sim: Move the draining interface into a separate base class

This patch moves the draining interface from SimObject to a separate
class that can be used by any object needing draining. However,
objects not visible to the Python code (i.e., objects not deriving
from SimObject) still depend on their parents informing them when to
drain. This patch also gets rid of the CountedDrainEvent (which isn't
really an event) and replaces it with a DrainManager.

src/python/m5/SimObject.py

@@ -123,7 +123,8 @@ class MetaSimObject(type):
                       'cxx_class' : str,
                       'cxx_type' : str,
                       'cxx_header' : str,
-                      'type' : str }
+                      'type' : str,
+                      'cxx_bases' : list }
     # Attributes that can be set any time
     keywords = { 'check' : FunctionType }
 
@@ -148,6 +149,8 @@ class MetaSimObject(type):
                 value_dict[key] = val
         if 'abstract' not in value_dict:
             value_dict['abstract'] = False
+        if 'cxx_bases' not in value_dict:
+            value_dict['cxx_bases'] = []
         cls_dict['_value_dict'] = value_dict
         cls = super(MetaSimObject, mcls).__new__(mcls, name, bases, cls_dict)
         if 'type' in value_dict:
@@ -414,6 +417,7 @@ class MetaSimObject(type):
         code('%module(package="m5.internal") param_$cls')
         code()
         code('%{')
+        code('#include "sim/sim_object.hh"')
         code('#include "params/$cls.hh"')
         for param in params:
             param.cxx_predecls(code)
@@ -458,7 +462,17 @@ using std::ptrdiff_t;
         code('%nodefault $classname;')
         code('class $classname')
         if cls._base:
-            code('    : public ${{cls._base.cxx_class}}')
+            bases = [ cls._base.cxx_class ] + cls.cxx_bases
+        else:
+            bases = cls.cxx_bases
+        base_first = True
+        for base in bases:
+            if base_first:
+                code('    : public ${{base}}')
+                base_first = False
+            else:
+                code('    , public ${{base}}')
+
         code('{')
         code('  public:')
         cls.export_methods(code)
@@ -581,30 +595,25 @@ class SimObject(object):
     abstract = True
     cxx_header = "sim/sim_object.hh"
 
+    cxx_bases = [ "Drainable" ]
+
     @classmethod
     def export_method_swig_predecls(cls, code):
         code('''
 %include <std_string.i>
+
+%import "python/swig/drain.i"
 ''')
 
     @classmethod
     def export_methods(cls, code):
         code('''
-    enum State {
-      Running,
-      Draining,
-      Drained
-    };
-
     void init();
     void loadState(Checkpoint *cp);
     void initState();
     void regStats();
     void resetStats();
     void startup();
-
-    unsigned int drain(Event *drain_event);
-    void resume();
 ''')
 
     # Initialize new instance.  For objects with SimObject-valued