Update shardGenerations for all indices on snapshot finalization (elastic#128650)

If an index is deleted after a snapshot has written its shardGenerations 
file but before the snapshot is finalized, we exclude this index from the 
snapshot because its indexMetadata is no longer available. However, 
the shardGenerations file is still valid in that it is the latest copy with all 
necessary information despite it containing an extra snapshot entry. 
This is OK. Instead of dropping this shardGenerations file, this PR 
changes to carry it forward by updating RepositoryData and relevant 
in-progress snapshots so that the next finalization builds on top of this one.

Co-authored-by: David Turner <david.turner@elastic.co>

Author: 2 people

docs/changelog/128650.yaml

@@ -0,0 +1,6 @@
+pr: 128650
+summary: Update shardGenerations for all indices on snapshot finalization
+area: Snapshot/Restore
+type: enhancement
+issues:
+  - 108907

server/src/main/java/org/elasticsearch/repositories/FinalizeSnapshotContext.java

@@ -27,7 +27,7 @@
  */
 public final class FinalizeSnapshotContext extends DelegatingActionListener<RepositoryData, RepositoryData> {
 
-    private final ShardGenerations updatedShardGenerations;
+    private final UpdatedShardGenerations updatedShardGenerations;
 
     /**
      * Obsolete shard generations map computed from the cluster state update that this finalization executed in
@@ -46,7 +46,7 @@ public final class FinalizeSnapshotContext extends DelegatingActionListener<Repo
     private final Runnable onDone;
 
     /**
-     * @param updatedShardGenerations updated shard generations
+     * @param updatedShardGenerations updated shard generations for both live and deleted indices
      * @param repositoryStateId       the unique id identifying the state of the repository when the snapshot began
      * @param clusterMetadata         cluster metadata
      * @param snapshotInfo            SnapshotInfo instance to write for this snapshot
@@ -57,7 +57,7 @@ public final class FinalizeSnapshotContext extends DelegatingActionListener<Repo
      *                                once all cleanup operations after snapshot completion have executed
      */
     public FinalizeSnapshotContext(
-        ShardGenerations updatedShardGenerations,
+        UpdatedShardGenerations updatedShardGenerations,
         long repositoryStateId,
         Metadata clusterMetadata,
         SnapshotInfo snapshotInfo,
@@ -78,7 +78,7 @@ public long repositoryStateId() {
         return repositoryStateId;
     }
 
-    public ShardGenerations updatedShardGenerations() {
+    public UpdatedShardGenerations updatedShardGenerations() {
         return updatedShardGenerations;
     }
 
@@ -120,4 +120,20 @@ public void onDone() {
     public void onResponse(RepositoryData repositoryData) {
         delegate.onResponse(repositoryData);
     }
+
+    /**
+     * A record used to track the new shard generations that have been written for each shard in a snapshot.
+     * An index may be deleted after the shard generation is written but before the snapshot is finalized.
+     * In this case, its shard generation is tracked in {@link #deletedIndices} because it's still a valid
+     * shard generation blob that exists in the repository and may be used by subsequent snapshots, even though
+     * the index will not be included in the snapshot being finalized. Otherwise, it is tracked in
+     * {@link #liveIndices}.
+     */
+    public record UpdatedShardGenerations(ShardGenerations liveIndices, ShardGenerations deletedIndices) {
+        public static final UpdatedShardGenerations EMPTY = new UpdatedShardGenerations(ShardGenerations.EMPTY, ShardGenerations.EMPTY);
+
+        public boolean hasShardGen(RepositoryShardId repositoryShardId) {
+            return liveIndices.hasShardGen(repositoryShardId) || deletedIndices.hasShardGen(repositoryShardId);
+        }
+    }
 }

