public static <K, V> Set<DistributedMember> getAllMembersForKey(final Region<K, V> r,
      final K key) {
    return Collections.unmodifiableSet(getAllForKey(r, key));
  }
Get all potential owners for a key. If the key exists in the Region, upon return there is no
guarantee that it has not moved nor does it guarantee all members are still alive.
<p>
This method is not a substitute for {@link Region#containsKey(Object)}.

@param <K> The key type of the region
@param <V> The value type of the region
@param r PartitionedRegion
@param key the key to evaluate
@throws IllegalStateException if the provided region is something other than a
        {@linkplain DataPolicy#PARTITION partitioned Region}
@return an unmodifiable set of all members
@since GemFire 6.0
geode-core/src/main/java/org/apache/geode/cache/partition/PartitionRegionHelper.java