MDEV-23009 SIGSEGV in get_field from acl_load (on optimized builds)

Classes that handle privilege tables (like Tables_priv_table)
could read some columns conditionally but they expect a certain
minimal number of colunms always to exist.

Add a check for a minimal required number of columns in privilege tables,
don't use a table that has fewer columns than required.

mysql-test/main/grant4.result

@@ -148,7 +148,7 @@ call mtr.add_suppression('mysql.user table is damaged');
 rename table mysql.user to mysql.user1;
 create table mysql.user (Host char(100), User char(100));
 flush privileges;
-ERROR HY000: Unknown error
+ERROR HY000: Fatal error: mysql.user table is damaged or in unsupported 3.20 format
 drop table mysql.user;
 rename table mysql.user1 to mysql.user;
 # switching back from mysql.user to mysql.global_priv

mysql-test/main/grant5.result

@@ -242,4 +242,10 @@ disconnect con1;
 connection default;
 drop database db;
 drop user foo;
+call mtr.add_suppression('mysql.host table is damaged');
+create table mysql.host (c1 int);
+insert mysql.host values (1);
+flush privileges;
+ERROR HY000: Fatal error: mysql.host table is damaged or in unsupported 3.20 format
+drop table mysql.host;
 # End of 10.4 tests

mysql-test/main/grant5.test

@@ -199,4 +199,14 @@ delete from db.t1 returning *;
 drop database db;
 drop user foo;
 
+#
+# MDEV-23009 SIGSEGV in get_field from acl_load (on optimized builds)
+#
+call mtr.add_suppression('mysql.host table is damaged');
+create table mysql.host (c1 int);
+insert mysql.host values (1);
+--error ER_UNKNOWN_ERROR
+flush privileges;
+drop table mysql.host;
+
 --echo # End of 10.4 tests

sql/sql_acl.cc

@@ -790,6 +790,15 @@ class Grant_table_base
   bool init_read_record(READ_RECORD* info) const
   {
     DBUG_ASSERT(m_table);
+
+    if (num_fields() < min_columns)
+    {
+      my_printf_error(ER_UNKNOWN_ERROR, "Fatal error: mysql.%s table is "
+                      "damaged or in unsupported 3.20 format",
+                      MYF(ME_ERROR_LOG), m_table->s->table_name.str);
+      return 1;
+    }
+
     bool result= ::init_read_record(info, m_table->in_use, m_table,
                                     NULL, NULL, 1, true, false);
     if (!result)
@@ -814,7 +823,7 @@ class Grant_table_base
  protected:
   friend class Grant_tables;
 
-  Grant_table_base() : start_priv_columns(0), end_priv_columns(0), m_table(0)
+  Grant_table_base() : min_columns(3), start_priv_columns(0), end_priv_columns(0), m_table(0)
   { }
 
   /* Compute how many privilege columns this table has. This method
@@ -843,6 +852,9 @@ class Grant_table_base
     }
   }
 
+
+  /* the min number of columns a table should have */
+  uint min_columns;
   /* The index at which privilege columns start. */
   uint start_priv_columns;
   /* The index after the last privilege column */
@@ -1241,7 +1253,7 @@ class User_table_tabular: public User_table
   friend class Grant_tables;
 
   /* Only Grant_tables can instantiate this class. */
-  User_table_tabular() {}
+  User_table_tabular() { min_columns= 13; /* As in 3.20.13 */ }
 
   /* The user table is a bit different compared to the other Grant tables.
      Usually, we only add columns to the grant tables when adding functionality.
@@ -1263,13 +1275,6 @@ class User_table_tabular: public User_table
 
   int setup_sysvars() const
   {
-    if (num_fields() < 13) // number of columns in 3.21
-    {
-      sql_print_error("Fatal error: mysql.user table is damaged or in "
-                      "unsupported 3.20 format.");
-      return 1;
-    }
-
     username_char_length= MY_MIN(m_table->field[1]->char_length(),
                                  USERNAME_CHAR_LENGTH);
     using_global_priv_table= false;
@@ -1701,7 +1706,7 @@ class Db_table: public Grant_table_base
  private:
   friend class Grant_tables;
 
-  Db_table() {}
+  Db_table() { min_columns= 9; /* as in 3.20.13 */ }
 };
 
 class Tables_priv_table: public Grant_table_base
@@ -1719,7 +1724,7 @@ class Tables_priv_table: public Grant_table_base
  private:
   friend class Grant_tables;
 
-  Tables_priv_table() {}
+  Tables_priv_table() { min_columns= 8; /* as in 3.22.26a */ }
 };
 
 class Columns_priv_table: public Grant_table_base
@@ -1736,7 +1741,7 @@ class Columns_priv_table: public Grant_table_base
  private:
   friend class Grant_tables;
 
-  Columns_priv_table() {}
+  Columns_priv_table() { min_columns= 7; /* as in 3.22.26a */ }
 };
 
 class Host_table: public Grant_table_base
@@ -1748,7 +1753,7 @@ class Host_table: public Grant_table_base
  private:
   friend class Grant_tables;
 
-  Host_table() {}
+  Host_table() { min_columns= 8; /* as in 3.20.13 */ }
 };
 
 class Procs_priv_table: public Grant_table_base
@@ -1766,7 +1771,7 @@ class Procs_priv_table: public Grant_table_base
  private:
   friend class Grant_tables;
 
-  Procs_priv_table() {}
+  Procs_priv_table() { min_columns=8; }
 };
 
 class Proxies_priv_table: public Grant_table_base
@@ -1783,7 +1788,7 @@ class Proxies_priv_table: public Grant_table_base
  private:
   friend class Grant_tables;
 
-  Proxies_priv_table() {}
+  Proxies_priv_table() { min_columns= 7; }
 };
 
 class Roles_mapping_table: public Grant_table_base
@@ -1797,7 +1802,7 @@ class Roles_mapping_table: public Grant_table_base
  private:
   friend class Grant_tables;
 
-  Roles_mapping_table() {}
+  Roles_mapping_table() { min_columns= 4; }
 };
 
 /**