Deriving sqlx::Type on an enum fails to decode a MySql enum

[cdhowie]

I have found these related issues/pull requests

This is reminiscent of #1241 but the underlying cause seems to be different.

Description

When testing if two enum types are compatible, sqlx checks if the flags are equal:

sqlx/sqlx-mysql/src/type_info.rs

Line 109 in b859914

return self.flags == other.flags;

This causes problems when the flags on the database column do not exactly match what the builtin __enum() typeinfo has on its flags:

sqlx/sqlx-mysql/src/type_info.rs

Line 40 in b859914

flags: ColumnFlags::ENUM,

For example, we have a column whose flags are NOT_NULL | ENUM | NO_DEFAULT_VALUE and we cannot therefore decode a #[derive(sqlx::Type)] enum from this column. It appears that if a column is marked "not null" or does not have a default value that decoding will therefore unnecessarily fail.

It seems to me that not all flags need to match in the enum case. Certain flags could be ignored, like NO_DEFAULT_VALUE and NOT_NULL.

In particular, this error gets raised:

error occurred while decoding column "Output": mismatched types; Rust type `db::Output` (as SQL type `ENUM`) is not compatible with SQL type `ENUM`

Reproduction steps

Derive sqlx::Type on an enum and attempt to decode it from a MySQL column that is marked either NOT NULL or does not have a default value.

Workaround

As much of MySqlTypeInfo is not public, the workaround is a bit kludgy, but it seems to work in our tests. Instead of deriving sqlx::Type we derive sqlx::Encode and sqlx::Decode, and implement sqlx::Type by hand, like so:

impl sqlx::Type<sqlx::MySql> for Output {
    fn type_info() -> <sqlx::MySql as sqlx::Database>::TypeInfo {
        sqlx::mysql::MySqlTypeInfo::__enum()
    }

    fn compatible(ty: &<sqlx::MySql as sqlx::Database>::TypeInfo) -> bool {
        sqlx::TypeInfo::name(ty) == "ENUM"
    }
}

SQLx version

0.8.3

Enabled SQLx features

chrono, mysql, runtime-tokio

Database server and version

MariaDB 10.11.8

Operating system

Linux (Ubuntu client, Debian server)

Rust version

1.84.1