server/src/main/java/org/elasticsearch/repositories/RepositoryData.java

@@ -25,6 +25,7 @@
 import org.elasticsearch.index.IndexVersions;
 import org.elasticsearch.logging.LogManager;
 import org.elasticsearch.logging.Logger;
+import org.elasticsearch.repositories.FinalizeSnapshotContext.UpdatedShardGenerations;
 import org.elasticsearch.snapshots.SnapshotId;
 import org.elasticsearch.snapshots.SnapshotInfo;
 import org.elasticsearch.snapshots.SnapshotState;
@@ -405,16 +406,16 @@ public Map<IndexId, Collection<String>> indexMetaDataToRemoveAfterRemovingSnapsh
      *
      * @param snapshotId       Id of the new snapshot
      * @param details          Details of the new snapshot
-     * @param shardGenerations Updated shard generations in the new snapshot. For each index contained in the snapshot an array of new
-     *                         generations indexed by the shard id they correspond to must be supplied.
+     * @param updatedShardGenerations Updated shard generations in the new snapshot, including both indices that are included
+     *                                in the given snapshot and those got deleted while finalizing.
      * @param indexMetaBlobs   Map of index metadata blob uuids
      * @param newIdentifiers   Map of new index metadata blob uuids keyed by the identifiers of the
      *                         {@link IndexMetadata} in them
      */
     public RepositoryData addSnapshot(
         final SnapshotId snapshotId,
         final SnapshotDetails details,
-        final ShardGenerations shardGenerations,
+        final UpdatedShardGenerations updatedShardGenerations,
         @Nullable final Map<IndexId, String> indexMetaBlobs,
         @Nullable final Map<String, String> newIdentifiers
     ) {
@@ -424,12 +425,13 @@ public RepositoryData addSnapshot(
             // the new master, so we make the operation idempotent
             return this;
         }
+        final var liveIndexIds = updatedShardGenerations.liveIndices().indices();
         Map<String, SnapshotId> snapshots = new HashMap<>(snapshotIds);
         snapshots.put(snapshotId.getUUID(), snapshotId);
         Map<String, SnapshotDetails> newSnapshotDetails = new HashMap<>(snapshotsDetails);
         newSnapshotDetails.put(snapshotId.getUUID(), details);
         Map<IndexId, List<SnapshotId>> allIndexSnapshots = new HashMap<>(indexSnapshots);
-        for (final IndexId indexId : shardGenerations.indices()) {
+        for (final IndexId indexId : liveIndexIds) {
             final List<SnapshotId> snapshotIds = allIndexSnapshots.get(indexId);
             if (snapshotIds == null) {
                 allIndexSnapshots.put(indexId, List.of(snapshotId));
@@ -445,11 +447,8 @@ public RepositoryData addSnapshot(
                 : "Index meta generations should have been empty but was [" + indexMetaDataGenerations + "]";
             newIndexMetaGenerations = IndexMetaDataGenerations.EMPTY;
         } else {
-            assert indexMetaBlobs.isEmpty() || shardGenerations.indices().equals(indexMetaBlobs.keySet())
-                : "Shard generations contained indices "
-                    + shardGenerations.indices()
-                    + " but indexMetaData was given for "
-                    + indexMetaBlobs.keySet();
+            assert indexMetaBlobs.isEmpty() || liveIndexIds.equals(indexMetaBlobs.keySet())
+                : "Shard generations contained indices " + liveIndexIds + " but indexMetaData was given for " + indexMetaBlobs.keySet();
             newIndexMetaGenerations = indexMetaDataGenerations.withAddedSnapshot(snapshotId, indexMetaBlobs, newIdentifiers);
         }
 
@@ -459,7 +458,7 @@ public RepositoryData addSnapshot(
             snapshots,
             newSnapshotDetails,
             allIndexSnapshots,
-            ShardGenerations.builder().putAll(this.shardGenerations).putAll(shardGenerations).build(),
+            ShardGenerations.builder().putAll(this.shardGenerations).update(updatedShardGenerations).build(),
             newIndexMetaGenerations,
             clusterUUID
         );

server/src/main/java/org/elasticsearch/repositories/ShardGenerations.java

@@ -28,6 +28,8 @@
 import java.util.regex.Pattern;
 import java.util.stream.Collectors;
 
