Example cases when it's needed:

• Alice uploads a new KDE release, after hours of uploading kdeartwork-wallpapers the network breaks, Bob runs librerelease on a single small package, db-update removes kdeartwork-wallpapers, Alice spends hours resending it.
• Alice uploads packages while new ones are building, rsync uploads some found after the signing is done, Bob uploads another package and db-update run by Bob fails due to missing signatures for Alice's packages.

There are more specific bugs in these cases and it's possible for others to be found. All of them involve different users running librerelease and db-update, so using a different staging directory for each user and making db-update run on only one such staging directory would solve them and doesn't need more complex changes.