executor: add executor::exit (#1155)

This allows blocking on executors from within a `threadpool::blocking` call.

tokio-executor/src/enter.rs

@@ -67,6 +67,42 @@ pub fn enter() -> Result<Enter, EnterError> {
     })
 }
 
+// Forces the current "entered" state to be cleared while the closure
+// is executed.
+//
+// # Warning
+//
+// This is hidden for a reason. Do not use without fully understanding
+// executors. Misuing can easily cause your program to deadlock.
+#[doc(hidden)]
+pub fn exit<F: FnOnce() -> R, R>(f: F) -> R {
+    // Reset in case the closure panics
+    struct Reset;
+    impl Drop for Reset {
+        fn drop(&mut self) {
+            ENTERED.with(|c| {
+                c.set(true);
+            });
+        }
+    }
+
+    ENTERED.with(|c| {
+        debug_assert!(c.get());
+        c.set(false);
+    });
+
+    let reset = Reset;
+    let ret = f();
+    ::std::mem::forget(reset);
+
+    ENTERED.with(|c| {
+        assert!(!c.get(), "closure claimed permanent executor");
+        c.set(true);
+    });
+
+    ret
+}
+
 impl Enter {
     /// Register a callback to be invoked if and when the thread
     /// ceased to act as an executor.

tokio-executor/src/lib.rs

@@ -1,5 +1,7 @@
 #![deny(missing_docs, missing_debug_implementations, warnings)]
 #![doc(html_root_url = "https://docs.rs/tokio-executor/0.1.7")]
+// Our MSRV doesn't allow us to fix these warnings yet
+#![allow(rust_2018_idioms)]
 
 //! Task execution related traits and utilities.
 //!
@@ -61,7 +63,7 @@ mod global;
 pub mod park;
 mod typed;
 
-pub use enter::{enter, Enter, EnterError};
+pub use enter::{enter, exit, Enter, EnterError};
 pub use error::SpawnError;
 pub use executor::Executor;
 pub use global::{spawn, with_default, DefaultExecutor};