MDEV-38740 Implement JSON as a pluggable data type

[hadeer-r]

• This PR implements JSON as a distinct pluggable data type in MariaDB using the MariaDB_DATA_TYPE_PLUGIN framework.
• Related issue: MDEV-38740
• This work is part of GSoC '26 and represents the initial progress and implementation milestones.

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Implementation Progress

• Implemented Type_handler_json, Field_json, and Type_collection_json, following XMLType structure while taking into account existing JSON type behavior.
• SHOW CREATE TABLE and DESCRIBE now explicitly display the column type as json instead of longtext.
• JSON columns natively accept CHARACTER SET and COLLATE attributes (e.g., JSON CHARACTER SET utf8) without throwing parse errors.
• send format=json metadata to clients over the wire protocol.

Testing
Migrated the built-in JSON tests (mysql-test/main/type_json.test) into the plugin test suite, splitting them into focused, modular files:

• type_json_basic.test (create, select, drop, CTAS)
• type_json_validation.test
• type_json_cast.test
• type_json_protocol.test

TODO
Implement implicit JSON validation through Field_json::store(). (Currently, inserting invalid JSON like 'a' succeeds unless an explicit CHECK constraint is added).

[gemini-code-assist]

Code Review

This pull request introduces a pluggable JSON data type plugin (type_json) for MariaDB, migrating the built-in JSON type handling to a modular plugin structure. The changes include adding the plugin implementation (plugin.cc, sql_type_json.cc, sql_type_json.h), updating parser rules in sql_yacc.yy, and adding comprehensive test suites. Feedback on the implementation highlights several critical issues: a potential null pointer dereference in Field_json::report_wrong_value when table is null, an incorrect charset mapping in make_conversion_table_field which should strictly use utf8mb4_bin, and a regression in aggregate_for_comparison that prevents comparisons between JSON and numeric or temporal types. Additionally, minor cleanups are recommended, such as removing commented-out method declarations and fixing a comment typo in plugin.cc.

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[gemini-code-assist]

In Field_json::report_wrong_value, the code accesses table->s->db.str and table->s->table_name.str without checking if table is NULL. In MariaDB, Field objects can sometimes have a NULL table pointer (e.g., when used as temporary or conversion fields). Accessing table->s directly in these cases will cause a server crash (segmentation fault).

int Field_json::report_wrong_value(const ErrConv &val)
{
  get_thd()->push_warning_truncated_value_for_field(
      Sql_condition::WARN_LEVEL_WARN, "json", val.ptr(),
      table ? table->s->db.str : "",
      table ? table->s->table_name.str : "",
      field_name.str);
  reset();
  return 1;
}

[gemini-code-assist]

In Type_handler_json::make_conversion_table_field, the charset of the conversion field is set to target->charset(). However, a Field_json must always use the utf8mb4_bin character set to ensure correct JSON representation and comparison behavior. If the target field has a different character set (e.g., latin1), creating a Field_json with that character set violates this invariant and can lead to encoding issues or assertion failures.

Field *Type_handler_json::make_conversion_table_field(
    MEM_ROOT *root, TABLE *table, uint metadata, const Field *target) const
{
  /* A JSON field uses 4 bytes for length as json file is a long blob*/
  uint pack_length= metadata & 0x00ff;
  if (pack_length != 4)
    return NULL;

  return new (root) Field_json(NULL, (uchar *) "", 1, Field::NONE,
                               &empty_clex_str, table->s, &my_charset_utf8mb4_bin);
}

[gemini-code-assist]

There are several commented-out method declarations in the Type_handler_json class. To maintain code cleanliness and readability, these unused, commented-out lines should be removed.

References

1. Keep the API of custom wrapper classes minimal by avoiding or removing unused methods.

[gemini-code-assist]

In Type_collection_json::aggregate_for_comparison, the aggregation only allows comparison between JSON and other string/blob/null types. Any comparison between JSON and numeric types (like INT, DOUBLE, DECIMAL) or temporal types (like DATETIME, DATE) will return NULL. This prevents implicit comparison between JSON columns and numbers/dates, which is a regression compared to the previous LONGTEXT-based JSON implementation. Consider allowing aggregation with other basic types or delegating to the standard string type collection.

[gemini-code-assist]

There is a typo/formatting issue at the end of the comment on line 28: */ is left over at the end of a single-line // comment. This should be cleaned up.

  MariaDB_PLUGIN_MATURITY_GAMMA // Maturity (see include/mysql/plugin.h)