feat(swarm): Make executor for connection tasks explicit (#3097)

Previously, the executor for connection tasks silently defaulted to a `futures::executor::ThreadPool`. This causes issues such as https://github.com/libp2p/rust-libp2p/issues/2230.

With this patch, we force the user to choose, which executor they want to run the connection tasks on which results in overall simpler API with less footguns.

Closes #3068.

core/CHANGELOG.md

@@ -4,8 +4,11 @@
 
 - Hide `prost::Error` from public API in `FromEnvelopeError::InvalidPeerRecord` and `signed_envelope::DecodingError`. See [PR 3058].
 
+- Move `Executor` to `libp2p-swarm`. See [PR 3097].
+
 [PR 3031]: https://github.com/libp2p/rust-libp2p/pull/3031
 [PR 3058]: https://github.com/libp2p/rust-libp2p/pull/3058
+[PR 3097]: https://github.com/libp2p/rust-libp2p/pull/3097
 
 # 0.37.0
 

core/src/lib.rs

@@ -83,23 +83,3 @@ pub use upgrade::{InboundUpgrade, OutboundUpgrade, ProtocolName, UpgradeError, U
 #[derive(thiserror::Error, Debug)]
 #[error(transparent)]
 pub struct DecodeError(prost::DecodeError);
-
-use std::{future::Future, pin::Pin};
-
-/// Implemented on objects that can run a `Future` in the background.
-///
-/// > **Note**: While it may be tempting to implement this trait on types such as
-/// >           [`futures::stream::FuturesUnordered`], please note that passing an `Executor` is
-/// >           optional, and that `FuturesUnordered` (or a similar struct) will automatically
-/// >           be used as fallback by libp2p. The `Executor` trait should therefore only be
-/// >           about running `Future`s in the background.
-pub trait Executor {
-    /// Run the given future in the background until it ends.
-    fn exec(&self, future: Pin<Box<dyn Future<Output = ()> + Send>>);
-}
-
-impl<F: Fn(Pin<Box<dyn Future<Output = ()> + Send>>)> Executor for F {
-    fn exec(&self, f: Pin<Box<dyn Future<Output = ()> + Send>>) {
-        self(f)
-    }
-}