This will be fixed in the next release. My solution is to get a shared lock when staging, and an exclusive lock when releasing.

I don't understand how locking would help: wouldn't it delay the second librestage until librerelease finishes, keeping the CPU idle instead of building packages for several hours in some cases?

The old solution was to make a list of staged files in librerelease once and then release them. It's safe to continue staging files until the next librerelease run releases them.

That is the case: it will block librestage.

Because sets of files can't be added to the staging directory atomically, there are a couple of race conditions so that I don't want to have the list of staged files.

Perhaps if librerelease first moved files to a temporary upload directory, and only held the lock while it was moving the files?

Although, this could also be fixed in the programs you use to build packages. Staging doesn't need to block building. treepkg/fullpkg could run the staging commands in the background. Also, they could be learn how to do parallel builds by spawning new chroot copies.

I'm closing this, and creating a "librerelease block librestage" bug. https://labs.parabola.nu/issues/410