Track Repository Gen. in BlobStoreRepository (#48944)

This is intended as a stop-gap solution/improvement to #38941 that
prevents repo modifications without an intermittent master failover
from causing inconsistent (outdated due to inconsistent listing of index-N blobs)
`RepositoryData` to be written.

Tracking the latest repository generation will move to the cluster state in a
separate pull request. This is intended as a low-risk change to be backported as
far as possible and motived by the recently increased chance of #38941 
causing trouble via SLM (see #47520).

Closes #47834
Closes #49048

Author: original-brownbear

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

@@ -113,6 +113,7 @@
 import java.util.concurrent.Executor;
 import java.util.concurrent.LinkedBlockingQueue;
 import java.util.concurrent.TimeUnit;
+import java.util.concurrent.atomic.AtomicLong;
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -366,7 +367,7 @@ public void deleteSnapshot(SnapshotId snapshotId, long repositoryStateId, boolea
         } else {
             try {
                 final Map<String, BlobMetaData> rootBlobs = blobContainer().listBlobs();
-                final RepositoryData repositoryData = getRepositoryData(latestGeneration(rootBlobs.keySet()));
+                final RepositoryData repositoryData = safeRepositoryData(repositoryStateId, rootBlobs);
                 // Cache the indices that were found before writing out the new index-N blob so that a stuck master will never
                 // delete an index that was created by another master node after writing this index-N blob.
                 final Map<String, BlobContainer> foundIndices = blobStore().blobContainer(indicesPath()).children();
@@ -377,6 +378,30 @@ public void deleteSnapshot(SnapshotId snapshotId, long repositoryStateId, boolea
         }
     }
 
+    /**
+     * Loads {@link RepositoryData} ensuring that it is consistent with the given {@code rootBlobs} as well of the assumed generation.
+     *
+     * @param repositoryStateId Expected repository generation
+     * @param rootBlobs         Blobs at the repository root
+     * @return RepositoryData
+     */
+    private RepositoryData safeRepositoryData(long repositoryStateId, Map<String, BlobMetaData> rootBlobs) {
+        final long generation = latestGeneration(rootBlobs.keySet());
+        final long genToLoad = latestKnownRepoGen.updateAndGet(known -> Math.max(known, repositoryStateId));
+        if (genToLoad > generation) {
+            // It's always a possibility to not see the latest index-N in the listing here on an eventually consistent blob store, just
+            // debug log it. Any blobs leaked as a result of an inconsistent listing here will be cleaned up in a subsequent cleanup or
+            // snapshot delete run anyway.
+            logger.debug("Determined repository's generation from its contents to [" + generation + "] but " +
+                "current generation is at least [" + genToLoad + "]");
+        }
+        if (genToLoad != repositoryStateId) {
+            throw new RepositoryException(metadata.name(), "concurrent modification of the index-N file, expected current generation [" +
+                repositoryStateId + "], actual current generation [" + genToLoad + "]");
+        }
+        return getRepositoryData(genToLoad);
+    }
+
     /**
      * After updating the {@link RepositoryData} each of the shards directories is individually first moved to the next shard generation
      * and then has all now unreferenced blobs in it deleted.
@@ -604,14 +629,8 @@ public void cleanup(long repositoryStateId, boolean writeShardGens, ActionListen
             if (isReadOnly()) {
                 throw new RepositoryException(metadata.name(), "cannot run cleanup on readonly repository");
             }
-            final RepositoryData repositoryData = getRepositoryData();
-            if (repositoryData.getGenId() != repositoryStateId) {
-                // Check that we are working on the expected repository version before gathering the data to clean up
-                throw new RepositoryException(metadata.name(), "concurrent modification of the repository before cleanup started, " +
-                    "expected current generation [" + repositoryStateId + "], actual current generation ["
-                    + repositoryData.getGenId() + "]");
-            }
             Map<String, BlobMetaData> rootBlobs = blobContainer().listBlobs();
+            final RepositoryData repositoryData = safeRepositoryData(repositoryStateId, rootBlobs);
             final Map<String, BlobContainer> foundIndices = blobStore().blobContainer(indicesPath()).children();
             final Set<String> survivingIndexIds =
                 repositoryData.getIndices().values().stream().map(IndexId::getId).collect(Collectors.toSet());
@@ -897,12 +916,36 @@ public void endVerification(String seed) {
         }
     }
 
