Implementing type reflection from mysql result

[danhuynhdev]

Add Mysql2::Result#field_types method to get field types from result.

Resolve: #1067

[sodabrew]

This looks good! Few smaller comments, and please rebase onto current master. (It appears there are no conflicts, but we'll get confirmation of unit tests.)

[danhuynhdev]

I've resolved the comments and squash the commits. Please feedback if any change is needed. cc: @sodabrew

[simi]

If I understand it well it will return nil for unknown type, right?

[sodabrew]

I wonder how we can force a unit test down the no-match path?

[danhuynhdev]

If we know a unknown to test for it, would it be unknown anymore 😆?

[danhuynhdev]

Anyway I've made it return 'unknown' for this case.

[simi]

I was looking into this @sodabrew and I have found out there's no easy way how to introduce custom column type in MySQL :/

But maybe there's some way how to create table including all MySQL data types (by some meta programming) and it would be possible to check if we cover all cases there.

[danhuynhdev]

It will fail if we include MYSQL_TYPE_JSON though. I cannot find any code to handle MYSQL_TYPE_JSON.

[sodabrew]

See #693 for the history on how JSON is supported; basically it's a fall-through to char[].

It seems to me that the type table should be updated to explicitly include JSON so that it's not "accidentally" supported but is explictly routed through the char[] path of the result handler.

What is the failure you're seeing if MYSQL_TYPE_JSON is included in this table?

[simi]

🤔 I was looking for any chance to keep the list of types updated when new type would be introduced (by exposing enum_field_types and ensuring we handle all supported types), but looking into enum_field_types history, it is rarely touched and seems pretty stable.

[sodabrew]

I wonder how we can force a unit test down the no-match path?

[danhuynhdev]

Seems like mysql2 cannot handle JSON type in query result. I've attempted to add JSON type to the list but it just failed right away.

[sodabrew]

The JSON type should be supported, what error are you seeing?

[danhuynhdev]

This is actually not an error on our side but on mysql's json support. I'm using mariadb on my local machine and they don't support JSON type. Also MYSQL_TYPE_JSON only exist on mysql 5.7.8 onward. Attempt to add it into the table will result in this error:

../../../../ext/mysql2/result.c:320:12: error: ‘MYSQL_TYPE_JSON’ undeclared (first use in this function)
       case MYSQL_TYPE_JSON:     // char[]

[danhuynhdev]

Not sure if we should add something like this for MYSQL_TYPE_JSON

#ifdef MYSQL_TYPE_JSON
      case MYSQL_TYPE_JSON:     // json
        rb_field_type = rb_str_new_cstr("json");
        break;
#endif

or just declare it if not exist

#ifnotdef MYSQL_TYPE_JSON
#define MYSQL_TYPE_JSON 245
#endif 

I'm inclined to do latter since it doesn't require user to install the correct header version on the client side.

[sodabrew]

I was going to say we should do the first one, but you're right that some clients may have been compiled with an older library, and the column type number is should always be the same for compatibility reasons.

[sodabrew]

This looks great! Thanks for updating your PR along with all the review comments.