Allow the `unfollow` action in the frozen phase (#81434)

* Allow the `unfollow` action in the frozen phase

The `unfollow` actions is injected before some of the actions that yield
the managed index unsafe to use in a CCR environment (eg. rollover,
searchable_snapshot, shrink).

This was not allowed in the `frozen` phase, however we do have the
`searchable_snapshot` action available.

This commit makes the `unfollow` action available in the frozen phase.

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

@@ -80,7 +80,7 @@ public class TimeseriesLifecycleType implements LifecycleType {
         FreezeAction.NAME,
         RollupV2.isEnabled() ? RollupILMAction.NAME : null
     ).filter(Objects::nonNull).collect(toList());
-    static final List<String> ORDERED_VALID_FROZEN_ACTIONS = Arrays.asList(SearchableSnapshotAction.NAME);
+    static final List<String> ORDERED_VALID_FROZEN_ACTIONS = Arrays.asList(UnfollowAction.NAME, SearchableSnapshotAction.NAME);
     static final List<String> ORDERED_VALID_DELETE_ACTIONS = Arrays.asList(WaitForSnapshotAction.NAME, DeleteAction.NAME);
 
     static final Set<String> VALID_HOT_ACTIONS = Sets.newHashSet(ORDERED_VALID_HOT_ACTIONS);

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

@@ -40,6 +40,7 @@ import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.ORDERED_V
 import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.ORDERED_VALID_WARM_ACTIONS;
 import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.VALID_COLD_ACTIONS;
 import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.VALID_DELETE_ACTIONS;
+import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.VALID_FROZEN_ACTIONS;
 import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.VALID_HOT_ACTIONS;
 import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.VALID_WARM_ACTIONS;
 import static org.elasticsearch.xpack.core.ilm.TimeseriesLifecycleType.WARM_PHASE;
@@ -174,6 +175,28 @@ public class TimeseriesLifecycleTypeTests extends ESTestCase {
         }
     }
 
+    public void testValidateFrozenPhase() {
+        LifecycleAction invalidAction = null;
+        Map<String, LifecycleAction> actions = randomSubsetOf(VALID_FROZEN_ACTIONS).stream()
+            .map(this::getTestAction)
+            .collect(Collectors.toMap(LifecycleAction::getWriteableName, Function.identity()));
+        if (randomBoolean()) {
+            invalidAction = getTestAction(randomFrom("rollover", "delete", "forcemerge", "shrink"));
+            actions.put(invalidAction.getWriteableName(), invalidAction);
+        }
+        Map<String, Phase> frozenPhase = Collections.singletonMap("frozen", new Phase("frozen", TimeValue.ZERO, actions));
+
+        if (invalidAction != null) {
+            Exception e = expectThrows(
+                IllegalArgumentException.class,
+                () -> TimeseriesLifecycleType.INSTANCE.validate(frozenPhase.values())
+            );
+            assertThat(e.getMessage(), equalTo("invalid action [" + invalidAction.getWriteableName() + "] defined in phase [frozen]"));
+        } else {
+            TimeseriesLifecycleType.INSTANCE.validate(frozenPhase.values());
+        }
+    }
+
     public void testValidateDeletePhase() {
         LifecycleAction invalidAction = null;
         Map<String, LifecycleAction> actions = VALID_DELETE_ACTIONS.stream()
@@ -358,6 +381,8 @@ public class TimeseriesLifecycleTypeTests extends ESTestCase {
     public void testUnfollowInjections() {
         assertTrue(isUnfollowInjected("hot", RolloverAction.NAME));
         assertTrue(isUnfollowInjected("warm", ShrinkAction.NAME));
+        assertTrue(isUnfollowInjected("cold", SearchableSnapshotAction.NAME));
+        assertTrue(isUnfollowInjected("frozen", SearchableSnapshotAction.NAME));
 
         assertFalse(isUnfollowInjected("hot", SetPriorityAction.NAME));
         assertFalse(isUnfollowInjected("warm", SetPriorityAction.NAME));