Work around for http2 hang with Or (#199)

This is a nasty hack that works around
https://github.com/hyperium/hyper/issues/2621.

Fixes https://github.com/tokio-rs/axum/issues/191

src/routing/mod.rs

@@ -547,7 +547,11 @@ where
 
     fn call(&mut self, request: Request<B>) -> Self::Future {
         let mut res = Response::new(crate::body::empty());
-        res.extensions_mut().insert(FromEmptyRouter { request });
+
+        if request.extensions().get::<OrDepth>().is_some() {
+            res.extensions_mut().insert(FromEmptyRouter { request });
+        }
+
         *res.status_mut() = self.status;
         EmptyRouterFuture {
             future: futures_util::future::ok(res),
@@ -565,6 +569,39 @@ struct FromEmptyRouter<B> {
     request: Request<B>,
 }
 
+/// We need to track whether we're inside an `Or` or not, and only if we then
+/// should we save the request into the response extensions.
+///
+/// This is to work around https://github.com/hyperium/hyper/issues/2621.
+///
+/// Since ours can be nested we have to track the depth to know when we're
+/// leaving the top most `Or`.
+///
+/// Hopefully when https://github.com/hyperium/hyper/issues/2621 is resolved we
+/// can remove this nasty hack.
+#[derive(Debug)]
+struct OrDepth(usize);
+
+impl OrDepth {
+    fn new() -> Self {
+        Self(1)
+    }
+
+    fn increment(&mut self) {
+        self.0 += 1;
+    }
+
+    fn decrement(&mut self) {
+        self.0 -= 1;
+    }
+}
+
+impl PartialEq<usize> for &mut OrDepth {
+    fn eq(&self, other: &usize) -> bool {
+        self.0 == *other
+    }
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct PathPattern(Arc<Inner>);