Limit request size

handler.go

@@ -43,6 +43,12 @@ type request struct {
 	Meta    map[string]string `json:"meta,omitempty"`
 }
 
+// Limit request size. Ideally this limit should be specific for each field
+// in the JSON request but as a simple defensive measure we just limit the
+// entire HTTP body.
+// Configured by WithMaxRequestSize.
+const DEFAULT_MAX_REQUEST_SIZE = 100 << 20 // 100 MiB
+
 type respError struct {
 	Code    int    `json:"code"`
 	Message string `json:"message"`
@@ -111,7 +117,33 @@ func (s *RPCServer) handleReader(ctx context.Context, r io.Reader, w io.Writer,
 	}
 
 	var req request
-	if err := json.NewDecoder(r).Decode(&req); err != nil {
+	// We read the entire request upfront in a buffer to be able to tell if the
+	// client sent more than maxRequestSize and report it back as an explicit error,
+	// instead of just silently truncating it and reporting a more vague parsing
+	// error.
+	bufferedRequest := new(bytes.Buffer)
+	// We use LimitReader to enforce maxRequestSize. Since it won't return an
+	// EOF we can't actually know if the client sent more than the maximum or
+	// not, so we read one byte more over the limit to explicitly query that.
+	// FIXME: Maybe there's a cleaner way to do this.
+	reqSize, err := bufferedRequest.ReadFrom(io.LimitReader(r, s.maxRequestSize+1))
+	if err != nil {
+		// ReadFrom will discard EOF so any error here is unexpected and should
+		// be reported.
+		rpcError(wf, &req, rpcParseError, xerrors.Errorf("reading request: %w", err))
+		return
+	}
+	if reqSize > s.maxRequestSize {
+		rpcError(wf, &req, rpcParseError,
+			// rpcParseError is the closest we have from the standard errors defined
+			// in [jsonrpc spec](https://www.jsonrpc.org/specification#error_object)
+			// to report the maximum limit.
+			xerrors.Errorf("request bigger than maximum %d allowed",
+				s.maxRequestSize))
+		return
+	}
+
+	if err := json.NewDecoder(bufferedRequest).Decode(&req); err != nil {
 		rpcError(wf, &req, rpcParseError, xerrors.Errorf("unmarshaling request: %w", err))
 		return
 	}

options_server.go

@@ -8,14 +8,16 @@ import (
 type ParamDecoder func(ctx context.Context, json []byte) (reflect.Value, error)
 
 type ServerConfig struct {
-	paramDecoders map[reflect.Type]ParamDecoder
+	paramDecoders  map[reflect.Type]ParamDecoder
+	maxRequestSize int64
 }
 
 type ServerOption func(c *ServerConfig)
 
 func defaultServerConfig() ServerConfig {
 	return ServerConfig{
-		paramDecoders: map[reflect.Type]ParamDecoder{},
+		paramDecoders:  map[reflect.Type]ParamDecoder{},
+		maxRequestSize: DEFAULT_MAX_REQUEST_SIZE,
 	}
 }
 
@@ -24,3 +26,9 @@ func WithParamDecoder(t interface{}, decoder ParamDecoder) ServerOption {
 		c.paramDecoders[reflect.TypeOf(t).Elem()] = decoder
 	}
 }
+
+func WithMaxRequestSize(max int64) ServerOption {
+	return func(c *ServerConfig) {
+		c.maxRequestSize = max
+	}
+}

server.go

@@ -22,6 +22,8 @@ type RPCServer struct {
 	methods map[string]rpcHandler
 
 	paramDecoders map[reflect.Type]ParamDecoder
+
+	maxRequestSize int64
 }
 
 // NewServer creates new RPCServer instance
@@ -32,8 +34,9 @@ func NewServer(opts ...ServerOption) *RPCServer {
 	}
 
 	return &RPCServer{
-		methods:       map[string]rpcHandler{},
-		paramDecoders: config.paramDecoders,
+		methods:        map[string]rpcHandler{},
+		paramDecoders:  config.paramDecoders,
+		maxRequestSize: config.maxRequestSize,
 	}
 }