SHOW EXPLAIN DELETE, post merge fixes

- Fix asserts, make sure that mysql_delete() operates on 
  thd->apc_target correctly* in all kinds of special cases 
  * - correctly means that one must switch it OFF iff it
  was switched ON.
- Added a few asserts to catch similar errors.

sql/my_apc.cc

@@ -70,6 +70,7 @@ void Apc_target::enable()
 void Apc_target::disable()
 {
   bool process= FALSE;
+  DBUG_ASSERT(enabled);
   mysql_mutex_lock(LOCK_thd_data_ptr);
   if (!(--enabled))
     process= TRUE;

sql/sql_parse.cc

@@ -807,7 +807,9 @@ bool do_command(THD *thd)
   my_net_set_read_timeout(net, thd->variables.net_read_timeout);
 
   DBUG_ASSERT(packet_length);
+  DBUG_ASSERT(!thd->apc_target.is_enabled());
   return_value= dispatch_command(command, thd, packet+1, (uint) (packet_length-1));
+  DBUG_ASSERT(!thd->apc_target.is_enabled());
 
 out:
   DBUG_RETURN(return_value);
@@ -1109,6 +1111,7 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
       ulong length= (ulong)(packet_end - beginning_of_next_stmt);
 
       log_slow_statement(thd);
+      DBUG_ASSERT(!thd->apc_target.is_enabled());
 
       /* Remove garbage at start of query */
       while (length > 0 && my_isspace(thd->charset(), *beginning_of_next_stmt))

sql/sql_update.cc

@@ -279,6 +279,7 @@ int mysql_update(THD *thd,
   Update_plan query_plan;
   query_plan.index= MAX_KEY;
   query_plan.using_filesort= FALSE;
+  bool apc_target_enabled= false; // means was enabled *by code this function*
   DBUG_ENTER("mysql_update");
 
   if (open_tables(thd, &table_list, &table_count, 0))
@@ -497,6 +498,7 @@ int mysql_update(THD *thd,
     goto exit_without_my_ok;
 
   thd->apc_target.enable();
+  apc_target_enabled= true;
   DBUG_EXECUTE_IF("show_explain_probe_update_exec_start", 
                   dbug_serve_apcs(thd, 1););
 
@@ -923,6 +925,7 @@ int mysql_update(THD *thd,
     thd->transaction.stmt.modified_non_trans_table= TRUE;
 
   thd->apc_target.disable(); //psergey-todo.
+  apc_target_enabled= false;
   end_read_record(&info);
   delete select;
   thd_proc_info(thd, "end");
@@ -996,6 +999,9 @@ int mysql_update(THD *thd,
   DBUG_RETURN((error >= 0 || thd->is_error()) ? 1 : 0);
 
 err:
+  if (apc_target_enabled)
+    thd->apc_target.disable();
+
   delete select;
   free_underlaid_joins(thd, select_lex);
   table->disable_keyread();
@@ -1003,6 +1009,7 @@ int mysql_update(THD *thd,
   DBUG_RETURN(1);
 
 exit_without_my_ok:
+  DBUG_ASSERT(!apc_target_enabled);
   thd->lex->upd_del_plan= &query_plan;
   
   select_send *result;