The logs printed by the builds include everything the build outputs itself, but also stuff from nixbuild.net. Also, if builds are running concurrently, the output will be interleaved (this is probably mostly an issue when you use the nix'
If you use the
--repeatoption of nix, which is meant for finding non-deterministic builds, all the repeated builds will run in parallel with the original build. If non-determinism is found, all builds will be cancelled. And, if the nix option
enforce-determinismis active, the final build result will be a failure.
If a build fails in a transient fashion (transient error reported by nix, or a failing builder server, or some bug in nixbuild.net), it will be restarted automatically. You will then see the build output repeated for the new run. A build can be restarted up to 3 times, after that it is failed.
There’s currently no feedback on the job scheduling. If your job have to wait for resources it will just hang. If you’re requesting resources that can’t be satisfied by the current config, your build will hang “forever”.
There is bug that sometimes (not very often) makes builds abort just when they are about to start. The builds will be restarted automatically, and the only thing you'll notice are log entries about the failure and the restart, and a slightly longer build time due to the double startup time.