Implementation of sqlx::Acquire is not general enough for MySqlConnection

[apognu]

This is, for me, a weird one, that I can only reproduce in a very specific situation, which may or may not be on sqlx's side.

My setup includes version 0.4.2 of sqlx, with Rocket from master (more on that below).

I am using sqlx as a State in Rocket, so I can recover a Pool<MySql> in every action function. The global idea of what I am trying to do is create database-layer functions that could either directly take &Pool<MySql>s or transactions into the pool (depending on where it is used, it may or may not be part of a transaction).

When used this way,, an error saying that the implementation of sqlx::Acquire is not general enough.

Here is a full minimal reproduction of the issue:

#[macro_use]
extern crate rocket;

use rocket::State;
use sqlx::{Acquire, MySql, Pool};

#[tokio::main]
async fn main() {}

#[get("/pool")]
async fn pool(pool: State<'_, Pool<MySql>>) {
    run_query(&*pool).await;
}

#[get("/transaction")]
async fn txn(pool: State<'_, Pool<MySql>>) {
    let mut txn = pool.begin().await.unwrap();

    run_query(&mut *txn).await;

    txn.commit().await.unwrap();
}

async fn run_query<'c, E>(pool: E)
where
    E: Acquire<'c, Database = MySql>,
{
    let mut conn = pool.acquire().await.unwrap();

    sqlx::query("SELECT 1").fetch_one(&mut *conn).await.unwrap();
}

The reported error is the following:

   Compiling rocket-sqlx-repro v0.1.0 (/.../rocket-sqlx-repro)
error: implementation of `sqlx::Acquire` is not general enough
  --> src/main.rs:10:1
   |
10 |   #[get("/pool")]
   |   ^^^^^^^^^^^^^^^ implementation of `sqlx::Acquire` is not general enough
   |
  ::: /.../src/github.com-1ecc6299db9ec823/sqlx-core-0.4.2/src/acquire.rs:8:1
   |
8  | / pub trait Acquire<'c> {
9  | |     type Database: Database;
10 | |
11 | |     type Connection: Deref<Target = <Self::Database as Database>::Connection> + DerefMut;
...  |
15 | |     fn begin(self) -> BoxFuture<'c, Result<Transaction<'c, Self::Database>, Error>>;
16 | | }
   | |_- trait `sqlx::Acquire` defined here
   |
   = note: `&Pool<MySql>` must implement `sqlx::Acquire<'0>`, for any lifetime `'0`...
   = note: ...but `&Pool<MySql>` actually implements `sqlx::Acquire<'1>`, for some specific lifetime `'1`
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: implementation of `sqlx::Acquire` is not general enough
  --> src/main.rs:16:1
   |
16 |   #[get("/transaction")]
   |   ^^^^^^^^^^^^^^^^^^^^^^ implementation of `sqlx::Acquire` is not general enough
   |
  ::: /.../src/github.com-1ecc6299db9ec823/sqlx-core-0.4.2/src/acquire.rs:8:1
   |
8  | / pub trait Acquire<'c> {
9  | |     type Database: Database;
10 | |
11 | |     type Connection: Deref<Target = <Self::Database as Database>::Connection> + DerefMut;
...  |
15 | |     fn begin(self) -> BoxFuture<'c, Result<Transaction<'c, Self::Database>, Error>>;
16 | | }
   | |_- trait `sqlx::Acquire` defined here
   |
   = note: `sqlx::Acquire<'1>` would have to be implemented for the type `&'0 mut MySqlConnection`, for any two lifetimes `'0` and `'1`...
   = note: ...but `sqlx::Acquire<'2>` is actually implemented for the type `&'2 mut MySqlConnection`, for some specific lifetime `'2`
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 2 previous errors

error: could not compile `rocket-sqlx-repro`

To learn more, run the command again with --verbose.

The exact same function works properly when used without the Rocket layer, but fails when wrapped in Rocket's macros. I reported the issue here first because the error mentioned sqlx::Acquire. But I really have no idea what is going on.

Feel free to close if you think this is a Rocket issue (and sorry if it is).

Thank you.

[abonander]

Related to #954 but this seems to concern a different trait impl, or at least one of the errors does.

[apognu]

Thank you for classifying this. I see you added db:mysql: if it is any help at all, the same error appears if I replace MySql with Postgres, so it might not be a MySQL-specific issue.

[jplatte]

I don't think this is related to #954, and probably a bug in user code. Can you provide a minimal verifiable example?

[jplatte]

Also, what version of Rust are you using?

