Don't return shutdown errors during shutdown

Author: nox

tokio-boring/src/lib.rs

@@ -254,6 +254,19 @@ where
                 return Poll::Pending;
             }
             Err(e) => {
+                // If boring returns PROTOCOL_IS_SHUTDOWN then the connection
+                // has already been shutdown and we can just return Ok(()), as
+                // this was exactly what we wanted to do anyway.
+                if e.code() == ErrorCode::SSL {
+                    if let Some(stack) = e.ssl_error() {
+                        if let Some(first) = stack.errors.first() {
+                            if first.code() as i32 == boring_sys::SSL_R_PROTOCOL_IS_SHUTDOWN {
+                                return Poll::Ready(Ok(()));
+                            }
+                        }
+                    }
+                }
+
                 return Poll::Ready(Err(e
                     .into_io_error()
                     .unwrap_or_else(|e| io::Error::new(io::ErrorKind::Other, e))));