+    // Tracks the latest known repository generation in a best-effort way to detect inconsistent listing of root level index-N blobs
+    // and concurrent modifications.
+    // Protected for use in MockEventuallyConsistentRepository
+    protected final AtomicLong latestKnownRepoGen = new AtomicLong(RepositoryData.EMPTY_REPO_GEN);
+
     @Override
     public RepositoryData getRepositoryData() {
-        try {
-            return getRepositoryData(latestIndexBlobId());
-        } catch (IOException ioe) {
-            throw new RepositoryException(metadata.name(), "Could not determine repository generation from root blobs", ioe);
+        // Retry loading RepositoryData in a loop in case we run into concurrent modifications of the repository.
+        while (true) {
+            final long generation;
+            try {
+                generation = latestIndexBlobId();
+            } catch (IOException ioe) {
+                throw new RepositoryException(metadata.name(), "Could not determine repository generation from root blobs", ioe);
+            }
+            final long genToLoad = latestKnownRepoGen.updateAndGet(known -> Math.max(known, generation));
+            if (genToLoad > generation) {
+                logger.info("Determined repository generation [" + generation
+                    + "] from repository contents but correct generation must be at least [" + genToLoad + "]");
+            }
+            try {
+                return getRepositoryData(genToLoad);
+            } catch (RepositoryException e) {
+                if (genToLoad != latestKnownRepoGen.get()) {
+                    logger.warn("Failed to load repository data generation [" + genToLoad +
+                        "] because a concurrent operation moved the current generation to [" + latestKnownRepoGen.get() + "]", e);
+                    continue;
+                }
+                throw e;
+            }
         }
     }
 
@@ -920,6 +963,12 @@ private RepositoryData getRepositoryData(long indexGen) {
                 return RepositoryData.snapshotsFromXContent(parser, indexGen);
             }
         } catch (IOException ioe) {
+            // If we fail to load the generation we tracked in latestKnownRepoGen we reset it.
+            // This is done as a fail-safe in case a user manually deletes the contents of the repository in which case subsequent
+            // operations must start from the EMPTY_REPO_GEN again
+            if (latestKnownRepoGen.compareAndSet(indexGen, RepositoryData.EMPTY_REPO_GEN)) {
+                logger.warn("Resetting repository generation tracker because we failed to read generation [" + indexGen + "]", ioe);
+            }
             throw new RepositoryException(metadata.name(), "could not read repository data from index blob", ioe);
         }
     }
@@ -945,11 +994,21 @@ protected void writeIndexGen(final RepositoryData repositoryData, final long exp
                                               "] - possibly due to simultaneous snapshot deletion requests");
         }
         final long newGen = currentGen + 1;
+        if (latestKnownRepoGen.get() >= newGen) {
+            throw new IllegalArgumentException(
+                "Tried writing generation [" + newGen + "] but repository is at least at generation [" + newGen + "] already");
+        }
         // write the index file
         final String indexBlob = INDEX_FILE_PREFIX + Long.toString(newGen);
         logger.debug("Repository [{}] writing new index generational blob [{}]", metadata.name(), indexBlob);
         writeAtomic(indexBlob,
             BytesReference.bytes(repositoryData.snapshotsToXContent(XContentFactory.jsonBuilder(), writeShardGens)), true);
+        final long latestKnownGen = latestKnownRepoGen.updateAndGet(known -> Math.max(known, newGen));
+        if (newGen < latestKnownGen) {
+            // Don't mess up the index.latest blob
+            throw new IllegalStateException(
+                "Wrote generation [" + newGen + "] but latest known repo gen concurrently changed to [" + latestKnownGen + "]");
+        }
         // write the current generation to the index-latest file
         final BytesReference genBytes;
         try (BytesStreamOutput bStream = new BytesStreamOutput()) {

server/src/test/java/org/elasticsearch/snapshots/SnapshotResiliencyTests.java

@@ -396,7 +396,7 @@ public void testConcurrentSnapshotCreateAndDelete() {
         final StepListener<CreateSnapshotResponse> createAnotherSnapshotResponseStepListener = new StepListener<>();
 
         continueOrDie(deleteSnapshotStepListener, acknowledgedResponse -> masterNode.client.admin().cluster()
-            .prepareCreateSnapshot(repoName, snapshotName).execute(createAnotherSnapshotResponseStepListener));
+            .prepareCreateSnapshot(repoName, snapshotName).setWaitForCompletion(true).execute(createAnotherSnapshotResponseStepListener));
         continueOrDie(createAnotherSnapshotResponseStepListener, createSnapshotResponse ->
             assertEquals(createSnapshotResponse.getSnapshotInfo().state(), SnapshotState.SUCCESS));
 