[apognu]

My first post contains all the code required to reproduce the issue. Bug was present with both Rust 1.49.0 and 1.51.0-nightly. It only occurs when used in a method decorated with Rocket's macro. Here's the minimal dependencies used in my example:

[dependencies]
rocket = { git = "https://github.com/SergioBenitez/Rocket" }
sqlx = { version = "0.4.2", features = ["mysql", "postgres", "runtime-tokio-rustls"] }
tokio = { version = "0.2.9" }

Do you any need any more information?

[jplatte]

Interesting, it works if you remove just the attributes. Also, I've been able to reduce this using cargo expand to just this:

use rocket::{data::Data, handler::HandlerFuture, request::Request, State};
use sqlx::{Acquire, MySql, Pool};

#[allow(unreachable_code)]
fn monomorphized_function<'_b>(__req: &'_b Request, __data: Data) -> HandlerFuture<'_b> {
    Box::pin(async move {
        let __rocket_param_pool: State<'_, Pool<MySql>> = todo!();
        let ___responder = pool(__rocket_param_pool).await;
        rocket::handler::Outcome::from(__req, ___responder)
    })
}

async fn pool(pool: State<'_, Pool<MySql>>) {
    run_query(&*pool).await;
}

async fn run_query<'c, E>(pool: E)
where
    E: Acquire<'c, Database = MySql>,
{
    let mut conn = pool.acquire().await.unwrap();

    sqlx::query("SELECT 1").fetch_one(&mut *conn).await.unwrap();
}

The reason I find this very weird is that rustc complains about the bound in run_query() at the usage of pool(). This smells like a bug in rustc.

[jplatte]

This seems to come down to lifetime parameters spilling into impl Future types that async fn are lowered to. I'm pretty sure I've got a fix about ready.

[jplatte]

Okay, I was wrong. This is related to Send. Conceptually this can be fixed by defining run_query as

fn run_query<'a, 'c, E>(pool: E) -> impl Future<Output = ()> + Send + 'a
where
    E: Acquire<'c, Database = MySql> + Send + 'a,
    E::Connection: Send,
{
    async move {
        let mut conn = pool.acquire().await.unwrap();
        sqlx::query("SELECT 1").fetch_one(&mut *conn).await.unwrap();
    }
}

However, that produces a rather unhelpful error message:

error[E0308]: mismatched types
  --> src/lib.rs:23:37
   |
23 | fn run_query<'a, 'c, E>(pool: E) -> impl Future<Output = ()> + Send + 'a
   |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
   |
   = note: expected type `std::marker::Send`
              found type `std::marker::Send`

I will try to minimize that error and create an issue at rust-lang/rust.

I've also found a fix to this! For some reason that I couldn't figure out it only works against the current git master of sqlx, not against 0.4.2: You have to add the Send requirement to the associated type Acquire::Connection (#1020):

diff --git a/sqlx-core/src/acquire.rs b/sqlx-core/src/acquire.rs
index 308e80da..dd8670ae 100644
--- a/sqlx-core/src/acquire.rs
+++ b/sqlx-core/src/acquire.rs
@@ -8,7 +8,7 @@ use std::ops::{Deref, DerefMut};
 pub trait Acquire<'c> {
     type Database: Database;
 
-    type Connection: Deref<Target = <Self::Database as Database>::Connection> + DerefMut;
+    type Connection: Deref<Target = <Self::Database as Database>::Connection> + DerefMut + Send;
 
     fn acquire(self) -> BoxFuture<'c, Result<Self::Connection, Error>>;

and then define run_query as

fn run_query<'a, 'c, E>(pool: E) -> impl Future<Output = ()> + Send + 'a
where
    E: Acquire<'c, Database = MySql> + Send + 'a,
{
    async move {
        let mut conn = pool.acquire().await.unwrap();
        sqlx::query("SELECT 1").fetch_one(&mut *conn).await.unwrap();
    }
}

(same as above, but without the bound on E::Connection).

Realistically though, I'd recommend just not using the Acquire trait, instead using a &mut MySqlConnection parameter and calling pool.acquire().await? before run_query:

#[macro_use]
extern crate rocket;

use rocket::State;
use sqlx::{MySql, MySqlConnection, Pool};

#[get("/pool")]
async fn pool(pool: State<'_, Pool<MySql>>) {
    let mut conn = pool.acquire().await.unwrap();
    run_query(&mut conn).await;
}

#[get("/transaction")]
async fn txn(pool: State<'_, Pool<MySql>>) {
    let mut txn = pool.begin().await.unwrap();

    run_query(&mut *txn).await;

    txn.commit().await.unwrap();
}

async fn run_query(conn: &mut MySqlConnection) {
    sqlx::query("SELECT 1").fetch_one(conn).await.unwrap();
}

[apognu]

Using &mut MySqlConnection does fix my issue, indeed. Thank you for the pointer.

I don't know if you wish to keep this open or not (if there is actually an issue, regardless of my particular problem). I'll leave the decision to you.

Have a nice day.