Remove uses of Pin::new_unchecked in h1 Dispatcher (#1374)

This removes the last uses of unsafe `Pin` functions in actix-web.

This PR adds a `Pin<Box<_>>` wrapper to `DispatcherState::Upgrade`,
`State::ExpectCall`, and `State::ServiceCall`.

The previous uses of the futures `State::ExpectCall` and `State::ServiceCall`
were Undefined Behavior - a future was obtained from `self.expect.call`
or `self.service.call`, pinned on the stack, and then immediately
returned from `handle_request`. The only alternative to using `Box::pin`
would be to refactor `handle_request` to write the futures directly into
their final location, or avoid polling them before they are returned.

The previous use of `DispatcherState::Upgrade` doesn't seem to be
unsound. However, having data pinned inside an enum that we
`std::mem::replace` would require some careful `unsafe` code to ensure
that we never call `std::mem::replace` when the active variant contains
pinned data. By using `Box::pin`, we any possibility of future
refactoring accidentally introducing undefined behavior.

Co-authored-by: Yuki Okushi <huyuumi.dev@gmail.com>

actix-http/src/h1/dispatcher.rs

@@ -66,7 +66,7 @@ where
     U::Error: fmt::Display,
 {
     Normal(InnerDispatcher<T, S, B, X, U>),
-    Upgrade(U::Future),
+    Upgrade(Pin<Box<U::Future>>),
     None,
 }
 
@@ -114,8 +114,8 @@ where
     B: MessageBody,
 {
     None,
-    ExpectCall(X::Future),
-    ServiceCall(S::Future),
+    ExpectCall(Pin<Box<X::Future>>),
+    ServiceCall(Pin<Box<S::Future>>),
     SendPayload(ResponseBody<B>),
 }
 
@@ -298,7 +298,7 @@ where
         let len = self.write_buf.len();
         let mut written = 0;
         while written < len {
-            match unsafe { Pin::new_unchecked(&mut self.io) }
+            match Pin::new(&mut self.io)
                 .poll_write(cx, &self.write_buf[written..])
             {
                 Poll::Ready(Ok(0)) => {
@@ -372,10 +372,10 @@ where
                     None => None,
                 },
                 State::ExpectCall(ref mut fut) => {
-                    match unsafe { Pin::new_unchecked(fut) }.poll(cx) {
+                    match fut.as_mut().poll(cx) {
                         Poll::Ready(Ok(req)) => {
                             self.send_continue();
-                            self.state = State::ServiceCall(self.service.call(req));
+                            self.state = State::ServiceCall(Box::pin(self.service.call(req)));
                             continue;
                         }
                         Poll::Ready(Err(e)) => {
@@ -387,7 +387,7 @@ where
                     }
                 }
                 State::ServiceCall(ref mut fut) => {
-                    match unsafe { Pin::new_unchecked(fut) }.poll(cx) {
+                    match fut.as_mut().poll(cx) {
                         Poll::Ready(Ok(res)) => {
                             let (res, body) = res.into().replace_body(());
                             self.state = self.send_response(res, body)?;
@@ -463,8 +463,8 @@ where
     ) -> Result<State<S, B, X>, DispatchError> {
         // Handle `EXPECT: 100-Continue` header
         let req = if req.head().expect() {
-            let mut task = self.expect.call(req);
-            match unsafe { Pin::new_unchecked(&mut task) }.poll(cx) {
+            let mut task = Box::pin(self.expect.call(req));
+            match task.as_mut().poll(cx) {
                 Poll::Ready(Ok(req)) => {
                     self.send_continue();
                     req
@@ -482,8 +482,8 @@ where
         };
 
         // Call service
-        let mut task = self.service.call(req);
-        match unsafe { Pin::new_unchecked(&mut task) }.poll(cx) {
+        let mut task = Box::pin(self.service.call(req));
+        match task.as_mut().poll(cx) {
             Poll::Ready(Ok(res)) => {
                 let (res, body) = res.into().replace_body(());
                 self.send_response(res, body)
@@ -681,20 +681,6 @@ where
     }
 }
 
-impl<T, S, B, X, U> Unpin for Dispatcher<T, S, B, X, U>
-where
-    T: AsyncRead + AsyncWrite + Unpin,
-    S: Service<Request = Request>,
-    S::Error: Into<Error>,
-    S::Response: Into<Response<B>>,
-    B: MessageBody,
-    X: Service<Request = Request, Response = Request>,
-    X::Error: Into<Error>,
-    U: Service<Request = (Request, Framed<T, Codec>), Response = ()>,
-    U::Error: fmt::Display,
-{
-}
-
 impl<T, S, B, X, U> Future for Dispatcher<T, S, B, X, U>
 where
     T: AsyncRead + AsyncWrite + Unpin,
@@ -771,7 +757,7 @@ where
                                 parts.write_buf = inner.write_buf;
                                 let framed = Framed::from_parts(parts);
                                 self.inner = DispatcherState::Upgrade(
-                                    inner.upgrade.unwrap().call((req, framed)),
+                                    Box::pin(inner.upgrade.unwrap().call((req, framed))),
                                 );
                                 return self.poll(cx);
                             } else {
@@ -823,7 +809,7 @@ where
                 }
             }
             DispatcherState::Upgrade(ref mut fut) => {
-                unsafe { Pin::new_unchecked(fut) }.poll(cx).map_err(|e| {
+                fut.as_mut().poll(cx).map_err(|e| {
                     error!("Upgrade handler error: {}", e);
                     DispatchError::Upgrade
                 })