gdb: introduce a per-interpreter event servicing method

This allows an interpreter to override internal calls to
gdb_do_one_event in case the former needs to handle alternate event
sources.

The default action is to call gdb_do_one_event and this is not overriden
in current internal interpreters. However this feature allows to easily
embed Tcl/Tk in insight that needs to concurrently handle Tcl events for
GUI handling.

In all cases, an interpreter event servicing method must call
gdb_do_one_event at some point.

All internal event servicing calls from gdb now direct to the
interpreter-specific method rather than gdb_do_one_event itself.

gdb/interps.h

@@ -23,6 +23,7 @@
 #define GDB_INTERPS_H
 
 #include "gdbsupport/intrusive_list.h"
+#include "gdbsupport/event-loop.h"
 
 struct bpstat;
 struct ui_out;
@@ -78,6 +79,13 @@ public:
   virtual void pre_command_loop ()
   {}
 
+  /* Service one event.
+     This gives the interpreter a chance to handle its own private
+     events that cannot be fed into the gdb event mechanism.
+     In all cases, this should call gdb_do_one_event at some point.  */
+  virtual int do_one_event (int mstimeout = -1)
+  { return gdb_do_one_event (mstimeout); }
+
   /* Returns true if this interpreter supports using the readline
      library; false if it uses GDB's own simplified readline
      emulation.  */

gdb/main.c

@@ -399,7 +399,7 @@ start_event_loop ()
 
       try
 	{
-	  result = gdb_do_one_event ();
+	  result = current_interpreter ()->do_one_event ();
 	}
       catch (const gdb_exception_forced_quit &ex)
 	{

gdb/top.c

@@ -415,7 +415,7 @@ wait_sync_command_done (void)
      point.  */
   scoped_enable_commit_resumed enable ("sync wait");
 
-  while (gdb_do_one_event () >= 0)
+  while (current_interpreter ()->do_one_event () >= 0)
     if (ui->prompt_state != PROMPT_BLOCKED)
       break;
 }
@@ -1031,7 +1031,7 @@ gdb_readline_wrapper (const char *prompt)
     (*after_char_processing_hook) ();
   gdb_assert (after_char_processing_hook == NULL);
 
-  while (gdb_do_one_event () >= 0)
+  while (current_interpreter ()->do_one_event () >= 0)
     if (gdb_readline_wrapper_done)
       break;