MWL#182: Explain running statements: address review feedback

- Make THD::check_killed() an inline function which makes calls to non-inline 
  functions only whern there are APC requests to be served.

sql/my_apc.h

@@ -53,6 +53,15 @@ public:
   void disable();
   
   void process_apc_requests();
+  /* 
+    A lightweight function, intended to be used in frequent checks like this:
+
+      if (apc_target.have_requests()) apc_target.process_apc_requests()
+  */
+  inline bool have_apc_requests()
+  {
+    return test(apc_calls);
+  }
   
   /* Functor class for calls you can schedule */
   class Apc_call

sql/sql_class.cc

@@ -2166,21 +2166,6 @@ void THD::rollback_item_tree_changes()
 }
 
 
-/*
-  Check if the thread has been killed, and also process "APC requests"
-
-  @retval true  The thread is killed, execution should be interrupted
-  @retval false Not killed, continue execution
-*/
-
-bool THD::check_killed()
-{
-  if (killed)
-    return TRUE;
-  apc_target.process_apc_requests(); 
-  return FALSE;
-}
-
 /*****************************************************************************
 ** Functions to provide a interface to select results
 *****************************************************************************/

sql/sql_class.h

@@ -2222,7 +2222,14 @@ public:
   */
   killed_state volatile killed;
 
-  bool check_killed();
+  inline bool check_killed()
+  {
+    if (killed)
+      return TRUE;
+    if (apc_target.have_apc_requests())
+      apc_target.process_apc_requests(); 
+    return FALSE;
+  }
 
   /* scramble - random string sent to client on handshake */
   char	     scramble[SCRAMBLE_LENGTH+1];