rpc: show more error detail for invalidMessageError (#30191)

Here we add distinct error messages for network timeouts and JSON parsing errors.
Note this specifically applies to HTTP connections serving a single RPC request.

Co-authored-by: Felix Lange <fjl@twurst.com>

rpc/server.go

@@ -18,7 +18,9 @@ package rpc
 
 import (
 	"context"
+	"errors"
 	"io"
+	"net"
 	"sync"
 	"sync/atomic"
 
@@ -151,8 +153,8 @@ func (s *Server) serveSingleRequest(ctx context.Context, codec ServerCodec) {
 
 	reqs, batch, err := codec.readBatch()
 	if err != nil {
-		if err != io.EOF {
+		if msg := messageForReadError(err); msg != "" {
-			resp := errorMessage(&invalidMessageError{"parse error"})
+			resp := errorMessage(&invalidMessageError{msg})
 			codec.writeJSON(ctx, resp, true)
 		}
 		return
@@ -164,6 +166,20 @@ func (s *Server) serveSingleRequest(ctx context.Context, codec ServerCodec) {
 	}
 }
 
+func messageForReadError(err error) string {
+	var netErr net.Error
+	if errors.As(err, &netErr) {
+		if netErr.Timeout() {
+			return "read timeout"
+		} else {
+			return "read error"
+		}
+	} else if err != io.EOF {
+		return "parse error"
+	}
+	return ""
+}
+
 // Stop stops reading new requests, waits for stopPendingRequestTimeout to allow pending
 // requests to finish, then closes all codecs which will cancel pending requests and
 // subscriptions.