Fix for Bug#5034 "prepared "select 1 into @arg15", second

execute crashes server": we were deleting lex->result
after each execute, but prepared statements assumed that
it's left intact.
The fix adds cleanup() method to select_result hierarchy,
so that result objects can be reused.
Plus we now need to delete result objects more wisely.


mysql-test/r/ps.result:
  Test results fixed: test case for bug#5034
mysql-test/t/ps.test:
  A test case for bug#5034, few followups
sql/sql_class.cc:
  - fix warning in THD::THD
  - implementation of cleanup() for select_result hierarchy
  - select_export::send_eof was identical to 
    select_dump::send_eof: moved to the base class select_to_file.
  - Statement::end_statement() to end lex, free items, and
    delete possible select_result
sql/sql_class.h:
  - select_result::cleanup() declaration
  -
sql/sql_insert.cc:
  - implementation of select_insert::cleanup(): currently
    we always create a new instance of select_insert/
    select_create on each execute.
sql/sql_lex.cc:
  - with more complicated logic of freeing lex->result it's 
    easier to have it non-zero only if it points to a valid
    result.
sql/sql_lex.h:
  Now st_lex::st_lex is not empty.
sql/sql_parse.cc:
  mysql_execute_command():
  - delete select_result *result only if it was created in
    this function.
  - use end_statement() to cleanup lex and thd in the end of
    each statement.
  - no need to save THD::lock if this is explain. This save
    apparently left from times when derived tables were 
    materialized here, not in open_and_lock_tables.
sql/sql_prepare.cc:
  - call result->cleanup() in reset_stmt_for_execute
  - now Statement is responsible for freeing its lex->result.
sql/sql_select.cc:
  handle_select():
  - don't delete result, it might be needed
    for next executions
  - result is never null

sql/sql_class.cc

@@ -155,10 +155,10 @@ bool foreign_key_prefix(Key *a, Key *b)
 ** Thread specific functions
 ****************************************************************************/
 
-THD::THD():user_time(0), current_arena(this), is_fatal_error(0),
-	   last_insert_id_used(0),
+THD::THD():user_time(0), current_arena(this), global_read_lock(0),
+           is_fatal_error(0), last_insert_id_used(0),
            insert_id_used(0), rand_used(0), time_zone_used(0),
-           in_lock_tables(0), global_read_lock(0), bootstrap(0)
+           in_lock_tables(0), bootstrap(0)
 {
   host= user= priv_user= db= ip=0;
   host_or_ip= "connecting host";
@@ -703,6 +703,12 @@ void select_result::send_error(uint errcode,const char *err)
   ::send_error(thd, errcode, err);
 }
 
+
+void select_result::cleanup()
+{
+  /* do nothing */
+}
+
 static String default_line_term("\n",default_charset_info);
 static String default_escaped("\\",default_charset_info);
 static String default_field_term("\t",default_charset_info);
@@ -808,6 +814,32 @@ void select_to_file::send_error(uint errcode,const char *err)
 }
 
 
+bool select_to_file::send_eof()
+{
+  int error= test(end_io_cache(&cache));
+  if (my_close(file,MYF(MY_WME)))
+    error= 1;
+  if (!error)
+    ::send_ok(thd,row_count);
+  file= -1;
+  return error;
+}
+
+
+void select_to_file::cleanup()
+{
+  /* In case of error send_eof() may be not called: close the file here. */
+  if (file >= 0)
+  {
+    (void) end_io_cache(&cache);
+    (void) my_close(file,MYF(0));
+    file= -1;
+  }
+  path[0]= '\0';
+  row_count= 0;
+}
+
+
 select_to_file::~select_to_file()
 {
   if (file >= 0)
@@ -1058,18 +1090,6 @@ err:
 }
 
 
-bool select_export::send_eof()
-{
-  int error=test(end_io_cache(&cache));
-  if (my_close(file,MYF(MY_WME)))
-    error=1;
-  if (!error)
-    ::send_ok(thd,row_count);
-  file= -1;
-  return error;
-}
-
-
 /***************************************************************************
 ** Dump  of select to a binary file
 ***************************************************************************/
@@ -1123,18 +1143,6 @@ err:
 }
 
 
-bool select_dump::send_eof()
-{
-  int error=test(end_io_cache(&cache));
-  if (my_close(file,MYF(MY_WME)))
-    error=1;
-  if (!error)
-    ::send_ok(thd,row_count);
-  file= -1;
-  return error;
-}
-
-
 select_subselect::select_subselect(Item_subselect *item_arg)
 {
   item= item_arg;
@@ -1301,6 +1309,13 @@ int select_dumpvar::prepare(List<Item> &list, SELECT_LEX_UNIT *u)
 }
 
 
+void select_dumpvar::cleanup()
+{
+  vars.empty();
+  row_count=0;
+}
+
+
 Item_arena::Item_arena(THD* thd)
   :free_list(0),
   state(INITIALIZED)
@@ -1405,6 +1420,21 @@ void Statement::restore_backup_statement(Statement *stmt, Statement *backup)
 }
 
 
+void Statement::end_statement()
+{
+  /* Cleanup SQL processing state to resuse this statement in next query. */
+  lex_end(lex);
+  delete lex->result;
+  lex->result= 0;
+  free_items(free_list);
+  free_list= 0;
+  /*
+    Don't free mem_root, as mem_root is freed in the end of dispatch_command
+    (once for any command).
+  */
+}
+
+
 void Item_arena::set_n_backup_item_arena(Item_arena *set, Item_arena *backup)
 {
   backup->set_item_arena(this);