MDEV-27744 LPAD in vcol created in ORACLE mode makes table corrupted in non-ORACLE

The crash happened with an indexed virtual column whose
value is evaluated using a function that has a different meaning
in sql_mode='' vs sql_mode=ORACLE:

- DECODE()
- LTRIM()
- RTRIM()
- LPAD()
- RPAD()
- REPLACE()
- SUBSTR()

For example:

CREATE TABLE t1 (
  b VARCHAR(1),
  g CHAR(1) GENERATED ALWAYS AS (SUBSTR(b,0,0)) VIRTUAL,
  KEY g(g)
);

So far we had replacement XXX_ORACLE() functions for all mentioned function,
e.g. SUBSTR_ORACLE() for SUBSTR(). So it was possible to correctly re-parse
SUBSTR_ORACLE() even in sql_mode=''.

But it was not possible to re-parse the MariaDB version of SUBSTR()
after switching to sql_mode=ORACLE. It was erroneously mis-interpreted
as SUBSTR_ORACLE().

As a result, this combination worked fine:

SET sql_mode=ORACLE;
CREATE TABLE t1 ... g CHAR(1) GENERATED ALWAYS AS (SUBSTR(b,0,0)) VIRTUAL, ...;
INSERT ...
FLUSH TABLES;
SET sql_mode='';
INSERT ...

But the other way around it crashed:

SET sql_mode='';
CREATE TABLE t1 ... g CHAR(1) GENERATED ALWAYS AS (SUBSTR(b,0,0)) VIRTUAL, ...;
INSERT ...
FLUSH TABLES;
SET sql_mode=ORACLE;
INSERT ...

At CREATE time, SUBSTR was instantiated as Item_func_substr and printed
in the FRM file as substr(). At re-open time with sql_mode=ORACLE, "substr()"
was erroneously instantiated as Item_func_substr_oracle.

Fix:

The fix proposes a symmetric solution. It provides a way to re-parse reliably
all sql_mode dependent functions to their original CREATE TABLE time meaning,
no matter what the open-time sql_mode is.

We take advantage of the same idea we previously used to resolve sql_mode
dependent data types.

Now all sql_mode dependent functions are printed by SHOW using a schema
qualifier when the current sql_mode differs from the function sql_mode:

SET sql_mode='';
CREATE TABLE t1 ... SUBSTR(a,b,c) ..;
SET sql_mode=ORACLE;
SHOW CREATE TABLE t1;   ->   mariadb_schema.substr(a,b,c)

SET sql_mode=ORACLE;
CREATE TABLE t2 ... SUBSTR(a,b,c) ..;
SET sql_mode='';
SHOW CREATE TABLE t1;   ->   oracle_schema.substr(a,b,c)

Old replacement names like substr_oracle() are still understood for
backward compatibility and used in FRM files (for downgrade compatibility),
but they are not printed by SHOW any more.

sql/sql_schema.h

@@ -19,6 +19,9 @@
 #include "mysqld.h"
 #include "lex_string.h"
 
+class Lex_ident_sys;
+class Create_func;
+
 class Schema
 {
   LEX_CSTRING m_name;
@@ -34,6 +37,24 @@ public:
     return src;
   }
 
+  /**
+    Find a native function builder, return an error if not found,
+    build an Item otherwise.
+  */
+  Item *make_item_func_call_native(THD *thd,
+                                   const Lex_ident_sys &name,
+                                   List<Item> *args) const;
+
+  /**
+    Find the native function builder associated with a given function name.
+    @param thd The current thread
+    @param name The native function name
+    @return The native function builder associated with the name, or NULL
+  */
+  virtual Create_func *find_native_function_builder(THD *thd,
+                                                    const LEX_CSTRING &name)
+                                                    const;
+
   // Builders for native SQL function with a special syntax in sql_yacc.yy
   virtual Item *make_item_func_replace(THD *thd,
                                        Item *subj,
@@ -77,5 +98,6 @@ public:
 
 
 extern Schema mariadb_schema;
+extern const Schema &oracle_schema_ref;
 
 #endif // SQL_SCHEMA_H_INCLUDED