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.

gdb/ChangeLog

@@ -1,3 +1,13 @@
+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.
+
 2015-01-22  Jan Kratochvil  <jan.kratochvil@redhat.com>
 
 	* NEWS (Changes since GDB 7.9): Add 'thread apply all' option

gdb/linux-nat.c

@@ -219,6 +219,9 @@ struct simple_pid_list *stopped_pids;
    event loop.  */
 static int linux_nat_event_pipe[2] = { -1, -1 };
 
+/* True if we're currently in async mode.  */
+#define linux_is_async_p() (linux_nat_event_pipe[0] != -1)
+
 /* Flush the event pipe.  */
 
 static void
@@ -4302,10 +4305,7 @@ linux_trad_target (CORE_ADDR (*register_u_offset)(struct gdbarch *, int, int))
 static int
 linux_nat_is_async_p (struct target_ops *ops)
 {
-  /* NOTE: palves 2008-03-21: We're only async when the user requests
-     it explicitly with the "set target-async" command.
-     Someday, linux will always be async.  */
-  return target_async_permitted;
+  return linux_is_async_p ();
 }
 
 /* target_can_async_p implementation.  */
@@ -4355,7 +4355,11 @@ static int async_terminal_is_ours = 1;
 static void
 linux_nat_terminal_inferior (struct target_ops *self)
 {
-  if (!target_is_async_p ())
+  /* Like target_terminal_inferior, use target_can_async_p, not
+     target_is_async_p, since at this point the target is not async
+     yet.  If it can async, then we know it will become async prior to
+     resume.  */
+  if (!target_can_async_p ())
     {
       /* Async mode is disabled.  */
       child_terminal_inferior (self);
@@ -4385,13 +4389,6 @@ linux_nat_terminal_inferior (struct target_ops *self)
 static void
 linux_nat_terminal_ours (struct target_ops *self)
 {
-  if (!target_is_async_p ())
-    {
-      /* Async mode is disabled.  */
-      child_terminal_ours (self);
-      return;
-    }
-
   /* GDB should never give the terminal to the inferior if the
      inferior is running in the background (run&, continue&, etc.),
      but claiming it sure should.  */
@@ -4444,7 +4441,7 @@ handle_target_event (int error, gdb_client_data client_data)
 static int
 linux_async_pipe (int enable)
 {
-  int previous = (linux_nat_event_pipe[0] != -1);
+  int previous = linux_is_async_p ();
 
   if (previous != enable)
     {

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+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.
+
 2015-01-22  Anders Granlund  <anders.granlund@ericsson.com>
 	    Simon Marchi  <simon.marchi@ericsson.com>
 

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;
+}

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
+    }
+}