_readline_wrapper/gdb_readline_wrapper_cleanup, which
force-disables target-async while the secondary prompt is active.  As
a result, when gdb_readline_wrapper returns, the target is left async,
even through it was sync to begin with.

That can result in weird bugs, like the one the test added by this
commit exposes.

Ref: https://sourceware.org/ml/gdb-patches/2015-01/msg00592.html

gdb/ChangeLog:
2015-01-23  Pedro Alves  <palves@redhat.com>

	* linux-nat.c (linux_is_async_p): New macro.
	(linux_nat_is_async_p):
	(linux_nat_terminal_inferior): Check whether the target can async
	instead of whether it is already async.
	(linux_nat_terminal_ours): Don't check whether the target is
	async.
	(linux_async_pipe): Use linux_is_async_p.

gdb/testsuite/ChangeLog:
2015-01-23  Pedro Alves  <palves@redhat.com>

	* gdb.threads/continue-pending-after-query.c: New file.
	* gdb.threads/continue-pending-after-query.exp: New file.

2 files changed, 138 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.threads/continue-pending-after-query.c b/gdb/testsuite/gdb.threads/continue-pending-after-query.c
new file mode 100644
index 0000000..9510ce8
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/continue-pending-after-query.c
@@ -0,0 +1,48 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2013-2015 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 <pthread.h>
+
+static int global;
+
+static void
+break_function (void)
+{
+  global = 42; /* set break here */
+}
+
+static void *
+thread_function (void *arg)
+{
+  break_function ();
+
+  return arg;
+}
+
+int
+main (void)
+{
+  pthread_t th;
+
+  pthread_create (&th, NULL, thread_function, NULL);
+
+  break_function ();
+
+  pthread_join (th, NULL);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.threads/continue-pending-after-query.exp b/gdb/testsuite/gdb.threads/continue-pending-after-query.exp
new file mode 100644
index 0000000..d4d50c9
--- /dev/null
+++ b/gdb/testsuite/gdb.threads/continue-pending-after-query.exp
@@ -0,0 +1,90 @@
+# This testcase is part of GDB, the GNU debugger.
+#
+# Copyright 2013-2015 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/>.
+
+# Regression test for a bug that would go like this:
+#
+# - Run to a breakpoint that is hit by two threads (A and B)
+#   simultaneously.
+#
+# - One of the breakpoint hits is processed (e.g., thread A) and
+#   causes a user-visible stop.  The other (thread B) is left pending.
+#
+# - The user deletes the breakpoint with "del", which causes a
+#   confirmation query.
+#
+# - By mistake, that would result in the target being left with async
+#   enabled, even though it wasn't to begin with.
+#
+# - GDB reacts to target async enablement by polling for target
+#   events.  As no thread is resumed the target replies
+#   TARGET_WAITKIND_NO_RESUMED.
+#
+# - The user continues the program, expecting it to exit.  The thread
+#   that has an event pending (thread B) is not really resumed.
+#
+# - But, nothing signals the event loop that there's a pending event
+#   waiting to be collected for thread B, so that event is never
+#   processed, thread B is never resumed and the program never exits.
+#
+# Ref: https://sourceware.org/ml/gdb-patches/2015-01/msg00592.html
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug pthreads}] == -1} {
+    return -1
+}
+
+proc test {} {
+    global srcfile gdb_prompt
+
+    if ![runto_main] {
+	return -1
+    }
+
+    delete_breakpoints
+
+    set bp_line [gdb_get_line_number "set break here" $srcfile]
+
+    gdb_breakpoint "break_function"
+    gdb_continue_to_breakpoint "cont to break_function" ".*$srcfile:$bp_line\r\n.*"
+
+    # Do something that causes a query/secondary prompt.
+
+    set test "delete breakpoints, answer prompt"
+    set saw_prompt 0
+    gdb_test_multiple "delete breakpoints" $test {
+	-re "Delete all breakpoints.*y or n.*$" {
+	    set saw_prompt 1
+	    send_gdb "y\n"
+	    exp_continue
+	}
+	-re "$gdb_prompt $" {
+	    gdb_assert $saw_prompt $test
+	}
+    }
+
+    gdb_continue_to_end "" "continue" 1
+}
+
+# Test a few times to make sure an event is left pending.  At the time
+# of writing, the bug always triggers, but that might naturally depend
+# on machine.
+for {set i 1} {$i <= 10} {incr i} {
+    with_test_prefix "iter $i" {
+	test
+    }
+}