@@ -1133,7 +1133,7 @@ protected void assertSnapshotOrGenericThread() {
                 } else {
                     return metaData -> {
                         final Repository repository = new MockEventuallyConsistentRepository(
-                            metaData, xContentRegistry(), deterministicTaskQueue.getThreadPool(), blobStoreContext);
+                            metaData, xContentRegistry(), deterministicTaskQueue.getThreadPool(), blobStoreContext, random());
                         repository.start();
                         return repository;
                     };

server/src/test/java/org/elasticsearch/snapshots/mockstore/MockEventuallyConsistentRepository.java

@@ -47,6 +47,7 @@
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Random;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicLong;
 import java.util.function.Function;
@@ -64,6 +65,8 @@
  */
 public class MockEventuallyConsistentRepository extends BlobStoreRepository {
 
+    private final Random random;
+
     private final Context context;
 
     private final NamedXContentRegistry namedXContentRegistry;
@@ -72,10 +75,12 @@ public MockEventuallyConsistentRepository(
         final RepositoryMetaData metadata,
         final NamedXContentRegistry namedXContentRegistry,
         final ThreadPool threadPool,
-        final Context context) {
+        final Context context,
+        final Random random) {
         super(metadata, namedXContentRegistry, threadPool, BlobPath.cleanPath());
         this.context = context;
         this.namedXContentRegistry = namedXContentRegistry;
+        this.random = random;
     }
 
     // Filters out all actions that are super-seeded by subsequent actions
@@ -107,6 +112,9 @@ protected BlobStore createBlobStore() {
      */
     public static final class Context {
 
+        // Eventual consistency is only simulated as long as this flag is false
+        private boolean consistent;
+
         private final List<BlobStoreAction> actions = new ArrayList<>();
 
         /**
@@ -117,6 +125,7 @@ public void forceConsistent() {
                 final List<BlobStoreAction> consistentActions = consistentView(actions);
                 actions.clear();
                 actions.addAll(consistentActions);
+                consistent = true;
             }
         }
     }
@@ -240,14 +249,14 @@ public Map<String, BlobMetaData> listBlobs() {
                 ensureNotClosed();
                 final String thisPath = path.buildAsString();
                 synchronized (context.actions) {
-                    return consistentView(context.actions).stream()
+                    return maybeMissLatestIndexN(consistentView(context.actions).stream()
                         .filter(
                             action -> action.path.startsWith(thisPath) && action.path.substring(thisPath.length()).indexOf('/') == -1
                                 && action.operation == Operation.PUT)
                         .collect(
                             Collectors.toMap(
                                 action -> action.path.substring(thisPath.length()),
-                                action -> new PlainBlobMetaData(action.path.substring(thisPath.length()), action.data.length)));
+                                action -> new PlainBlobMetaData(action.path.substring(thisPath.length()), action.data.length))));
                 }
             }
 
@@ -268,9 +277,21 @@ public Map<String, BlobContainer> children() {
 
             @Override
             public Map<String, BlobMetaData> listBlobsByPrefix(String blobNamePrefix) {
-                return Maps.ofEntries(
-                    listBlobs().entrySet().stream().filter(entry -> entry.getKey().startsWith(blobNamePrefix)).collect(Collectors.toList())
-                );
+                return maybeMissLatestIndexN(
+                    Maps.ofEntries(listBlobs().entrySet().stream().filter(entry -> entry.getKey().startsWith(blobNamePrefix))
+                        .collect(Collectors.toList())));
+            }
+
+            // Randomly filter out the latest /index-N blob from a listing to test that tracking of it in latestKnownRepoGen
+            // overrides an inconsistent listing
+            private Map<String, BlobMetaData> maybeMissLatestIndexN(Map<String, BlobMetaData> listing) {
+                // Only filter out latest index-N at the repo root and only as long as we're not in a forced consistent state
+                if (path.parent() == null && context.consistent == false && random.nextBoolean()) {
+                    final Map<String, BlobMetaData> filtered = new HashMap<>(listing);
+                    filtered.remove(BlobStoreRepository.INDEX_FILE_PREFIX + latestKnownRepoGen.get());
+                    return Map.copyOf(filtered);
+                }
+                return listing;
             }
 
