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Integer underflow when repacking tapes where number of files in storage class is less than number of existing tape copies

Summary

In Scheduler::expandRepackRequest():

filesToArchive is defined like this:

//We are in the case where we possibly need to create new copies (if the number of copies the storage class of the current ArchiveFile
//is greater than the number of tape files we have in the current ArchiveFile)
uint64_t nbFilesAlreadyArchived = getNbFilesAlreadyArchived(archiveFile);
uint64_t nbCopiesInStorageClass = sc.nbCopies;
uint64_t filesToArchive = nbCopiesInStorageClass - nbFilesAlreadyArchived;

If nbFilesAlreadyArchived < nbCopiesInStorageClass, filesToArchive is an integer underflow.

This causes the following exception:

if(retrieveSubRequest.copyNbsToRearchive.size() < filesToArchive){
          deleteRepackBuffer(std::move(dir),lc);
          throw ExpandRepackRequestException("In Scheduler::expandRepackRequest(): Missing archive routes for the creation of the new copies of the files");
}

Steps to reproduce

  1. Reduce the number of copies in a dual-copy storage class from 2 to 1:
$ cta-admin sc ch -n backup_zvault -c 1
  1. Repack a tape with files in that storage class.

What is the current bug behaviour?

Repack stops with exception, In Scheduler::expandRepackRequest(): Missing archive routes for the creation of the new copies of the files.

What is the expected correct behaviour?

Repack should repack one copy of the file.