Create an r2d2 pool of SyncConnectionWrapper

[JMLX42]

Hello,

Here is my existing code:

pub type Connection = SqliteConnection;

pub struct Database {
    connection: Pool<ConnectionManager<Connection>>,
}

impl Database {
    pub fn new(database_url: Url) -> Self {
        let manager = ConnectionManager::<Connection>::new(database_url);
        let connection = r2d2::Pool::builder()
            .max_size(16)
            .build(manager)
            .expect("Failed to create pool.");

        Self { connection }
    }

Now I need to run some async transactions. So I'm thinking the best way to do this is to use a pool of async connections. Since I am using SQLite, here is what I tried:

pub type Connection = SyncConnectionWrapper<SqliteConnection>;

pub struct Database {
    connection: Pool<ConnectionManager<Connection>>,
}

impl Database {
    pub fn new(database_url: Url) -> Self {
        let manager = ConnectionManager::<Connection>::new(database_url);
        let connection = r2d2::Pool::builder()
            .max_size(16)
            .build(manager)
            .expect("Failed to create pool.");

        Self { connection }
    }

But it doesn't work:

error[E0277]: the trait bound `diesel_async::sync_connection_wrapper::SyncConnectionWrapper<diesel::SqliteConnection>: diesel::r2d2::R2D2Connection` is not satisfied
   --> crates/gltf_live_database/src/lib.rs:72:5
    |
70  | #[derive(Clone)]
    |          ----- in this derive macro expansion
71  | pub struct Database {
72  |     connection: Pool<ConnectionManager<Connection>>,
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `diesel::r2d2::R2D2Connection` is not implemented for `diesel_async::sync_connection_wrapper::SyncConnectionWrapper<diesel::SqliteConnection>`, which is required by `diesel::r2d2::ConnectionManager<diesel_async::sync_connection_wrapper::SyncConnectionWrapper<diesel::SqliteConnection>>: r2d2::ManageConnection`
    |
    = help: the trait `diesel::r2d2::R2D2Connection` is implemented for `diesel::SqliteConnection`
    = note: required for `diesel::r2d2::ConnectionManager<diesel_async::sync_connection_wrapper::SyncConnectionWrapper<diesel::SqliteConnection>>` to implement `r2d2::ManageConnection`
note: required by a bound in `r2d2::Pool`
   --> /home/jmlx/.cargo/registry/src/index.crates.io-6f17d22bba15001f/r2d2-0.8.10/src/lib.rs:319:8
    |
317 | pub struct Pool<M>(Arc<SharedPool<M>>)
    |            ---- required by a bound in this struct
318 | where
319 |     M: ManageConnection;
    |        ^^^^^^^^^^^^^^^^ required by this bound in `Pool`
    = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)

I' ve found the example on how to use SyncConnectionWrapper<SqliteConnection>, but nothing regarding an r2d2 pool of SyncConnectionWrapper<SqliteConnection>.

Please advise,

[weiznich]

This is expected behavior. The SyncConnectionWrapper exposes an async connection implementation. r2d2 is a sync connection pool implementation and therefore fundamentally incompatible with async connection implementations. You might want to use an async connection pool like deadpool, bb8 or mobc instead.