executor, threadpool: forward port fix from #1155 (#1433)

Add executor::exit, allowing other executors inside threadpool::blocking.

tokio-threadpool/src/blocking.rs

@@ -4,6 +4,7 @@ use futures_core::ready;
 use std::error::Error;
 use std::fmt;
 use std::task::Poll;
+use tokio_executor;
 
 /// Error raised by `blocking`.
 pub struct BlockingError {
@@ -140,7 +141,10 @@ where
     ready!(res)?;
 
     // Currently in blocking mode, so call the inner closure
-    let ret = f();
+    //
+    // "Exit" the current executor in case the blocking function wants
+    // to call a different executor.
+    let ret = tokio_executor::exit(move || f());
 
     // Try to transition out of blocking mode. This is a fast path that takes
     // back ownership of the worker if the worker handoff didn't complete yet.

tokio-threadpool/tests/blocking.rs

@@ -39,6 +39,26 @@ fn basic() {
     rx2.recv().unwrap();
 }
 
+#[test]
+fn other_executors_can_run_inside_blocking() {
+    let _ = ::env_logger::try_init();
+
+    let pool = Builder::new().pool_size(1).max_blocking(1).build();
+
+    let (tx, rx) = mpsc::channel();
+
+    pool.spawn(async move {
+        let res = blocking(|| {
+            let _e = tokio_executor::enter().expect("nested blocking enter");
+            tx.send(()).unwrap();
+        });
+
+        assert_ready!(res).unwrap();
+    });
+
+    rx.recv().unwrap();
+}
+
 #[test]
 fn notify_task_on_capacity() {
     const BLOCKING: usize = 10;