+import static org.elasticsearch.repositories.FinalizeSnapshotContext.UpdatedShardGenerations;
+
 /**
  * Represents the current {@link ShardGeneration} for each shard in a repository.
  */
@@ -231,6 +233,14 @@ public Builder putAll(ShardGenerations shardGenerations) {
             return this;
         }
 
+        public Builder update(UpdatedShardGenerations updatedShardGenerations) {
+            putAll(updatedShardGenerations.liveIndices());
+            // For deleted indices, we only update the generations if they are present in the existing generations, i.e.
+            // they are referenced by other snapshots.
+            updateIfPresent(updatedShardGenerations.deletedIndices());
+            return this;
+        }
+
         public Builder put(IndexId indexId, int shardId, SnapshotsInProgress.ShardSnapshotStatus status) {
             // only track generations for successful shard status values
             return put(indexId, shardId, status.state().failed() ? null : status.generation());
@@ -244,6 +254,20 @@ public Builder put(IndexId indexId, int shardId, ShardGeneration generation) {
             return this;
         }
 
+        private void updateIfPresent(ShardGenerations shardGenerations) {
+            shardGenerations.shardGenerations.forEach((indexId, gens) -> {
+                final Map<Integer, ShardGeneration> existingShardGens = generations.get(indexId);
+                if (existingShardGens != null) {
+                    for (int i = 0; i < gens.size(); i++) {
+                        final ShardGeneration gen = gens.get(i);
+                        if (gen != null) {
+                            existingShardGens.put(i, gen);
+                        }
+                    }
+                }
+            });
+        }
+
         private boolean noDuplicateIndicesWithSameName(IndexId newId) {
             for (IndexId id : generations.keySet()) {
                 if (id.getName().equals(newId.getName()) && id.equals(newId) == false) {
@@ -254,6 +278,9 @@ private boolean noDuplicateIndicesWithSameName(IndexId newId) {
         }
 
         public ShardGenerations build() {
+            if (generations.isEmpty()) {
+                return EMPTY;
+            }
             return new ShardGenerations(generations.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, entry -> {
                 final Set<Integer> shardIds = entry.getValue().keySet();
                 assert shardIds.isEmpty() == false;

server/src/main/java/org/elasticsearch/repositories/blobstore/BlobStoreRepository.java

@@ -1749,11 +1749,10 @@ int sizeInBytes() {
     public void finalizeSnapshot(final FinalizeSnapshotContext finalizeSnapshotContext) {
         assert ThreadPool.assertCurrentThreadPool(ThreadPool.Names.SNAPSHOT);
         final long repositoryStateId = finalizeSnapshotContext.repositoryStateId();
-        final ShardGenerations shardGenerations = finalizeSnapshotContext.updatedShardGenerations();
         final SnapshotInfo snapshotInfo = finalizeSnapshotContext.snapshotInfo();
         assert repositoryStateId > RepositoryData.UNKNOWN_REPO_GEN
             : "Must finalize based on a valid repository generation but received [" + repositoryStateId + "]";
-        final Collection<IndexId> indices = shardGenerations.indices();
+        final Collection<IndexId> indices = finalizeSnapshotContext.updatedShardGenerations().liveIndices().indices();
         final SnapshotId snapshotId = snapshotInfo.snapshotId();
         // Once we are done writing the updated index-N blob we remove the now unreferenced index-${uuid} blobs in each shard
         // directory if all nodes are at least at version SnapshotsService#SHARD_GEN_IN_REPO_DATA_VERSION
@@ -1867,7 +1866,7 @@ record RootBlobUpdateResult(RepositoryData oldRepositoryData, RepositoryData new
                     existingRepositoryData.addSnapshot(
                         snapshotId,
                         snapshotDetails,
-                        shardGenerations,
+                        finalizeSnapshotContext.updatedShardGenerations(),
                         metadataWriteResult.indexMetas(),
                         metadataWriteResult.indexMetaIdentifiers()
                     ),