Tidy up SnapshotShardsUpdateContext usage (#72537)

Removes a few bits of duplication (e.g. passing in the cluster state
_and_ the snapshots-in-progress entries from the cluster state) and some
other small cleanups too.

server/src/main/java/org/elasticsearch/snapshots/SnapshotsService.java

@@ -2468,8 +2468,7 @@ public class SnapshotsService extends AbstractLifecycleComponent implements Clus
      */
     static final ClusterStateTaskExecutor<ShardSnapshotUpdate> SHARD_STATE_EXECUTOR = (currentState, tasks) ->
             ClusterStateTaskExecutor.ClusterTasksResult.<ShardSnapshotUpdate>builder().successes(tasks).build(
-                    new SnapshotShardsUpdateContext(currentState, tasks).applyToEntries(
-                            currentState.custom(SnapshotsInProgress.TYPE, SnapshotsInProgress.EMPTY).entries()).updatedState());
+                    new SnapshotShardsUpdateContext(currentState, tasks).computeUpdatedState());
 
     private static boolean isQueued(@Nullable ShardSnapshotStatus status) {
         return status != null && status.state() == ShardState.QUEUED;
@@ -2490,9 +2489,6 @@ public class SnapshotsService extends AbstractLifecycleComponent implements Clus
         // current cluster state
         private final ClusterState currentState;
 
-        // snapshot entries computed by applying tasks to existing snapshot entries
-        private final List<SnapshotsInProgress.Entry> entries = new ArrayList<>();
-
         // updates outstanding to be applied to existing snapshot entries
         private final List<ShardSnapshotUpdate> unconsumedUpdates;
 
@@ -2504,29 +2500,29 @@ public class SnapshotsService extends AbstractLifecycleComponent implements Clus
             unconsumedUpdates = new ArrayList<>(updates);
         }
 
-        public ClusterState updatedState() {
+        ClusterState computeUpdatedState() {
+            final List<SnapshotsInProgress.Entry> oldEntries
+                    = currentState.custom(SnapshotsInProgress.TYPE, SnapshotsInProgress.EMPTY).entries();
+            final List<SnapshotsInProgress.Entry> newEntries = new ArrayList<>(oldEntries.size());
+            for (SnapshotsInProgress.Entry entry : oldEntries) {
+                newEntries.add(applyToEntry(entry));
+            }
+
             if (changedCount > 0) {
                 logger.trace("changed cluster state triggered by [{}] snapshot state updates and resulted in starting " +
                         "[{}] shard snapshots", changedCount, startedCount);
-                return ClusterState.builder(currentState).putCustom(SnapshotsInProgress.TYPE, SnapshotsInProgress.of(entries)).build();
+                return ClusterState.builder(currentState).putCustom(SnapshotsInProgress.TYPE, SnapshotsInProgress.of(newEntries)).build();
             }
             return currentState;
         }
 
-        SnapshotShardsUpdateContext applyToEntries(List<SnapshotsInProgress.Entry> snapshots) {
-            for (SnapshotsInProgress.Entry entry : snapshots) {
-                entries.add(applyToEntry(entry));
-            }
-            return this;
-        }
-
         private SnapshotsInProgress.Entry applyToEntry(SnapshotsInProgress.Entry entry) {
             // Completed snapshots do not require any updates so we just add them to the output list and keep going.
             // Also we short circuit if there are no more unconsumed updates to apply.
             if (entry.state().completed() || unconsumedUpdates.isEmpty()) {
                 return entry;
             }
-            return new EntryContext(entry, unconsumedUpdates.iterator()).updatedEntry();
+            return new EntryContext(entry).computeUpdatedEntry();
         }
 
         // Per snapshot entry state
@@ -2543,12 +2539,14 @@ public class SnapshotsService extends AbstractLifecycleComponent implements Clus
             // builder for updated shard clone status mappings if any could be computed
             private ImmutableOpenMap.Builder<RepositoryShardId, ShardSnapshotStatus> clonesBuilder = null;
 
-            EntryContext(SnapshotsInProgress.Entry entry, Iterator<ShardSnapshotUpdate> iterator) {
+            EntryContext(SnapshotsInProgress.Entry entry) {
                 this.entry = entry;
-                this.iterator = iterator;
+                this.iterator = unconsumedUpdates.iterator();
             }
 
-            SnapshotsInProgress.Entry updatedEntry() {
+            SnapshotsInProgress.Entry computeUpdatedEntry() {
+                assert shardsBuilder == null && clonesBuilder == null : "update context was already used";
+
                 // loop over all the shard updates that are potentially applicable to the current snapshot entry
                 while (iterator.hasNext()) {
                     final ShardSnapshotUpdate update = iterator.next();