[CCR] AutoFollowCoordinator should tolerate that auto follow patterns may be removed

AutoFollowCoordinator should take into account that after auto following
an index and while updating that a leader index has been followed, that
the auto follow pattern may have been removed via delete auto follow patters
api.

Closes elastic#35480

Author: martijnvg

client/rest-high-level/src/test/java/org/elasticsearch/client/CCRIT.java

@@ -128,7 +128,6 @@ public void testIndexFollowing() throws Exception {
         assertThat(unfollowResponse.isAcknowledged(), is(true));
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/35937")
     public void testAutoFollowing() throws Exception {
         CcrClient ccrClient = highLevelClient().ccr();
         PutAutoFollowPatternRequest putAutoFollowPatternRequest =

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/action/AutoFollowCoordinator.java

@@ -394,6 +394,13 @@ static Function<ClusterState, ClusterState> recordLeaderIndexAsFollowFunction(St
             return currentState -> {
                 AutoFollowMetadata currentAutoFollowMetadata = currentState.metaData().custom(AutoFollowMetadata.TYPE);
                 Map<String, List<String>> newFollowedIndexUUIDS = new HashMap<>(currentAutoFollowMetadata.getFollowedLeaderIndexUUIDs());
+                if (newFollowedIndexUUIDS.containsKey(name) == false) {
+                    // A delete auto follow pattern request can have removed the auto follow pattern while we want to update
+                    // the auto follow metadata with the fact that an index was successfully auto followed. If this
+                    // happens, we can just skip this step.
+                    return currentState;
+                }
+
                 newFollowedIndexUUIDS.compute(name, (key, existingUUIDs) -> {
                     assert existingUUIDs != null;
                     List<String> newUUIDs = new ArrayList<>(existingUUIDs);