rt: fix Runtime::reactor() as used by tokio-core (#721)

* rt: fix `Runtime::reactor()` as used by tokio-core

Up until Tokio v0.1.11, the handle returned by `Runtime::reactor()`
pointed to a reactor instance running in a background thread. The thread
was eagerly spawned.

As of v0.1.12, a reactor instance is created per runtime worker thread.
`Runtime::reactor()` was deprecated and updated to point to the reactor
for one of the worker threads.

A problem occurs when attempting to use the reactor before spawning a
task. Worker threads are spawned lazily, which means that the reactor
referenced by `Runtime::reactor()` is not yet running.

This patch changes `Runtime::reactor` back to a dedicated reactor
running on a background thread. However, the background thread is now
spawned lazily when the deprecated function is first called.

Fixes #720

* Fix comment

Co-Authored-By: carllerche <[email protected]>

src/runtime/builder.rs

@@ -273,9 +273,6 @@ impl Builder {
         // Get a handle to the clock for the runtime.
         let clock = self.clock.clone();
 
-        // Get a handle to the first reactor.
-        let reactor = reactor_handles[0].clone();
-
         let pool = self.threadpool_builder
             .around_worker(move |w, enter| {
                 let index = w.id().to_usize();
@@ -299,9 +296,14 @@ impl Builder {
             })
             .build();
 
+        // To support deprecated `reactor()` function
+        let reactor = Reactor::new()?;
+        let reactor_handle = reactor.handle();
+
         Ok(Runtime {
             inner: Some(Inner {
-                reactor,
+                reactor_handle,
+                reactor: Mutex::new(Some(reactor)),
                 pool,
             }),
         })