net: avoid race in driver shutdown

Fixes: tokio-rs#2924

tokio/src/io/driver/mod.rs

@@ -32,7 +32,7 @@ pub(crate) struct Driver {
     poll: mio::Poll,
 
     /// State shared between the reactor and the handles.
-    inner: Arc<Inner>,
+    inner: Option<Arc<Inner>>,
 }
 
 /// A reference to an I/O driver
@@ -106,11 +106,11 @@ impl Driver {
             events: Some(mio::Events::with_capacity(1024)),
             resources: slab,
             poll,
-            inner: Arc::new(Inner {
+            inner: Some(Arc::new(Inner {
                 registry,
                 io_dispatch: allocator,
                 waker,
-            }),
+            })),
         })
     }
 
@@ -122,7 +122,7 @@ impl Driver {
     /// to bind them to this event loop.
     pub(crate) fn handle(&self) -> Handle {
         Handle {
-            inner: Arc::downgrade(&self.inner),
+            inner: Arc::downgrade(self.inner.as_ref().expect("driver closed")),
         }
     }
 
@@ -181,6 +181,8 @@ impl Driver {
 
 impl Drop for Driver {
     fn drop(&mut self) {
+        let inner = self.inner.take().expect("driver closed");
+        drop(inner);
         self.resources.for_each(|io| {
             // If a task is waiting on the I/O resource, notify it. The task
             // will then attempt to use the I/O resource and fail due to the