Return "invalid request" if JSON parses, but isn't a Request

src/transport.rs

@@ -1,5 +1,8 @@
 //! Generic server for multiplexing bidirectional streams through a transport.
 
+#[cfg(feature = "runtime-agnostic")]
+use std::error::Error as StdError;
+
 #[cfg(feature = "runtime-agnostic")]
 use async_codec_lite::{FramedRead, FramedWrite};
 #[cfg(feature = "runtime-agnostic")]
@@ -15,7 +18,7 @@ use futures::{future, join, stream, FutureExt, Sink, SinkExt, Stream, StreamExt,
 use log::error;
 use tower::Service;
 
-use crate::codec::LanguageServerCodec;
+use crate::codec::{LanguageServerCodec, ParseError};
 use crate::jsonrpc::{Error, Id, Message, Request, Response};
 use crate::service::{ClientSocket, RequestStream, ResponseSink};
 
@@ -149,7 +152,7 @@ where
                     }
                     Err(err) => {
                         error!("failed to decode message: {}", err);
-                        let res = Response::from_error(Id::Null, Error::parse_error());
+                        let res = Response::from_error(Id::Null, to_jsonrpc_error(err));
                         responses_tx.send(Message::Response(res)).await.unwrap();
                     }
                 }
@@ -172,6 +175,24 @@ fn display_sources(error: &dyn std::error::Error) -> String {
     }
 }
 
+#[cfg(feature = "runtime-tokio")]
+#[inline]
+fn to_jsonrpc_error(err: ParseError) -> Error {
+    match err {
+        ParseError::Body(err) if err.is_data() => Error::invalid_request(),
+        _ => Error::parse_error(),
+    }
+}
+
+#[cfg(feature = "runtime-agnostic")]
+#[inline]
+fn to_jsonrpc_error(err: impl StdError) -> Error {
+    match err.source().and_then(|e| e.downcast_ref()) {
+        Some(ParseError::Body(err)) if err.is_data() => Error::invalid_request(),
+        _ => Error::parse_error(),
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::task::{Context, Poll};