The trait bound `futures::Future, Error=Box<

Question

I have the following (simplified) code snippet and I'm trying to run a future on a CpuPool:

use futures::{self, Future, IntoFuture};
use std::error;
use futures_cpupool::CpuPool;

pub struct Store<T: 'static + Send + Sync> {
    inner: Arc<StoreInner<T, C, SerDe, P>>,
}

struct StoreInner<T: 'static + Send + Sync> {
    read_thread_pool: CpuPool,
}

impl<T: 'static + Send + Sync> Store<T> {
    pub fn get(self, obj_id: String) -> Box<Future<Item = Arc<T>, Error = Box<error::Error + Send>>>
    where
        T: for<'de> BinaryDeserialize<'de>,
    {
        let latest_version_id =
            futures::future::ok(()).and_then(move |_| self.get_latest_version_id(&obj_id));
        let latest_version = latest_version_id.and_then(
            move |version_id| -> Box<Future<Item = Arc<T>, Error = Box<error::Error + Send>>> {
                self.get_version(&obj_id, version_id)
            },
        );
        Box::new(self.inner.read_thread_pool.spawn(latest_version))
    }
}

However, I get the following error when I try to compile.

error[E0277]: the trait bound `futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>: std::marker::Send` is not satisfied
   --> src/store/mod.rs:181:46
    |
181 |         Box::new(self.inner.read_thread_pool.spawn(Box::new(latest_version)))
    |                                              ^^^^^ `futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>` cannot be sent between threads safely
    |
    = help: the trait `std::marker::Send` is not implemented for `futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>`
    = note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>`
    = note: required because it appears within the type `std::boxed::Box<futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>`
    = note: required because it appears within the type `futures::future::chain::Chain<futures::AndThen<futures::FutureResult<(), std::boxed::Box<std::error::Error + std::marker::Send>>, std::boxed::Box<futures::Future<Item=serde::export::Option<std::string::String>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:177:67: 177:115 self:_, obj_id:_]>, std::boxed::Box<futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:178:56: 180:10 self:_, obj_id:_]>`
    = note: required because it appears within the type `futures::AndThen<futures::AndThen<futures::FutureResult<(), std::boxed::Box<std::error::Error + std::marker::Send>>, std::boxed::Box<futures::Future<Item=serde::export::Option<std::string::String>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:177:67: 177:115 self:_, obj_id:_]>, std::boxed::Box<futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:178:56: 180:10 self:_, obj_id:_]>`
    = note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<futures::AndThen<futures::AndThen<futures::FutureResult<(), std::boxed::Box<std::error::Error + std::marker::Send>>, std::boxed::Box<futures::Future<Item=serde::export::Option<std::string::String>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:177:67: 177:115 self:_, obj_id:_]>, std::boxed::Box<futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:178:56: 180:10 self:_, obj_id:_]>>`
    = note: required because it appears within the type `std::boxed::Box<futures::AndThen<futures::AndThen<futures::FutureResult<(), std::boxed::Box<std::error::Error + std::marker::Send>>, std::boxed::Box<futures::Future<Item=serde::export::Option<std::string::String>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:177:67: 177:115 self:_, obj_id:_]>, std::boxed::Box<futures::Future<Item=std::sync::Arc<T>, Error=std::boxed::Box<std::error::Error + std::marker::Send>>>, [closure@src/store/mod.rs:178:56: 180:10 self:_, obj_id:_]>>`

The code looks innocent and both Item and Error in the future are Send. I have no idea why I get this error.

Answer 1

I think the problem here is that the Future trait is not Send just because its Item and Error are both Send - you have to specify it explicitly.

If Future was a struct, I think the compiler could automatically derive Send, but it's not, so it can't.

I fixed up the compilation errors and added + Send to the the Future types you've defined, and it now compiles (apart from the fact there's no main function):

extern crate futures;
extern crate futures_cpupool;

use futures::Future;
use std::error;
use std::sync::Arc;
use futures_cpupool::CpuPool;

pub struct Store {
    inner: Arc<StoreInner>,
}

struct StoreInner {
    read_thread_pool: CpuPool,
}

impl Store {
    pub fn get<T: 'static + Send + Sync>(
        self,
        obj_id: String,
    ) -> Box<Future<Item = Arc<T>, Error = Box<error::Error + Send>> + Send> {
        let latest_version_id =
            futures::future::ok(()).and_then(move |_| self.get_latest_version_id(&obj_id));
        let latest_version =
            latest_version_id
                .and_then(
                    move |version_id| -> Box<
                        Future<Item = Arc<T>, Error = Box<error::Error + Send>> + Send,
                    > { self.get_version(&obj_id, version_id) },
                );
        Box::new(self.inner.read_thread_pool.spawn(latest_version))
    }

    pub fn get_latest_version_id(
        &self,
        obj_id: &String,
    ) -> Box<Future<Item = String, Error = Box<error::Error + Send>> + Send> {
        unimplemented!();
    }

    pub fn get_version<T: 'static + Send + Sync>(
        &self,
        obj_id: &String,
        version_id: String,
    ) -> Box<Future<Item = Arc<T>, Error = Box<error::Error + Send>> + Send> {
        unimplemented!();
    }
}

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