Remove index-N Rebuild in Shard Snapshot Updates (#45740)

* There is no point in listing out every shard over and over when the `index-N` blob in the shard contains a list of all the files
   * Rebuilding the `index-N` from the `snap-${uuid}.dat` blobs does not provide any material benefit. It only would in the corner case of a corrupted `index-N` but otherwise uncorrupted blobs since we neither check the correctness of the content of all segment blobs nor do we do a similar recovery at the root of the repository.
   * Also, at least in version `6.x` we only mark a shard snapshot as successful after writing out the updated `index-N` blob so all snapshots that would work with `7.x` and newer must have correct `index-N` blobs

=> Removed the rebuilding of the `index-N` content from `snap-${uuid}.dat` files and moved to only listing `index-N` when taking a snapshot instead of listing all files
=> Removed check of file existence against physical blob listing
=> Kept full listing on the delete side to retain full cleanup of blobs that aren't referenced by the `index-N`

Author: original-brownbear

server/src/main/java/org/elasticsearch/index/snapshots/blobstore/BlobStoreIndexShardSnapshots.java

@@ -44,6 +44,8 @@
  */
 public class BlobStoreIndexShardSnapshots implements Iterable<SnapshotFiles>, ToXContentFragment {
 
+    public static final BlobStoreIndexShardSnapshots EMPTY = new BlobStoreIndexShardSnapshots(Collections.emptyList());
+
     private final List<SnapshotFiles> shardSnapshots;
     private final Map<String, FileInfo> files;
     private final Map<String, List<FileInfo>> physicalFiles;

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

@@ -843,7 +843,7 @@ public void snapshotShard(Store store, MapperService mapperService, SnapshotId s
             final BlobContainer shardContainer = shardContainer(indexId, shardId);
             final Map<String, BlobMetaData> blobs;
             try {
-                blobs = shardContainer.listBlobs();
+                blobs = shardContainer.listBlobsByPrefix(INDEX_FILE_PREFIX);
             } catch (IOException e) {
                 throw new IndexShardSnapshotFailedException(shardId, "failed to list blobs", e);
             }
@@ -888,7 +888,7 @@ public void snapshotShard(Store store, MapperService mapperService, SnapshotId s
                     List<BlobStoreIndexShardSnapshot.FileInfo> filesInfo = snapshots.findPhysicalIndexFiles(fileName);
                     if (filesInfo != null) {
                         for (BlobStoreIndexShardSnapshot.FileInfo fileInfo : filesInfo) {
-                            if (fileInfo.isSame(md) && snapshotFileExistsInBlobs(fileInfo, blobs)) {
+                            if (fileInfo.isSame(md)) {
                                 // a commit point file with the same name, size and checksum was already copied to repository
                                 // we will reuse it for this snapshot
                                 existingFileInfo = fileInfo;
@@ -1159,23 +1159,7 @@ private Tuple<BlobStoreIndexShardSnapshots, Long> buildBlobStoreIndexShardSnapsh
         } else if (blobKeys.isEmpty() == false) {
             logger.warn("Could not find a readable index-N file in a non-empty shard snapshot directory [{}]", shardContainer.path());
         }
-
-        // We couldn't load the index file - falling back to loading individual snapshots
-        List<SnapshotFiles> snapshots = new ArrayList<>();
-        for (String name : blobKeys) {
-            try {
-                BlobStoreIndexShardSnapshot snapshot = null;
-                if (name.startsWith(SNAPSHOT_PREFIX)) {
-                    snapshot = indexShardSnapshotFormat.readBlob(shardContainer, name);
-                }
-                if (snapshot != null) {
-                    snapshots.add(new SnapshotFiles(snapshot.snapshot(), snapshot.indexFiles()));
-                }
-            } catch (IOException e) {
-                logger.warn(() -> new ParameterizedMessage("Failed to read blob [{}]", name), e);
-            }
-        }
-        return new Tuple<>(new BlobStoreIndexShardSnapshots(snapshots), latest);
+        return new Tuple<>(BlobStoreIndexShardSnapshots.EMPTY, latest);
     }
 
     /**