How to access a future through parking_lot guard

[dusty-phillips]

I have a repro here:

https://github.com/dusty-phillips/async-graphql/blob/parking_lot_repro/examples/parking_lot.rs

In short, I'm using the parking_lot crate to get an RwLock. I've put the lock inside an Arc, and tried to access it from both a query and a mutation. However, I'm getting the following compile errors:

master* $ cargo run --example parking_lot
   Compiling async-graphql v0.10.8 (/home/dusty/beanstalk/async-graphql)
error: future cannot be sent between threads safely
  --> examples/parking_lot.rs:26:1
   |
26 | #[Object]
   | ^^^^^^^^^ future returned by `__resolve_field` is not `Send`
   |
   = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `*mut ()`
note: future is not `Send` as this value is used across an await
  --> examples/parking_lot.rs:30:9
   |
30 |         self.thing.read().get_the_value().await
   |         -----------------^^^^^^^^^^^^^^^^^^^^^^
   |         |
   |         await occurs here, with `self.thing.read()` maybe used later
   |         has type `lock_api::rwlock::RwLockReadGuard<'_, parking_lot::raw_rwlock::RawRwLock, Thing>`
31 |     }
   |     - `self.thing.read()` is later dropped here
   = note: required for the cast to the object type `dyn std::future::Future<Output = std::result::Result<serde_json::value::Value, anyhow::Error>> + std::marker::Send`

error: future cannot be sent between threads safely
  --> examples/parking_lot.rs:38:1
   |
38 | #[Object]
   | ^^^^^^^^^ future returned by `__resolve_field` is not `Send`
   |
   = help: within `impl std::future::Future`, the trait `std::marker::Send` is not implemented for `*mut ()`
note: future is not `Send` as this value is used across an await
  --> examples/parking_lot.rs:42:9
   |
42 |         self.thing.write().set_the_value(thing_value).await;
   |         ------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^- `self.thing.write()` is later dropped here
   |         |
   |         await occurs here, with `self.thing.write()` maybe used later
   |         has type `lock_api::rwlock::RwLockWriteGuard<'_, parking_lot::raw_rwlock::RawRwLock, Thing>`
   = note: required for the cast to the object type `dyn std::future::Future<Output = std::result::Result<serde_json::value::Value, anyhow::Error>> + std::marker::Send`

error: aborting due to 2 previous errors

error: could not compile `async-graphql`.

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

I've tried moving the guard out, but it doesn't seem to work. Intuitively, I can imagine it might not be possible to hold a lock across an await, but there is no compile error when I try to access it in the main() function (line 72) I don't get a compile error there. So the issue seems to be something in the Object macro, but I don't know how to debug. Any suggestions?

[sunli829]

This could be the reason:
Async_graphql internally calls MutationRoot::set_value with an indirect boxing Pin<Box<dyn Future<Output = Result<()>> + 'a + Send>>, and RwLockGuard does not implement Send.

Synchronization lock is not recommended, use async_std::sync::RwLock to solve this problem.

use async_std::sync::RwLock;

#[Object]
impl MutationRoot {
    #[field]
    async fn set_value(&self, thing_value: String) -> bool {
        self.thing.write().await.set_the_value(thing_value).await;
        true
    }
}

[dusty-phillips]

Ah, thanks. That works for my immediate problem, although it shows that I'm going to have a much worse time working with a deeply nested parking_lot lock in a dependent library later. 🤢