Make tokio::run panic if called from inside tokio::run (#646)

This is implemented by creating an `Enter` instance from within `run`.

This patch also introduces `Enter::block_on`.

Fixes #504

src/runtime/mod.rs

@@ -125,6 +125,7 @@ use reactor::{Background, Handle};
 
 use std::io;
 
+use tokio_executor::enter;
 use tokio_threadpool as threadpool;
 
 use futures;
@@ -210,7 +211,9 @@ where F: Future<Item = (), Error = ()> + Send + 'static,
 {
     let mut runtime = Runtime::new().unwrap();
     runtime.spawn(future);
-    runtime.shutdown_on_idle().wait().unwrap();
+    enter().expect("nested tokio::run")
+        .block_on(runtime.shutdown_on_idle())
+        .unwrap();
 }
 
 impl Runtime {

tests/runtime.rs

@@ -390,3 +390,15 @@ mod from_block_on_all {
         test(|f| { tokio::spawn(f); })
     }
 }
+
+#[test]
+fn run_in_run() {
+    use std::panic;
+
+    tokio::run(lazy(|| {
+        panic::catch_unwind(|| {
+            tokio::run(lazy(|| { Ok::<(), ()>(()) }))
+        }).unwrap_err();
+        Ok::<(), ()>(())
+    }));
+}

tokio-executor/src/enter.rs

@@ -3,6 +3,8 @@ use std::cell::Cell;
 use std::error::Error;
 use std::fmt;
 
+use futures::{self, Future};
+
 thread_local!(static ENTERED: Cell<bool> = Cell::new(false));
 
 /// Represents an executor context.
@@ -80,6 +82,13 @@ impl Enter {
     pub fn make_permanent(mut self) {
         self.permanent = true;
     }
+
+    /// Blocks the thread on the specified future, returning the value with
+    /// which that future completes.
+    pub fn block_on<F: Future>(&mut self, f: F) -> Result<F::Item, F::Error> {
+        futures::executor::spawn(f).wait_future()
+    }
+
 }
 
 impl fmt::Debug for Enter {