[PATCH 1/4] 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.  Ideally we would check this flag
in target_can_async_p and target_is_async_p, however, there are a few
places in GDB where we call the can_async_p and is_async_p methods
directly without calling through the target_* wrapper.

As a result each target must check the target_async_permitted flag in
their own method implementations.

However, as the target_can_async_p and target_is_async_p functions are
also called in some places, what we can do is add an assert in these
functions that ensures targets are checking target_async_permitted
correctly.

So, the rule is: in target_can_async_p and target_is_async_p, if the
global target_async_permitted is false, then the result of calling the
target_ops method (::can_async_p or ::is_async_p) should also be
false.

Right now, I believe that only the linux_nat_target and the
remote_target support async mode, and these targets do, correctly
check target_async_permitted.  But if, in future, additional targets
add support for async mode, then these asserts should ensure that the
targets are checking target_async_permitted correctly.

Further, the target_async method is used to enable or disable async
mode for a target.  My claim here is that we should only try to enable
async mode for targets if target_async_permitted is true, and if the
target supports async mode.  I've added an assertion to this effect in
the target_async function.

There should be no user visible changes after this commit.
---
 gdb/target.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/gdb/target.c b/gdb/target.c
index 8fe27c775ea..f8e56cceb08 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -391,7 +391,9 @@ target_can_lock_scheduler ()
 bool
 target_can_async_p ()
 {
-  return current_inferior ()->top_target ()->can_async_p ();
+  bool result = current_inferior ()->top_target ()->can_async_p ();
+  gdb_assert (target_async_permitted || !result);
+  return result;
 }
 
 /* See target.h.  */
@@ -399,7 +401,9 @@ target_can_async_p ()
 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
@@ -4328,6 +4332,9 @@ 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 both allowed, and the target supports async mode.  */
+  gdb_assert (!enable || (target_async_permitted && target_can_async_p ()));
   infrun_async (enable);
   current_inferior ()->top_target ()->async (enable);
 }
-- 
2.25.4