BuildOperationRef oldState = get();
        try {
            set(state);
            return block.call();
        } finally {
            set(oldState);
        }
    }

    public void with(@Nullable BuildOperationRef state, Runnable block) {
        BuildOperationRef oldState = get();
        try {
            set(state);
            block.run();

	// we don't need fields for every kind of resource.
	id       int64
	oldState uint8
	newState uint8
}

func goStateTransition(id GoID, from, to GoState) StateTransition {
	return StateTransition{
		Resource: ResourceID{Kind: ResourceGoroutine, id: int64(id)},
		oldState: uint8(from),
		newState: uint8(to),
	}
}