[PATCHv2 3/6] gdb: add asserts in target.c for target_async_permitted

[Andrew Burgess <aburgess@redhat.com>]

The target_async_permitted flag allows a user to override whether a
target can act in async mode or not.  In previous commits I have moved
the checking of this flag out of the various ::can_async_p methods and
into the common target.c code.

In this commit I will add some additional assertions into
target_is_async_p and target_async.  The rules these assertions are
checking are:

  1. A target that returns false for target_can_async_p should never
  become "async enabled", and so ::is_async_p should always return
  false.  This is being checked in target_is_async_p.

  2. GDB should never try to enable async mode for a target that
  returns false for target_can_async_p, this is checked in
  target_async.

There are a few places where we call the ::is_async_p method directly,
in these cases we will obviously not pass through the assert in
target_is_async_p, however, there are also plenty of places where we
do call target_is_async_p so if GDB starts to misbehave we should
catch it quickly enough.

There should be no user visible changes after this commit.
---
 gdb/target.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/gdb/target.c b/gdb/target.c
index d693b670350..941433f8953 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -411,7 +411,9 @@ target_can_async_p (struct target_ops *target)
 bool
 target_is_async_p ()
 {
-  return current_inferior ()->top_target ()->is_async_p ();
+  bool result = current_inferior ()->top_target ()->is_async_p ();
+  gdb_assert (target_async_permitted || !result);
+  return result;
 }
 
 exec_direction_kind
@@ -4340,6 +4342,9 @@ maintenance_print_target_stack (const char *cmd, int from_tty)
 void
 target_async (int enable)
 {
+  /* If we are trying to enable async mode then it must be the case that
+     async mode is possible for this target.  */
+  gdb_assert (!enable || target_can_async_p ());
   infrun_async (enable);
   current_inferior ()->top_target ()->async (enable);
 }
-- 
2.25.4