[BUG] Remote cluster state download/restore is failing for Data stream

[dhwanilpatel]

Describe the bug
Remote cluster state download/restore is failing for Data stream. It throws the NPE and kills the process. It is able to sucessfully download the global metadata and index metadata seperatly, it is failing when we combine both in Single Metadata to return. Index Metadata which we got from remote is map of (index uuid -> index metadata). During building metadata of data stream, it finds the index metadata using index name in indices map, which returns null and causes NPE.

[2023-10-19T10:39:18,292][ERROR][o.o.b.Bootstrap          ] [2e85817919de4d9540903aefd5bf46f1] Exception
java.lang.IllegalStateException: Unable to restore remote index metadata
        at org.opensearch.index.recovery.RemoteStoreRestoreService.restore(RemoteStoreRestoreService.java:154)
        at org.opensearch.gateway.GatewayMetaState.start(GatewayMetaState.java:178)
        at org.opensearch.node.Node.start(Node.java:1444)
        at org.opensearch.bootstrap.Bootstrap.start(Bootstrap.java:339)
        at org.opensearch.bootstrap.Bootstrap.init(Bootstrap.java:413)
        at org.opensearch.bootstrap.OpenSearch.init(OpenSearch.java:180)
        at org.opensearch.bootstrap.OpenSearch.execute(OpenSearch.java:171)
        at org.opensearch.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:104)
        at org.opensearch.cli.Command.mainWithoutErrorHandling(Command.java:138)
        at org.opensearch.cli.Command.main(Command.java:101)
        at org.opensearch.bootstrap.OpenSearch.main(OpenSearch.java:137)
        at org.opensearch.bootstrap.OpenSearch.main(OpenSearch.java:103)
Caused by: java.lang.NullPointerException
        at java.base/java.util.Objects.requireNonNull(Objects.java:208)
        at java.base/java.util.ImmutableCollections$List12.<init>(ImmutableCollections.java:556)
        at java.base/java.util.List.of(List.java:1043)
        at java.base/java.util.ImmutableCollections.listCopy(ImmutableCollections.java:172)
        at java.base/java.util.List.copyOf(List.java:1068)
        at org.opensearch.cluster.metadata.IndexAbstraction$DataStream.<init>(IndexAbstraction.java:348)
        at org.opensearch.cluster.metadata.Metadata$Builder.buildIndicesLookup(Metadata.java:1652)
        at org.opensearch.cluster.metadata.Metadata$Builder.buildMetadataWithRecomputedIndicesLookups(Metadata.java:1601)
        at org.opensearch.cluster.metadata.Metadata$Builder.build(Metadata.java:1489)
        at org.opensearch.gateway.remote.RemoteClusterStateService.getLatestMetadata(RemoteClusterStateService.java:734)
        at org.opensearch.index.recovery.RemoteStoreRestoreService.restore(RemoteStoreRestoreService.java:149)
        ... 11 more

Fix
Index Metadata we get from remote store is map of (Index uuid -> Index Metadata). We need to convert it to (Index name -> Index Metadata).

OpenSearch/server/src/main/java/org/opensearch/gateway/remote/RemoteClusterStateService.java

Lines 722 to 728 in 41a12e2

	// Fetch Global Metadata
	Metadata globalMetadata = getGlobalMetadata(clusterName, clusterUUID, clusterMetadataManifest.get());
	// Fetch Index Metadata
	Map<String, IndexMetadata> indices = getIndexMetadataMap(clusterName, clusterUUID, clusterMetadataManifest.get());
	return Metadata.builder(globalMetadata).indices(indices).build();

Meta issue #10523