/**
 * Attempt to read the collection and chunk metadata. May not read all the updates if the metadata
 * for the collection is being updated concurrently.
 *
 * TODO: this is unsafe without an index on the relevant chunks collection. Will be used when
 * SERVER-27714 is done.
 *
 * If the epoch changes while reading the chunks, returns an empty object.
 */
StatusWith<CollectionAndChangedChunks> getImcompletePersistediMetadataSinceVersion(
    OperationContext* opCtx,
    const NamespaceString& nss,
    ChunkVersion version) {

    try {
        CollectionAndChangedChunks collAndChunks = getPersistedMetadataSinceVersion(
            opCtx,
            nss,
            version,
            false);
        if (collAndChunks.changedChunks.empty()) {
            // Found a collections entry, but the chunks are being updated.
            return CollectionAndChangedChunks();
        }

        // Make sure the collections entry epoch has not changed since we began reading chunks --
        // an epoch change between reading the collections entry and reading the chunk metadata
        // would invalidate the chunks.

        auto afterShardCollectionsEntry = uassertStatusOK(readShardCollectionsEntry(opCtx, nss));
        if (shardCollectionEntry.getEpoch() != afterShardCollectionsEntry.getEpoch()) {
            // The collection was dropped and recreated since we began. Return empty results.
            return CollectionAndChangedChunks();
        }

        return collAndChunks;
    } catch (const DBException& ex) {
        Status status = ex.toStatus();
        if (status == ErrorCodes::NamespaceNotFound) {
            return CollectionAndChangedChunks();
        }
        return Status(
                ErrorCodes::OperationFailed,
                str::stream() << "Failed to reload local metadata due to '"
                              << status.toString()
                              << "'.");
    }
}