Inject an unfollow action before executing a downsample action in ILM (#128788)

When an index alias/data stream is replicated via CCR, the follower index needs to unfollow the leader index before executing actions that will remove the original index.
The downsample action is also an action that removes the original index so the expected behaviour is that ILM would automatically unfollow.

Co-authored-by: Niels Bauman <[email protected]>

Author: Anirudh2112

docs/changelog/128788.yaml

@@ -0,0 +1,6 @@
+pr: 105773
+summary: Inject an unfollow action before executing a downsample action in ILM
+area: ILM+SLM
+type: bug
+issues:
+ - 105773

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/TimeseriesLifecycleType.java

@@ -132,7 +132,8 @@ public List<Phase> getOrderedPhases(Map<String, Phase> phases) {
                 if (actions.containsKey(UnfollowAction.NAME) == false
                     && (actions.containsKey(RolloverAction.NAME)
                         || actions.containsKey(ShrinkAction.NAME)
-                        || actions.containsKey(SearchableSnapshotAction.NAME))) {
+                        || actions.containsKey(SearchableSnapshotAction.NAME)
+                        || actions.containsKey(DownsampleAction.NAME))) {
                     Map<String, LifecycleAction> actionMap = new HashMap<>(phase.getActions());
                     actionMap.put(UnfollowAction.NAME, UnfollowAction.INSTANCE);
                     phase = new Phase(phase.getName(), phase.getMinimumAge(), actionMap);

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ilm/TimeseriesLifecycleTypeTests.java

@@ -476,6 +476,9 @@ public void testUnfollowInjections() {
         assertTrue(isUnfollowInjected("cold", SearchableSnapshotAction.NAME));
         assertTrue(isUnfollowInjected("frozen", SearchableSnapshotAction.NAME));
 
+        assertTrue(isUnfollowInjected("warm", DownsampleAction.NAME));
+        assertTrue(isUnfollowInjected("cold", DownsampleAction.NAME));
+
         assertFalse(isUnfollowInjected("hot", SetPriorityAction.NAME));
         assertFalse(isUnfollowInjected("warm", SetPriorityAction.NAME));
         assertFalse(isUnfollowInjected("warm", AllocateAction.NAME));