Bug#25252847: FOREIGN KEY CONSTRAINTS SHOULD BE CHECKED IN SQL LAYER

The problem was that several storage engine agnostic foreign
key constraint checks are currently done inside InnoDB.
This requires InnoDB to maintain FK metadata.

As a step towards removing this requirement, this patch adds
two FK checks to the SQL layer. These checks are:
- Disallow FKs on temporary tables.
- Disallow index prefixes on FK columns.

These two restrictions are already documented, so this patch
does not introduce changes of behavior.

Author: jhauglid

sql/key_spec.cc

@@ -88,7 +88,8 @@ bool foreign_key_prefix(const Key_spec *a, const Key_spec *b)
 }
 
 
-bool Foreign_key_spec::validate(THD *thd, List<Create_field> &table_fields) const
+bool Foreign_key_spec::validate(THD *thd, const char *table_name,
+                                List<Create_field> &table_fields) const
 {
   DBUG_ENTER("Foreign_key_spec::validate");
 
@@ -126,6 +127,13 @@ bool Foreign_key_spec::validate(THD *thd, List<Create_field> &table_fields) cons
   }
   for (const Key_part_spec *column : columns)
   {
+    // Index prefixes on foreign keys columns are not supported.
+    if (column->length > 0)
+    {
+      my_error(ER_CANNOT_ADD_FOREIGN, MYF(0), table_name);
+      DBUG_RETURN(true);
+    }
+
     it.rewind();
     while ((sql_field= it++) &&
            my_strcasecmp(system_charset_info,

sql/key_spec.h

@@ -200,12 +200,14 @@ class Foreign_key_spec: public Key_spec
     on which the FK is created.
 
     @param thd                  Thread handle
+    @param table_name           Table name (for error reporting)
     @param table_fields         List of columns
 
     @retval false   Key valid
     @retval true    Key invalid
  */
-  bool validate(THD *thd, List<Create_field> &table_fields) const;
+  bool validate(THD *thd, const char *table_name,
+                List<Create_field> &table_fields) const;
 };
 
 /**

sql/sql_table.cc

@@ -4693,6 +4693,7 @@ static const char* generate_fk_name(const char *table_name,
   Prepare FOREIGN_KEY struct with info about a foreign key.
 
   @param thd                 Thread handle.
+  @param create_info         Create info from parser.
   @param table_name          Table name.
   @param create_list         New columns.
   @param existing_fks        Array of pre-existing FKs.
@@ -4706,6 +4707,7 @@ static const char* generate_fk_name(const char *table_name,
 */
 
 static bool prepare_foreign_key(THD *thd,
+                                HA_CREATE_INFO *create_info,
                                 const char *table_name,
                                 List<Create_field> *create_list,
                                 FOREIGN_KEY *existing_fks,
@@ -4717,7 +4719,14 @@ static bool prepare_foreign_key(THD *thd,
 {
   DBUG_ENTER("prepare_foreign_key");
 
-  if (fk_key->validate(thd, *create_list))
+  // FKs are not supported for temporary tables.
+  if (create_info->options & HA_LEX_CREATE_TMP_TABLE)
+  {
+    my_error(ER_CANNOT_ADD_FOREIGN, MYF(0), table_name);
+    DBUG_RETURN(true);
+  }
+
+  if (fk_key->validate(thd, table_name, *create_list))
     DBUG_RETURN(true);
 
   if (fk_key->name.str)
@@ -5298,7 +5307,7 @@ bool mysql_prepare_create_table(THD *thd,
 
     if (key->type == KEYTYPE_FOREIGN)
     {
-      if (prepare_foreign_key(thd, error_table_name,
+      if (prepare_foreign_key(thd, create_info, error_table_name,
                               &alter_info->create_list,
                               existing_fks, existing_fks_count,
                               fk_key_info_buffer, fk_number,