The default Body type is used in the client in a Pool, so it has to be
Send. On the server, we can use a !Send type if the executor is single
threaded.
Expand the existing example to show that.
The `hyper::rt::Executor` trait allows defining custom executors to be
used with hyper's `Client` and `Server`.
Closes#1944
BREAKING CHANGE: Any type passed to the `executor` builder methods must
now implement `hyper::rt::Executor`.
`hyper::rt::spawn` usage should be replaced with `tokio::task::spawn`.
`hyper::rt::run` usage should be replaced with `#[tokio::main]` or
managing a `tokio::runtime::Runtime` manually.
Until this commit, servers have required that `Service` and their
`Future` to be `Send`, since the server needs to spawn some internal
tasks to an executor, and by default, that is `tokio::spawn`, which
could be spawning to a threadpool. This was true even if the user were
certain there was no threadpool involved, and was instead using a
different single-threaded runtime, like
`tokio::runtime::current_thread`.
This changes makes all the server pieces generic over an `E`, which is
essentially `Executor<PrivateTypes<Server::Future>>`. There's a new set
of internal traits, `H2Exec` and `NewSvcExec`, which allow for the type
signature to only show the generics that the user is providing. The
traits cannot be implemented explicitly, but there are blanket
implementations for `E: Executor<SpecificType>`. If the user provides
their own executor, it simply needs to have a generic `impl<F>
Executor<F> for MyExec`. That impl can have bounds deciding whether to
require `F: Send`. If the executor does require `Send`, and the
`Service` futures are `!Send`, there will be compiler errors.
To prevent a breaking change, all the types that gained the `E` generic
have a default type set, which is the original `tokio::spawn` executor.
