Predefine some common server errors.

error.go

@@ -46,6 +46,16 @@ var errServerStopped = errors.New("the server has been stopped")
 // explicit call to its Close method.
 var errClientStopped = errors.New("the client has been stopped")
 
+// errEmptyMethod is the error reported for an empty request method name.
+var errEmptyMethod = &Error{Code: code.InvalidRequest, Message: "empty method name"}
+
+// errChannelClosed is the error reported to a pending callback when the client
+// channel has closed before the call completed.
+var errChannelClosed = &Error{Code: code.Cancelled, Message: "client channel terminated"}
+
+// errEmptyBatch is the error reported for an empty request batch.
+var errEmptyBatch = &Error{Code: code.InvalidRequest, Message: "empty request batch"}
+
 // ErrConnClosed is returned by a server's push-to-client methods if they are
 // called after the client connection is closed.
 var ErrConnClosed = errors.New("client connection is closed")

server.go

@@ -286,7 +286,7 @@ func (s *Server) checkAndAssign(next jmessages) tasks {
  } else if !s.versionOK(req.V) {
  t.err = ErrInvalidVersion
  } else if req.M == "" {
- t.err = &Error{Code: code.InvalidRequest, Message: "empty method name"}
+ t.err = errEmptyMethod
  } else if s.setContext(t, id) {
  t.m = s.assign(t.ctx, req.M)
  if t.m == nil {
@@ -572,7 +572,7 @@ func (s *Server) stop(err error) {
  for id, rsp := range s.call {
  rsp.ch <- &jmessage{
  ID: json.RawMessage(id),
- E: &Error{Message: "client channel terminated", Code: code.Cancelled},
+ E: errChannelClosed,
  }
  delete(s.call, id)
  }
@@ -615,7 +615,7 @@ func (s *Server) read(ch receiver) {
  } else if derr != nil { // parse failure; report and continue
  s.pushError(derr)
  } else if len(in) == 0 {
- s.pushError(&Error{Code: code.InvalidRequest, Message: "empty request batch"})
+ s.pushError(errEmptyBatch)
  } else {
  s.log("Received request batch of size %d (qlen=%d)", len(in), s.inq.Len())
  s.inq.PushBack(in)