A cleanup for MDEV-20042 Implement EXTRA2_FIELD_DATA_TYPE_INFO in FRM

Adding error reporting (ER_UNKNOWN_DATA_TYPE) when a handler name read
from EXTRA2_FIELD_DATA_TYPE_INFO is not known to the server.

mysql-test/main/frm-debug.result

@@ -11,11 +11,14 @@ SET SESSION debug_dbug="+d,frm_data_type_info";
 SET SESSION debug_dbug="+d,frm_data_type_info_emulate";
 CREATE TABLE t1 (c01 INT, c02 CHAR(20), c03 TEXT, c04 DOUBLE);
 Warnings:
-Note	1105	build_frm_image: Field data type info length: 12
+Note	1105	build_frm_image: Field data type info length: 14
 Note	1105	DBUG: [0] name='c01' type_info=''
-Note	1105	DBUG: [1] name='c02' type_info='char'
-Note	1105	DBUG: [2] name='c03' type_info='blob'
+Note	1105	DBUG: [1] name='c02' type_info='xchar'
+Note	1105	DBUG: [2] name='c03' type_info='xblob'
 Note	1105	DBUG: [3] name='c04' type_info=''
-DROP TABLE t1;
 SET SESSION debug_dbug="-d,frm_data_type_info_emulate";
 SET SESSION debug_dbug="-d,frm_data_type_info";
+FLUSH TABLES;
+SHOW CREATE TABLE t1;
+ERROR HY000: Unknown data type: 'xchar'
+DROP TABLE t1;

mysql-test/main/frm-debug.test

@@ -14,6 +14,9 @@ SET SESSION debug_dbug="-d,frm_data_type_info";
 SET SESSION debug_dbug="+d,frm_data_type_info";
 SET SESSION debug_dbug="+d,frm_data_type_info_emulate";
 CREATE TABLE t1 (c01 INT, c02 CHAR(20), c03 TEXT, c04 DOUBLE);
-DROP TABLE t1;
 SET SESSION debug_dbug="-d,frm_data_type_info_emulate";
 SET SESSION debug_dbug="-d,frm_data_type_info";
+FLUSH TABLES;
+--error ER_UNKNOWN_DATA_TYPE
+SHOW CREATE TABLE t1;
+DROP TABLE t1;

sql/sql_type.cc

@@ -200,13 +200,31 @@ Type_handler::handler_by_name(const LEX_CSTRING &name)
 }
 
 
+#ifndef DBUG_OFF
+static const Type_handler *frm_data_type_info_emulate(const LEX_CSTRING &name)
+{
+  if (Name(STRING_WITH_LEN("xchar")).eq(name))
+    return &type_handler_string;
+  if (Name(STRING_WITH_LEN("xblob")).eq(name))
+     return &type_handler_blob;
+  return NULL;
+}
+#endif
+
+
 const Type_handler *
 Type_handler::handler_by_name_or_error(const LEX_CSTRING &name)
 {
   const Type_handler *h= handler_by_name(name);
   if (!h)
+  {
+    DBUG_EXECUTE_IF("frm_data_type_info_emulate",
+      if ((h= frm_data_type_info_emulate(name)))
+        return h;
+    );
     my_error(ER_UNKNOWN_DATA_TYPE, MYF(0),
              ErrConvString(name.str, name.length, system_charset_info).ptr());
+  }
   return h;
 }
 
@@ -8845,7 +8863,8 @@ bool Type_handler::Column_definition_data_type_info_image(Binary_string *to,
   // Have *some* columns write type info (let's use string fields as an example)
   DBUG_EXECUTE_IF("frm_data_type_info_emulate",
                   if (cmp_type() == STRING_RESULT)
-                    return to->append(name().lex_cstring()););
+                    return to->append("x", 1) ||
+                           to->append(name().lex_cstring()););
   return false;
 }
 

sql/table.cc

@@ -1632,7 +1632,7 @@ public:
   {
     return m_count;
   }
-  const Elem element(uint i) const
+  const Elem& element(uint i) const
   {
     DBUG_ASSERT(i < m_count);
     return m_array[i];
@@ -2340,12 +2340,31 @@ int TABLE_SHARE::init_from_binary_frm_image(THD *thd, bool write,
 
         if (field_data_type_info_array.count())
         {
+          const LEX_CSTRING &info= field_data_type_info_array.
+                                     element(i).type_info();
           DBUG_EXECUTE_IF("frm_data_type_info",
             push_warning_printf(thd, Sql_condition::WARN_LEVEL_NOTE,
             ER_UNKNOWN_ERROR, "DBUG: [%u] name='%s' type_info='%.*s'",
             i, share->fieldnames.type_names[i],
-            (uint) field_data_type_info_array.element(i).type_info().length,
-            field_data_type_info_array.element(i).type_info().str););
+            (uint) info.length, info.str););
+
+          if (info.length)
+          {
+            const Type_handler *h= Type_handler::handler_by_name_or_error(info);
+            /*
+              This code will eventually be extended here:
+              - If the handler was not found by name, we could
+                still open the table using the fallback type handler "handler",
+                at least for a limited set of commands.
+              - If the handler was found by name, we could check
+                that "h" and "handler" have the same type code
+                (and maybe some other properties) to make sure
+                that the FRM data is consistent.
+            */
+            if (!h)
+              goto err;
+            handler= h;
+          }
         }
       }