rpc: redefine ErrNotificationsUnsupported with error code -32601

rpc/errors.go

@@ -59,7 +59,6 @@ var (
 
 const (
 	errcodeDefault                  = -32000
-	errcodeNotificationsUnsupported = -32001
 	errcodeTimeout                  = -32002
 	errcodePanic                    = -32603
 	errcodeMarshalError             = -32603
@@ -77,6 +76,30 @@ func (e *methodNotFoundError) Error() string {
 	return fmt.Sprintf("the method %s does not exist/is not available", e.method)
 }
 
+type notificationsUnsupportedError struct{}
+
+func (e notificationsUnsupportedError) Error() string {
+	return "notifications not supported"
+}
+
+func (e notificationsUnsupportedError) ErrorCode() int { return -32601 }
+
+// Is checks for equivalence to another error. Here we define that all errors with code
+// -32601 (method not found) are equivalent to notificationsUnsupportedError. This is
+// done to enable the following pattern:
+//
+//	sub, err := client.Subscribe(...)
+//	if errors.Is(err, rpc.ErrNotificationsUnsupported) {
+//		// server doesn't support subscriptions
+//	}
+func (e notificationsUnsupportedError) Is(other error) bool {
+	if other == (notificationsUnsupportedError{}) {
+		return true
+	}
+	rpcErr, ok := other.(Error)
+	return ok && rpcErr.ErrorCode() == -32601
+}
+
 type subscriptionNotFoundError struct{ namespace, subscription string }
 
 func (e *subscriptionNotFoundError) ErrorCode() int { return -32601 }

rpc/handler.go

@@ -475,10 +475,7 @@ func (h *handler) handleCall(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage
 // handleSubscribe processes *_subscribe method calls.
 func (h *handler) handleSubscribe(cp *callProc, msg *jsonrpcMessage) *jsonrpcMessage {
 	if !h.allowSubscribe {
-		return msg.errorResponse(&internalServerError{
-			code:    errcodeNotificationsUnsupported,
-			message: ErrNotificationsUnsupported.Error(),
-		})
+		return msg.errorResponse(ErrNotificationsUnsupported)
 	}
 
 	// Subscription method name is first argument.

rpc/subscription.go

@@ -33,7 +33,8 @@ import (
 
 var (
 	// ErrNotificationsUnsupported is returned when the connection doesn't support notifications
-	ErrNotificationsUnsupported = errors.New("notifications not supported")
+	ErrNotificationsUnsupported = notificationsUnsupportedError{}
+
 	// ErrSubscriptionNotFound is returned when the notification for the given id is not found
 	ErrSubscriptionNotFound = errors.New("subscription not found")
 )