Give warnings if open_stat_table_for_ddl() fails

The warning is given in case of table not found or if there is a lock
timeout. The warning is needed as in case of a lock timeout then the
persistent table stats are going to be wrong.

mysql-test/main/stat_tables_missing.result

@@ -2,6 +2,9 @@ create table t1 (a int);
 alter table mysql.column_stats rename to mysql.column_stats1;
 flush tables;
 alter table t1 change a b varchar(100);
+Warnings:
+Warning	1177	Got error 1146 when trying to open statistics table `table_stats` for updating statistics
+Warning	1177	Got error 1146 when trying to open statistics table `table_stats` for updating statistics
 show create table t1;
 Table	Create Table
 t1	CREATE TABLE `t1` (

sql/sql_base.cc

@@ -73,6 +73,8 @@ No_such_table_error_handler::handle_condition(THD *,
                                               Sql_condition ** cond_hdl)
 {
   *cond_hdl= NULL;
+  if (!first_error)
+    first_error= sql_errno;
   if (sql_errno == ER_NO_SUCH_TABLE || sql_errno == ER_NO_SUCH_TABLE_IN_ENGINE)
   {
     m_handled_errors++;
@@ -95,7 +97,6 @@ bool No_such_table_error_handler::safely_trapped_errors()
   return ((m_handled_errors > 0) && (m_unhandled_errors == 0));
 }
 
-
 /**
   This internal handler is used to trap ER_NO_SUCH_TABLE and
   ER_WRONG_MRG_TABLE errors during CHECK/REPAIR TABLE for MERGE

sql/sql_base.h

@@ -643,7 +643,7 @@ class No_such_table_error_handler : public Internal_error_handler
 {
 public:
   No_such_table_error_handler()
-    : m_handled_errors(0), m_unhandled_errors(0)
+    : m_handled_errors(0), m_unhandled_errors(0), first_error(0)
   {}
 
   bool handle_condition(THD *thd,
@@ -658,11 +658,12 @@ class No_such_table_error_handler : public Internal_error_handler
     trapped and no other errors have been seen. FALSE otherwise.
   */
   bool safely_trapped_errors();
+  bool any_error() { return m_handled_errors == 0 || m_unhandled_errors == 0; }
+  uint got_error() { return first_error; }
 
 private:
   int m_handled_errors;
   int m_unhandled_errors;
+  uint first_error;
 };
-
-
 #endif /* SQL_BASE_INCLUDED */

sql/sql_statistics.cc

@@ -284,6 +284,10 @@ static int open_stat_tables(THD *thd, TABLE_LIST *tables, bool for_write)
   @details
   This is used by DDLs. When a column or index is dropped or renamed,
   stat tables need to be adjusted accordingly.
+
+  This function should not generate any errors as the callers are not checking
+  the result of delete_statistics_for_table()
+
 */
 static inline int open_stat_table_for_ddl(THD *thd, TABLE_LIST *table,
                                           const LEX_CSTRING *stat_tab_name)
@@ -293,6 +297,14 @@ static inline int open_stat_table_for_ddl(THD *thd, TABLE_LIST *table,
   thd->push_internal_handler(&nst_handler);
   int res= open_system_tables_for_read(thd, table);
   thd->pop_internal_handler();
+  if (res && nst_handler.any_error())
+  {
+    push_warning_printf(thd, Sql_condition::WARN_LEVEL_WARN,
+                        ER_CHECK_NO_SUCH_TABLE,
+                        "Got error %d when trying to open statistics "
+                        "table %`s for updating statistics",
+                        nst_handler.got_error(), stat_table_name->str);
+  }
   return res;
 }