  Given a graph $G$, guards are placed on vertices of $G$. Then vertices are
subject to an infinite sequence of attacks so that each attack must be defended
by a guard moving from a neighboring vertex. The m-eternal domination number is
the minimum number of guards such that the graph can be defended indefinitely.
In this paper we study the m-eternal domination number of cactus graphs, that
is, connected graphs where each edge lies in at most two cycles, and we
consider three variants of the m-eternal domination number: first variant
allows multiple guards to occupy a single vertex, second variant does not allow
it, and in the third variant additional "eviction" attacks must be defended. We
provide a new upper bound for the m-eternal domination number of cactus graphs,
and for a subclass of cactus graphs called Christmas cactus graphs, where each
vertex lies in at most two cycles, we prove that these three numbers are equal.
Moreover, we present a linear-time algorithm for computing them.
