docs and assertions

Author: original-brownbear

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

@@ -13,6 +13,7 @@
 import org.elasticsearch.common.util.concurrent.CountDown;
 import org.elasticsearch.snapshots.SnapshotId;
 import org.elasticsearch.snapshots.SnapshotInfo;
+import org.elasticsearch.threadpool.ThreadPool;
 
 import java.util.Collection;
 import java.util.List;
@@ -46,12 +47,13 @@ public final class GetSnapshotInfoContext implements ActionListener<SnapshotInfo
      * Listener resolved when fetching {@link SnapshotInfo} has completed. If resolved successfully, no more calls to
      * {@link #consumer} will be made. Only resolves exceptionally if {@link #abortOnFailure} is true in case one or more
      * {@link SnapshotInfo} failed to be fetched.
+     * This listener is always invoked on the {@link ThreadPool.Names#SNAPSHOT_META} pool.
      */
     private final ActionListener<Void> doneListener;
 
     /**
      * {@link BiConsumer} invoked for each {@link SnapshotInfo} that is fetched with this instance and the {@code SnapshotInfo} as
-     * arguments.
+     * arguments. This consumer is always invoked on the {@link ThreadPool.Names#SNAPSHOT_META} pool.
      */
     private final BiConsumer<GetSnapshotInfoContext, SnapshotInfo> consumer;
 
@@ -102,6 +104,7 @@ public boolean done() {
 
     @Override
     public void onResponse(SnapshotInfo snapshotInfo) {
+        assert Repository.assertSnapshotMetaThread();
         try {
             consumer.accept(this, snapshotInfo);
         } catch (Exception e) {
@@ -121,6 +124,7 @@ public void onResponse(SnapshotInfo snapshotInfo) {
 
     @Override
     public void onFailure(Exception e) {
+        assert Repository.assertSnapshotMetaThread();
         if (abortOnFailure) {
             if (counter.fastForward()) {
                 failDoneListener(e);

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

@@ -25,6 +25,7 @@
 import org.elasticsearch.indices.recovery.RecoveryState;
 import org.elasticsearch.snapshots.SnapshotId;
 import org.elasticsearch.snapshots.SnapshotInfo;
+import org.elasticsearch.threadpool.ThreadPool;
 
 import java.io.IOException;
 import java.util.Collection;
@@ -81,28 +82,23 @@ default Repository create(RepositoryMetadata metadata, Function<String, Reposito
      * Reads a single snapshot description from the repository
      *
      * @param snapshotId snapshot id to read description for
-     * @param listener   listener to resolve with snapshot description
+     * @param listener   listener to resolve with snapshot description (is resolved on the {@link ThreadPool.Names#SNAPSHOT_META} pool)
      */
     default void getSnapshotInfo(SnapshotId snapshotId, ActionListener<SnapshotInfo> listener) {
-        getSnapshotInfo(
-            new GetSnapshotInfoContext(
-                List.of(snapshotId),
-                true,
-                () -> false,
-                (context, snapshotInfo) -> listener.onResponse(snapshotInfo),
-                new ActionListener<>() {
-                    @Override
-                    public void onResponse(Void o) {
-                        // ignored
-                    }
+        getSnapshotInfo(new GetSnapshotInfoContext(List.of(snapshotId), true, () -> false, (context, snapshotInfo) -> {
+            assert Repository.assertSnapshotMetaThread();
+            listener.onResponse(snapshotInfo);
+        }, new ActionListener<>() {
+            @Override
+            public void onResponse(Void o) {
+                // ignored
+            }
 
-                    @Override
-                    public void onFailure(Exception e) {
-                        listener.onFailure(e);
-                    }
-                }
-            )
-        );
+            @Override
+            public void onFailure(Exception e) {
+                listener.onFailure(e);
+            }
+        }));
     }
 
     /**
@@ -324,4 +320,11 @@ void cloneShardSnapshot(
     default Map<String, Object> adaptUserMetadata(Map<String, Object> userMetadata) {
         return userMetadata;
     }
+
+    static boolean assertSnapshotMetaThread() {
+        final String threadName = Thread.currentThread().getName();
+        assert threadName.contains('[' + ThreadPool.Names.SNAPSHOT_META + ']') || threadName.startsWith("TEST-")
+            : "Expected current thread [" + Thread.currentThread() + "] to be a snapshot meta thread.";
+        return true;
+    }
 }

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

@@ -289,6 +289,7 @@ private void startRestore(
         BiConsumer<ClusterState, Metadata.Builder> updater,
         ActionListener<RestoreCompletionResponse> listener
     ) throws IOException {
+        assert Repository.assertSnapshotMetaThread();
         final SnapshotId snapshotId = snapshotInfo.snapshotId();
         final String repositoryName = repository.getMetadata().name();
         final Snapshot snapshot = new Snapshot(repositoryName, snapshotId);