Check latest repoData before applying workaround for missing shardGen file (#112979) (#113155)

It is expected that the old master may attempt to read a shardGen file
that is deleted by the new master. This PR checks the latest repo data
before applying the workaround (or throwing AssertionError) for missing
shardGen files.

Relates: #112337 Resolves: #112811
(cherry picked from commit 99b5ed87c2bed31af1c5b298892cd6b0dddf9177)

# Conflicts:
#	muted-tests.yml

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

@@ -596,7 +596,13 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
                 existingSnapshots = tuple.v1();
             } else {
                 newGen = ShardGeneration.newGeneration();
-                existingSnapshots = buildBlobStoreIndexShardSnapshots(index, Collections.emptySet(), shardContainer, shardGeneration).v1();
+                existingSnapshots = buildBlobStoreIndexShardSnapshots(
+                    index,
+                    shardNum,
+                    Collections.emptySet(),
+                    shardContainer,
+                    shardGeneration
+                ).v1();
                 existingShardGen = shardGeneration;
             }
             SnapshotFiles existingTargetFiles = null;
@@ -1309,6 +1315,7 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
                         newGen = -1L;
                         blobStoreIndexShardSnapshots = buildBlobStoreIndexShardSnapshots(
                             indexId,
+                            shardId,
                             originalShardBlobs,
                             shardContainer,
                             originalRepositoryData.shardGenerations().getShardGen(indexId, shardId)
@@ -3203,6 +3210,7 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
             snapshotStatus.ensureNotAborted();
             Tuple<BlobStoreIndexShardSnapshots, ShardGeneration> tuple = buildBlobStoreIndexShardSnapshots(
                 context.indexId(),
+                shardId.id(),
                 blobs,
                 shardContainer,
                 generation
@@ -3848,7 +3856,7 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
             blobs = shardContainer.listBlobsByPrefix(OperationPurpose.SNAPSHOT_METADATA, SNAPSHOT_INDEX_PREFIX).keySet();
         }
 
-        return buildBlobStoreIndexShardSnapshots(indexId, blobs, shardContainer, shardGen).v1();
+        return buildBlobStoreIndexShardSnapshots(indexId, shardId, blobs, shardContainer, shardGen).v1();
     }
 
     /**
@@ -3856,6 +3864,7 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
      * the given list of blobs in the shard container.
      *
      * @param indexId    {@link IndexId} identifying the corresponding index
+     * @param shardId    The 0-based shard id, see also {@link ShardId#id()}
      * @param blobs      list of blobs in repository
      * @param generation shard generation or {@code null} in case there was no shard generation tracked in the {@link RepositoryData} for
      *                   this shard because its snapshot was created in a version older than
@@ -3864,6 +3873,7 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
      */
     private Tuple<BlobStoreIndexShardSnapshots, ShardGeneration> buildBlobStoreIndexShardSnapshots(
         IndexId indexId,
+        int shardId,
         Set<String> blobs,
         BlobContainer shardContainer,
         @Nullable ShardGeneration generation
@@ -3883,6 +3893,21 @@ public abstract class BlobStoreRepository extends AbstractLifecycleComponent imp
                     generation
                 );
             } catch (NoSuchFileException noSuchFileException) {
+                // Master may have concurrently mutated the shard generation. This can happen when master fails over
+                // which is "expected". We do not need to apply the following workaround for missing file in this case.
+                final RepositoryData currentRepositoryData;
+                try {
+                    final long latestGeneration = latestIndexBlobId();
+                    currentRepositoryData = getRepositoryData(latestGeneration);
+                } catch (Exception e) {
+                    noSuchFileException.addSuppressed(e);
+                    throw noSuchFileException;
+                }
+                final ShardGeneration latestShardGen = currentRepositoryData.shardGenerations().getShardGen(indexId, shardId);
+                if (latestShardGen == null || latestShardGen.equals(generation) == false) {
+                    throw noSuchFileException;
+                }
+
                 // This shouldn't happen (absent an external force deleting blobs from the repo) but in practice we've found bugs in the way
                 // we manipulate shard generation UUIDs under concurrent snapshot load which can lead to incorrectly deleting a referenced
                 // shard-level `index-UUID` blob during finalization. We definitely want to treat this as a test failure (see the `assert`