Hyper client does not clean up idle connections with non-tokio executor

Version

Hyper 0.13.10

Platform
The output of uname -a (UNIX), or version and 32 or 64-bit (Windows)

Fuchsia, as well as on Linux and OS-X

Description

Fuchsia uses hyper as our http client in a few places. Inside the Fuchsia operating system, we use a Fuchsia-specific executor. Our host-side development tools also use async-executor instead of tokio. Because of this, we've discovered that our client connection pool is not closing idle connections. This is because the pool implementation only runs the periodic idle connection cleaner if the runtime feature, which pulls in tokio, is enabled:

https://github.com/hyperium/hyper/blob/master/src/client/pool.rs#L385

We'd be happy to work around this on our side, but unfortunately hyper doesn't expose the pool implementation, or any method to remove idle connections.

One possible way we could support this is to add a function Client::close_idle_connections(), which under the covers calls PoolInner::clear_expired. We could set up a task using our timer interfaces to periodically close these connections. Would you be interested in patch that implements this? Or would you prefer a separate mechanism to expose this functionality?

Start in src/client/pool.rs around the runtime-gated idle cleaner and PoolInner::clear_expired. Compare the proposed Client::close_idle_connections API with the existing pool behavior and determine how a non-tokio executor could trigger cleanup; done means idle connections can be closed without requiring the runtime feature.