fix: don't create static variables containing a tokio::runtime::Handle

There was a bug in the approach implemented in #30.

The issue could manifest as a panic with message:

"A Tokio 1.x context was found, but it is being shutdown."

This would occur in the following scenario:

1. `.compat()` was used within a tokio context other than the fallback runtime.
2. That runtime was shutdown.
3. `.compat()` was used again.

The root cause being that we had a `Handle` stored in a `static` global.
`Handle`s are weak by design, and cannot prevent the runtime from shutting down.

I reverted most of the changes in #30 and instead added a `get_runtime_handle` function.
This function attempts to acquire an existing runtime handle with `try_current` before creating a fallback runtime.
This should be safe because `try_current` will only succeed within the scope of a tokio runtime context,
and it cannot outlive that scope.

src/lib.rs

@@ -208,7 +208,7 @@ pin_project! {
             if this.inner.is_some() {
                 // If the inner future wasn't moved out using into_inner,
                 // enter the tokio context while the inner value is dropped.
-                let _guard = TOKIO1.handle.enter();
+                let _guard = get_runtime_handle().enter();
                 this.project().inner.set(None);
             }
         }
@@ -326,7 +326,7 @@ impl<T: Future> Future for Compat<T> {
     type Output = T::Output;
 
     fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let _guard = TOKIO1.handle.enter();
+        let _guard = get_runtime_handle().enter();
         self.get_pin_mut().poll(cx)
     }
 }
@@ -453,38 +453,21 @@ impl<T: futures_io::AsyncSeek> tokio::io::AsyncSeek for Compat<T> {
     }
 }
 
-static TOKIO1: Lazy<GlobalRuntime> = Lazy::new(|| {
-    let mut fallback_rt = None;
-    let handle = tokio::runtime::Handle::try_current().unwrap_or_else(|_| {
-        thread::Builder::new()
-            .name("async-compat/tokio-1".into())
-            .spawn(move || TOKIO1.fallback_rt.as_ref().unwrap().block_on(Pending))
-            .unwrap();
-        let rt = tokio::runtime::Builder::new_current_thread()
-            .enable_all()
-            .build()
-            .expect("cannot start tokio-1 runtime");
-
-        let handle = rt.handle().clone();
-
-        fallback_rt = Some(rt);
-
-        handle
-    });
+fn get_runtime_handle() -> tokio::runtime::Handle {
+    tokio::runtime::Handle::try_current().unwrap_or_else(|_| TOKIO1.handle().clone())
+}
 
-    GlobalRuntime {
-        handle,
-        fallback_rt,
-    }
+static TOKIO1: Lazy<tokio::runtime::Runtime> = Lazy::new(|| {
+    thread::Builder::new()
+        .name("async-compat/tokio-1".into())
+        .spawn(|| TOKIO1.block_on(Pending))
+        .unwrap();
+    tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .expect("cannot start tokio-1 runtime")
 });
 
-struct GlobalRuntime {
-    /// The handle used for all `Compat` futures.
-    handle: tokio::runtime::Handle,
-    /// Only used if we couldn't acquire a handle to a runtime on creation.
-    fallback_rt: Option<tokio::runtime::Runtime>,
-}
-
 struct Pending;
 
 impl Future for Pending {
@@ -497,29 +480,31 @@ impl Future for Pending {
 
 #[cfg(test)]
 mod tests {
+    use super::Lazy;
     use crate::{CompatExt, TOKIO1};
 
     #[test]
-    fn existing_tokio_runtime_is_reused_by_compat() {
+    fn fallback_runtime_is_created_if_and_only_if_outside_tokio_context() {
+        // Use compat inside of a tokio context.
         tokio::runtime::Builder::new_multi_thread()
             .enable_all()
             .build()
             .unwrap()
-            .block_on(async { println!("foo") }.compat());
+            .block_on(use_tokio().compat());
 
-        assert!(TOKIO1.fallback_rt.is_none());
-    }
+        // We didn't need to create the fallback runtime, because we used compat
+        // inside of an existing tokio context.
+        assert!(Lazy::get(&TOKIO1).is_none());
 
-    #[test]
-    fn tokio_runtime_is_reused_even_after_it_exits() {
-        tokio::runtime::Builder::new_multi_thread()
-            .enable_all()
-            .build()
-            .unwrap()
-            .block_on(async { println!("foo") });
+        // Use compat outside of a tokio context.
+        futures::executor::block_on(use_tokio().compat());
 
-        futures::executor::block_on(async { println!("foo") }.compat());
+        // We must have created the fallback runtime, because we used compat
+        // outside of a tokio context.
+        assert!(Lazy::get(&TOKIO1).is_some());
+    }
 
-        assert!(TOKIO1.fallback_rt.is_none());
+    async fn use_tokio() {
+        tokio::time::sleep(std::time::Duration::from_micros(1)).await
     }
 }