Increase store ref before snapshotting index commit (#84776)

Snapshotted commits should also hold a reference to the store, so they 
are always usable; otherwise, callers need to manage the store's 
references manually. This change applies only to InternalEngine as we
already do this in ReadOnlyEngine.

docs/changelog/84776.yaml

@@ -0,0 +1,5 @@
+pr: 84776
+summary: Increase store ref before snapshotting index commit
+area: Engine
+type: bug
+issues: []

server/src/main/java/org/elasticsearch/index/engine/InternalEngine.java

@@ -2169,14 +2169,40 @@ public IndexCommitRef acquireLastIndexCommit(final boolean flushFirst) throws En
             flush(false, true);
             logger.trace("finish flush for snapshot");
         }
-        final IndexCommit lastCommit = combinedDeletionPolicy.acquireIndexCommit(false);
-        return new Engine.IndexCommitRef(lastCommit, () -> releaseIndexCommit(lastCommit));
+        store.incRef();
+        boolean success = false;
+        try {
+            final IndexCommit lastCommit = combinedDeletionPolicy.acquireIndexCommit(false);
+            final IndexCommitRef commitRef = new IndexCommitRef(
+                lastCommit,
+                () -> IOUtils.close(() -> releaseIndexCommit(lastCommit), store::decRef)
+            );
+            success = true;
+            return commitRef;
+        } finally {
+            if (success == false) {
+                store.decRef();
+            }
+        }
     }
 
     @Override
     public IndexCommitRef acquireSafeIndexCommit() throws EngineException {
-        final IndexCommit safeCommit = combinedDeletionPolicy.acquireIndexCommit(true);
-        return new Engine.IndexCommitRef(safeCommit, () -> releaseIndexCommit(safeCommit));
+        store.incRef();
+        boolean success = false;
+        try {
+            final IndexCommit safeCommit = combinedDeletionPolicy.acquireIndexCommit(true);
+            final IndexCommitRef commitRef = new IndexCommitRef(
+                safeCommit,
+                () -> IOUtils.close(() -> releaseIndexCommit(safeCommit), store::decRef)
+            );
+            success = true;
+            return commitRef;
+        } finally {
+            if (success == false) {
+                store.decRef();
+            }
+        }
     }
 
     private void releaseIndexCommit(IndexCommit snapshot) throws IOException {

server/src/main/java/org/elasticsearch/index/shard/LocalShardSnapshot.java

@@ -22,27 +22,16 @@
 import java.io.Closeable;
 import java.io.IOException;
 import java.util.Collection;
-import java.util.concurrent.atomic.AtomicBoolean;
 
 final class LocalShardSnapshot implements Closeable {
     private final IndexShard shard;
     private final Store store;
     private final Engine.IndexCommitRef indexCommit;
-    private final AtomicBoolean closed = new AtomicBoolean(false);
 
     LocalShardSnapshot(IndexShard shard) {
         this.shard = shard;
-        store = shard.store();
-        store.incRef();
-        boolean success = false;
-        try {
-            indexCommit = shard.acquireLastIndexCommit(true);
-            success = true;
-        } finally {
-            if (success == false) {
-                store.decRef();
-            }
-        }
+        this.store = shard.store();
+        this.indexCommit = shard.acquireLastIndexCommit(true);
     }
 
     Index getIndex() {
@@ -110,13 +99,7 @@ public void close() throws IOException {
 
     @Override
     public void close() throws IOException {
-        if (closed.compareAndSet(false, true)) {
-            try {
-                indexCommit.close();
-            } finally {
-                store.decRef();
-            }
-        }
+        indexCommit.close();
     }
 
     IndexMetadata getIndexMetadata() {

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -5664,6 +5664,7 @@ public void testAcquireIndexCommit() throws Exception {
         final AtomicLong globalCheckpoint = new AtomicLong(SequenceNumbers.NO_OPS_PERFORMED);
         final Engine.IndexCommitRef snapshot;
         final boolean closeSnapshotBeforeEngine = randomBoolean();
+        final int expectedDocs;
         try (InternalEngine engine = createEngine(store, createTempDir(), globalCheckpoint::get)) {
             int numDocs = between(1, 20);
             for (int i = 0; i < numDocs; i++) {
@@ -5679,6 +5680,7 @@ public void testAcquireIndexCommit() throws Exception {
             } else {
                 snapshot = engine.acquireLastIndexCommit(flushFirst);
             }
+            expectedDocs = flushFirst && safeCommit == false ? numDocs : 0;
             int moreDocs = between(1, 20);
             for (int i = 0; i < moreDocs; i++) {
                 index(engine, numDocs + i);
@@ -5687,7 +5689,7 @@ public void testAcquireIndexCommit() throws Exception {
             engine.flush();
             // check that we can still read the commit that we captured
             try (IndexReader reader = DirectoryReader.open(snapshot.getIndexCommit())) {
-                assertThat(reader.numDocs(), equalTo(flushFirst && safeCommit == false ? numDocs : 0));
+                assertThat(reader.numDocs(), equalTo(expectedDocs));
             }
             assertThat(DirectoryReader.listCommits(engine.store.directory()), hasSize(2));
 
@@ -5699,8 +5701,17 @@ public void testAcquireIndexCommit() throws Exception {
             }
         }
 
+        if (randomBoolean()) {
+            IOUtils.close(store);
+        }
+
         if (closeSnapshotBeforeEngine == false) {
-            snapshot.close(); // shouldn't throw AlreadyClosedException
+            // check that we can still read the commit that we captured
+            try (DirectoryReader reader = DirectoryReader.open(snapshot.getIndexCommit())) {
+                assertThat(reader.numDocs(), equalTo(expectedDocs));
+            } finally {
+                snapshot.close();
+            }
         }
     }
 

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/repository/CcrRestoreSourceServiceTests.java

@@ -167,8 +167,10 @@ public void testGetSessionReader() throws IOException {
 
         byte[] expectedBytes = new byte[(int) fileMetadata.length()];
         byte[] actualBytes = new byte[(int) fileMetadata.length()];
-        Engine.IndexCommitRef indexCommitRef = indexShard1.acquireSafeIndexCommit();
-        try (IndexInput indexInput = indexCommitRef.getIndexCommit().getDirectory().openInput(fileName, IOContext.READONCE)) {
+        try (
+            Engine.IndexCommitRef indexCommitRef = indexShard1.acquireSafeIndexCommit();
+            IndexInput indexInput = indexCommitRef.getIndexCommit().getDirectory().openInput(fileName, IOContext.READONCE)
+        ) {
             indexInput.seek(0);
             indexInput.readBytes(expectedBytes, 0, (int) fileMetadata.length());
         }