gateway: Skip unknown events while reconnecting (#255)

This change skips events that are unknown while the bot reconnects. This
is an event that is particularly rare as it requires unimplemented
events being called in the time before a bot's HELLO -> RESUME events
are called. This change explicitly returns unknown events as a special
time defined in wsutil/op.go and ignores them from reaching gateway/op.go

gateway/gateway.go

@@ -474,7 +474,8 @@ func (g *Gateway) start(ctx context.Context) error {
 	// Expect either READY or RESUMED before continuing.
 	wsutil.WSDebug("Waiting for either READY or RESUMED.")
 
-	// WaitForEvent should
+	// WaitForEvent should until the bot becomes ready or resumes (if a
+	// previous ready event has already been called).
 	err := wsutil.WaitForEvent(ctx, g, ch, func(op *wsutil.OP) bool {
 		switch op.EventName {
 		case "READY":

gateway/op.go

@@ -85,10 +85,10 @@ func (g *Gateway) HandleOP(op *wsutil.OP) error {
 		// Check if we know the event
 		fn, ok := EventCreator[op.EventName]
 		if !ok {
-			return fmt.Errorf(
-				"unknown event %s: %s",
-				op.EventName, string(op.Data),
-			)
+			return &wsutil.UnknownEventError{
+				Name: op.EventName,
+				Data: op.Data,
+			}
 		}
 
 		// Make a new pointer to the event

utils/wsutil/op.go

@@ -66,6 +66,25 @@ func AssertEvent(ev Event, code OPCode, v interface{}) (*OP, error) {
 	return op, nil
 }
 
+// UnknownEventError is required by HandleOP if an event is encountered that is
+// not known. Internally, unknown events are logged and ignored. It is not a
+// fatal error.
+type UnknownEventError struct {
+	Name string
+	Data json.Raw
+}
+
+// Error formats the unknown event error to with the event name and payload
+func (err UnknownEventError) Error() string {
+	return fmt.Sprintf("unknown event %s: %s", err.Name, string(err.Data))
+}
+
+// IsBrokenConnection returns true if the error is a broken connection error.
+func IsUnknownEvent(err error) bool {
+	var uevent *UnknownEventError
+	return errors.As(err, &uevent)
+}
+
 type EventHandler interface {
 	HandleOP(op *OP) error
 }
@@ -98,6 +117,11 @@ func WaitForEvent(ctx context.Context, h EventHandler, ch <-chan Event, fn func(
 			// also prevent a race condition with things that need Ready after
 			// Open().
 			if err := h.HandleOP(o); err != nil {
+				// Explicitly ignore events we don't know.
+				if IsUnknownEvent(err) {
+					WSError(err)
+					continue
+				}
 				return err
 			}