Fix potential hang if extensions contain same StreamRef

If a user stored a `StreamRef` to the same stream in the request or
response extensions, they would be dropped while the internal store lock
was held. That would lead to a deadlock, since dropping a stream ref
will try to take the store lock to clean up.

Clear extensions of Request and Response before locking store, prevent
this.

Fixes hyperium/hyper#2621

src/proto/streams/streams.rs

@@ -207,13 +207,16 @@ where
 
     pub fn send_request(
         &mut self,
-        request: Request<()>,
+        mut request: Request<()>,
         end_of_stream: bool,
         pending: Option<&OpaqueStreamRef>,
     ) -> Result<StreamRef<B>, SendError> {
         use super::stream::ContentLength;
         use http::Method;
 
+        // Clear before taking lock, incase extensions contain a StreamRef.
+        request.extensions_mut().clear();
+
         // TODO: There is a hazard with assigning a stream ID before the
         // prioritize layer. If prioritization reorders new streams, this
         // implicitly closes the earlier stream IDs.
@@ -1062,9 +1065,11 @@ impl<B> StreamRef<B> {
 
     pub fn send_response(
         &mut self,
-        response: Response<()>,
+        mut response: Response<()>,
         end_of_stream: bool,
     ) -> Result<(), UserError> {
+        // Clear before taking lock, incase extensions contain a StreamRef.
+        response.extensions_mut().clear();
         let mut me = self.opaque.inner.lock().unwrap();
         let me = &mut *me;
 
@@ -1082,7 +1087,12 @@ impl<B> StreamRef<B> {
         })
     }
 
-    pub fn send_push_promise(&mut self, request: Request<()>) -> Result<StreamRef<B>, UserError> {
+    pub fn send_push_promise(
+        &mut self,
+        mut request: Request<()>,
+    ) -> Result<StreamRef<B>, UserError> {
+        // Clear before taking lock, incase extensions contain a StreamRef.
+        request.extensions_mut().clear();
         let mut me = self.opaque.inner.lock().unwrap();
         let me = &mut *me;