             @Override

server/src/test/java/org/elasticsearch/snapshots/mockstore/MockEventuallyConsistentRepositoryTests.java

@@ -50,7 +50,7 @@ public void testReadAfterWriteConsistently() throws IOException {
         MockEventuallyConsistentRepository.Context blobStoreContext = new MockEventuallyConsistentRepository.Context();
         try (BlobStoreRepository repository = new MockEventuallyConsistentRepository(
             new RepositoryMetaData("testRepo", "mockEventuallyConsistent", Settings.EMPTY),
-            xContentRegistry(), mock(ThreadPool.class), blobStoreContext)) {
+            xContentRegistry(), mock(ThreadPool.class), blobStoreContext, random())) {
             repository.start();
             final BlobContainer blobContainer = repository.blobStore().blobContainer(repository.basePath());
             final String blobName = randomAlphaOfLength(10);
@@ -70,7 +70,7 @@ public void testReadAfterWriteAfterReadThrows() throws IOException {
         MockEventuallyConsistentRepository.Context blobStoreContext = new MockEventuallyConsistentRepository.Context();
         try (BlobStoreRepository repository = new MockEventuallyConsistentRepository(
             new RepositoryMetaData("testRepo", "mockEventuallyConsistent", Settings.EMPTY),
-            xContentRegistry(), mock(ThreadPool.class), blobStoreContext)) {
+            xContentRegistry(), mock(ThreadPool.class), blobStoreContext, random())) {
             repository.start();
             final BlobContainer blobContainer = repository.blobStore().blobContainer(repository.basePath());
             final String blobName = randomAlphaOfLength(10);
@@ -86,7 +86,7 @@ public void testReadAfterDeleteAfterWriteThrows() throws IOException {
         MockEventuallyConsistentRepository.Context blobStoreContext = new MockEventuallyConsistentRepository.Context();
         try (BlobStoreRepository repository = new MockEventuallyConsistentRepository(
             new RepositoryMetaData("testRepo", "mockEventuallyConsistent", Settings.EMPTY),
-            xContentRegistry(), mock(ThreadPool.class), blobStoreContext)) {
+            xContentRegistry(), mock(ThreadPool.class), blobStoreContext, random())) {
             repository.start();
             final BlobContainer blobContainer = repository.blobStore().blobContainer(repository.basePath());
             final String blobName = randomAlphaOfLength(10);
@@ -104,7 +104,7 @@ public void testOverwriteRandomBlobFails() throws IOException {
         MockEventuallyConsistentRepository.Context blobStoreContext = new MockEventuallyConsistentRepository.Context();
         try (BlobStoreRepository repository = new MockEventuallyConsistentRepository(
             new RepositoryMetaData("testRepo", "mockEventuallyConsistent", Settings.EMPTY),
-            xContentRegistry(), mock(ThreadPool.class), blobStoreContext)) {
+            xContentRegistry(), mock(ThreadPool.class), blobStoreContext, random())) {
             repository.start();
             final BlobContainer container = repository.blobStore().blobContainer(repository.basePath());
             final String blobName = randomAlphaOfLength(10);
@@ -121,7 +121,7 @@ public void testOverwriteShardSnapBlobFails() throws IOException {
         MockEventuallyConsistentRepository.Context blobStoreContext = new MockEventuallyConsistentRepository.Context();
         try (BlobStoreRepository repository = new MockEventuallyConsistentRepository(
             new RepositoryMetaData("testRepo", "mockEventuallyConsistent", Settings.EMPTY),
-            xContentRegistry(), mock(ThreadPool.class), blobStoreContext)) {
+            xContentRegistry(), mock(ThreadPool.class), blobStoreContext, random())) {
             repository.start();
             final BlobContainer container =
                 repository.blobStore().blobContainer(repository.basePath().add("indices").add("someindex").add("0"));
@@ -143,7 +143,7 @@ public void testOverwriteSnapshotInfoBlob() {
             new ThreadPool.Info(ThreadPool.Names.SNAPSHOT, ThreadPool.ThreadPoolType.FIXED, randomIntBetween(1, 10)));
         try (BlobStoreRepository repository = new MockEventuallyConsistentRepository(
             new RepositoryMetaData("testRepo", "mockEventuallyConsistent", Settings.EMPTY),
-            xContentRegistry(), threadPool, blobStoreContext)) {
+            xContentRegistry(), threadPool, blobStoreContext, random())) {
             repository.start();
 
             // We create a snap- blob for snapshot "foo" in the first generation