fixup! internal/driver: Fix off-by-one error with max volume attachments

internal/driver/controllerserver.go

@@ -265,11 +265,17 @@ func (linodeCS *LinodeControllerServer) ControllerPublishVolume(ctx context.Cont
 	}
 
 	// Check to see if there is room to attach this volume to the instance.
-	if canAttach, err := linodeCS.canAttach(ctx, instance); err != nil {
+	if canAttach, err := linodeCS.canAttach(ctx, instance); errors.Is(err, errNilInstance) {
+		return &csi.ControllerPublishVolumeResponse{}, status.Error(codes.Internal, "cannot determine volume attachments for a nil instance")
+	} else if err != nil {
 		return &csi.ControllerPublishVolumeResponse{}, status.Error(codes.Internal, err.Error())
 	} else if !canAttach {
+		// If we can, try and add a little more information to the error message
+		// for the caller.
 		limit, err := linodeCS.maxVolumeAttachments(ctx, instance)
-		if err != nil {
+		if errors.Is(err, errNilInstance) {
+			return &csi.ControllerPublishVolumeResponse{}, status.Error(codes.Internal, "cannot calculate max volume attachments for a nil instance")
+		} else if err != nil {
 			return &csi.ControllerPublishVolumeResponse{}, status.Error(codes.ResourceExhausted, "max number of volumes already attached to instance")
 		}
 		return &csi.ControllerPublishVolumeResponse{}, status.Errorf(codes.ResourceExhausted, "max number of volumes (%d) already attached to instance", limit)
@@ -325,12 +331,19 @@ func (s *LinodeControllerServer) canAttach(ctx context.Context, instance *linode
 	return len(volumes) < limit, nil
 }
 
+var (
+	// errNilInstance is a general-purpose error used to indicate a nil
+	// [github.com/linode/linodego.Instance] was passed as an argument to a
+	// function.
+	errNilInstance = errors.New("nil instance")
+)
+
 // maxVolumeAttachments returns the maximum number of volumes that can be
 // attached to a single Linode instance, minus any currently-attached instance
 // disks.
 func (s *LinodeControllerServer) maxVolumeAttachments(ctx context.Context, instance *linodego.Instance) (int, error) {
 	if instance == nil || instance.Specs == nil {
-		return 0, errors.New("nil instance")
+		return 0, errNilInstance
 	}
 
 	disks, err := s.CloudProvider.ListInstanceDisks(ctx, instance.ID, nil)