Re: How to use -list-target-features MI command

[Jan Vrany <jan.vrany@fit.cvut.cz>]

Hi,

On Mon, 2018-04-23 at 14:45 +0100, Pedro Alves wrote:
> 
> > One way to hack around it would be to use for example the new
> > "starti" command, which breaks at (before?) the first instruction
> > of the program, issue -list-target-features, and then continue. 
> > There might be some better/cleaner solution, but that's what comes
> > to mind right now.
> 
> IMO the clean solution is to connect to the native target
> explicitly, with "-target-select native", before -exec-run,
> instead of letting -exec-run automatically connect to
> the native target if not connected to any other target yet.
> 
>  -gdb-set mi-async on
>  ^done
>  (gdb) 
>  -target-select native
>  ^connected
>  (gdb) 
>  -list-target-features
>  ^done,features=["async"]
>  (gdb) 
>  -exec-run ...
> 
> I'd also suggest that frontends consider using 
>   "set auto-connect-native-target off"
> https://sourceware.org/gdb/onlinedocs/gdb/Starting.html
> 

Thanks! This makes sense. The only problem I see is that it only works
reliably if one does not use CLI. In my case, I want user to be able 
to use the CLI. 


> I don't see a way right now that you could know about that.  There
would
> need to be a new asynchronous event emitted when the target stack 
> changes.

So gave it a go and added a new event. Seems to work fine (for me):

diff --git a/gdb/mi/mi-interp.c b/gdb/mi/mi-interp.c
index 8bfb1298a5..f140a3cca2 100644
--- a/gdb/mi/mi-interp.c
+++ b/gdb/mi/mi-interp.c
@@ -22,6 +22,7 @@
 #include "event-top.h"
 #include "event-loop.h"
 #include "inferior.h"
+#include "target.h"
 #include "infrun.h"
 #include "ui-out.h"
 #include "top.h"
@@ -85,6 +86,7 @@ static void mi_command_param_changed (const char
*param, const char *value);
 static void mi_memory_changed (struct inferior *inf, CORE_ADDR
memaddr,
 			       ssize_t len, const bfd_byte *myaddr);
 static void mi_on_sync_execution_done (void);
+static void mi_current_target_changed (struct target_ops *target);
 
 static int report_initial_inferior (struct inferior *inf, void
*closure);
 
@@ -1280,6 +1282,38 @@ mi_user_selected_context_changed
(user_selected_what selection)
     }
 }
 
+static void 
+mi_current_target_changed (struct target_ops *target) 
+{
+  SWITCH_THRU_ALL_UIS ()
+    {
+      struct mi_interp *mi = as_mi_interp (top_level_interpreter ());
+
+      if (mi == NULL)
+	      continue;
+
+      target_terminal::scoped_restore_terminal_state term_state;
+      target_terminal::ours_for_output ();
+
+      fprintf_unfiltered (mi->event_channel,
+			  "target-changed,features=[");
+      if (mi_async_p ())
+        fprintf_unfiltered (mi->event_channel,
+          "\"async\"");
+      if (target_can_execute_reverse)
+        {
+          if (mi_async_p ()) 
+            fprintf_unfiltered (mi->event_channel,",\"reverse\"");
+          else
+            fprintf_unfiltered (mi->event_channel,"\"reverse\"");
+        }
+      fprintf_unfiltered (mi->event_channel,
+        "]");
+      gdb_flush (mi->event_channel);
+    }
+}	
+
+
 static int
 report_initial_inferior (struct inferior *inf, void *closure)
 {
@@ -1382,4 +1416,5 @@ _initialize_mi_interp (void)
   gdb::observers::sync_execution_done.attach
(mi_on_sync_execution_done);
   gdb::observers::user_selected_context_changed.attach
     (mi_user_selected_context_changed);
+  gdb::observers::current_target_changed.attach
(mi_current_target_changed);
 }
diff --git a/gdb/observable.c b/gdb/observable.c
index 5539b9837b..f99a39b909 100644
--- a/gdb/observable.c
+++ b/gdb/observable.c
@@ -74,6 +74,7 @@ DEFINE_OBSERVABLE (inferior_call_pre);
 DEFINE_OBSERVABLE (inferior_call_post);
 DEFINE_OBSERVABLE (register_changed);
 DEFINE_OBSERVABLE (user_selected_context_changed);
+DEFINE_OBSERVABLE (current_target_changed);
 
 } /* namespace observers */
 } /* namespace gdb */
diff --git a/gdb/observable.h b/gdb/observable.h
index 34447b90bb..3f97cc5cb1 100644
--- a/gdb/observable.h
+++ b/gdb/observable.h
@@ -228,6 +228,9 @@ extern observable<struct frame_info *, int>
register_changed;
    frame has changed.  */
 extern observable<user_selected_what> user_selected_context_changed;
 
+/* The current target has changed. */
+extern observable<struct target_ops *> current_target_changed;
+
 } /* namespace observers */
 
 } /* namespace gdb */
diff --git a/gdb/target.c b/gdb/target.c
index e8d4ae7ea8..5cb182c849 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -49,6 +49,7 @@
 #include "byte-vector.h"
 #include "terminal.h"
 #include <algorithm>
+#include <observable.h>
 
 static void generic_tls_error (void) ATTRIBUTE_NORETURN;
 
@@ -719,6 +720,8 @@ update_current_target (void)
 
   if (targetdebug)
     setup_target_debug ();
+
+  gdb::observers::current_target_changed.notify(&current_target);
 }
 
 /* Push a new target type into the stack of the existing target
accessors,