A simple nix binary cache upload daemon.
Often, when building nix packages, one would then use a post-build hook to sign and upload the built paths artifacts. An example on how to do this is shown in official documentation.
There are caveats however. Notably, doing this blocks the build loop so your machine can sit there signing paths/uploading results while not doing useful building work in meantime.
The documentation suggests:
A more advanced implementation might pass the store paths to a user-supplied daemon or queue for processing the store paths outside of the build loop.
This is that "daemon".
The design is very simple: start the process, making note od the process ID. In
the post-build hook, simply feed the paths to the stdin of the process. That's
it. The process then takes care of signing and uploading all the paths, without
blocking the builds.
Once done, send SIGTERM to the process. It'll exit once it's done with whatever left-over work.