gdb/infrun: stop all threads if there exists a non-stop target

Stop all threads not only if the current target is non-stop, but also
if there exists a non-stop target.

The multi-target patch (5b6d1e4fa4 "Multi-target support") made the
following change to gdb/inf-child.c:

void
 inf_child_target::maybe_unpush_target ()
 {
-  if (!inf_child_explicitly_opened && !have_inferiors ())
+  if (!inf_child_explicitly_opened)
     unpush_target (this);
 }

If we are in all-stop mode with multiple inferiors, and an exit event
is received from an inferior, target_mourn_inferior() gets to this
point and without the have_inferiors() check, the target is unpushed.
This leads to having exec_ops as the top target.

Here is a test scenario.  Two executables, ./a.out returns
immediately; ./sleepy just sleeps.

  $ gdb ./sleepy
  (gdb) start
  ...
  (gdb) add-inferior -exec ./a.out
  ...
  (gdb) inferior 2
  [Switching to inferior 2..
  (gdb) start
  ...
  (gdb) set schedule-multiple on
  (gdb) set debug infrun 1
  (gdb) continue

At this point, the exit event is received from ./a.out.  Normally,
this would lead to stop_all_threads() to also stop ./sleepy, but this
doesn't happen, because target_is_non_stop_p() returns false.  And it
returns false because the top target is no longer the process target;
it is the exec_ops.

This patch modifies 'stop_waiting' to call 'stop_all_threads' if there
exists a non-stop target, not just when the current top target is
non-stop.

Tested on X86_64 Linux.

gdb/ChangeLog:
2020-04-01  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* infrun.c (stop_all_threads): Update assertion, plus when
	stopping threads, take into account that we might be trying
	to stop an all-stop target.
	(stop_waiting): Call 'stop_all_threads' if there exists a
	non-stop target.

gdb/testsuite/ChangeLog:
2020-04-01  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.multi/stop-all-on-exit.c: New test.
	* gdb.multi/stop-all-on-exit.exp: New file.

gdb/infrun.c

@@ -4711,7 +4711,7 @@ stop_all_threads (void)
   int pass;
   int iterations = 0;
 
-  gdb_assert (target_is_non_stop_p ());
+  gdb_assert (exists_non_stop_target ());
 
   if (debug_infrun)
     fprintf_unfiltered (gdb_stdlog, "infrun: stop_all_threads\n");
@@ -4742,6 +4742,17 @@ stop_all_threads (void)
 	     to tell the target to stop.  */
 	  for (thread_info *t : all_non_exited_threads ())
 	    {
+	      /* For a single-target setting with an all-stop target,
+		 we would not even arrive here.  For a multi-target
+		 setting, until GDB is able to handle a mixture of
+		 all-stop and non-stop targets, simply skip all-stop
+		 targets' threads.  This should be fine due to the
+		 protection of 'check_multi_target_resumption'.  */
+
+	      switch_to_thread_no_regs (t);
+	      if (!target_is_non_stop_p ())
+		continue;
+
 	      if (t->executing)
 		{
 		  /* If already stopping, don't request a stop again.
@@ -4753,7 +4764,6 @@ stop_all_threads (void)
 					    "infrun:   %s executing, "
 					    "need stop\n",
 					    target_pid_to_str (t->ptid).c_str ());
-		      switch_to_thread_no_regs (t);
 		      target_stop (t->ptid);
 		      t->stop_requested = 1;
 		    }
@@ -7894,9 +7904,9 @@ stop_waiting (struct execution_control_state *ecs)
   /* Let callers know we don't want to wait for the inferior anymore.  */
   ecs->wait_some_more = 0;
 
-  /* If all-stop, but the target is always in non-stop mode, stop all
+  /* If all-stop, but there exists a non-stop target, stop all
      threads now that we're presenting the stop to the user.  */
-  if (!non_stop && target_is_non_stop_p ())
+  if (!non_stop && exists_non_stop_target ())
     stop_all_threads ();
 }