Allow "building" with max-jobs=0 & no remote builders configured (or improve error message to specify what is missing)

[Zocker1999NET]

Describe the bug

Despite everything being available for evaluation & realization (by utilizing substituter), Nix is unable to complete a build with --max-jobs 0 and no remote builders configured.

Steps To Reproduce

1. Configure system.autoUpgrade similar to my setup
   • (just in case, the timings are configured there)
2. provide a substitutor where the newest nixosConfiguration of the target is already built
3. upgrade target manually to newest nixosConfiguration (optional, but even then this bug occurs)
4. execute systemctl start nixos-upgrade.service and observe journal

Expected behavior

nixos-upgrade.service should be able to fully complete the "upgrade" process i.e. nix should be able to "build" everything because everything is provided by the local store and/or substituter.

Metadata

From the VM trying to perform the autoUpgrade: nix-env (Nix) 2.31.2

Additional context

The rationale for my setup is some VMs are not capable enough of building their own configuration (e.g. because they have at max 1 GiB RAM available). So I have a special builder & substituter VM, which has plenty of resources available (but is less prioritized in case of shortages). From former manual upgrades, I know that my substituter does build everything necessary for upgrading my VMs, and that the VMs are capable of accessing the substituter.

If the behavior I expect is the also the one which is currently implemented, then the error message that Nix gives is not really helpful. Because in that case I would expect that Nix at least states what derivation is missing & must therefore be built. As e.g. this is the log of one of my test VMs trying to perform a system.autoUpgrade:

Mar 01 00:26:27 immich systemd[1]: Starting NixOS Upgrade...
Mar 01 00:26:28 immich nixos-upgrade-start[14887]: building the system configuration...
Mar 01 00:26:37 immich nixos-upgrade-start[14896]: error:
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while calling the 'head' builtin
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          at /nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/lib/attrsets.nix:1696:13:
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          1695|           if length values == 1 || pred here (elemAt values 1) (head values) then
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          1696|             head values
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:              |             ^
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          1697|           else
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating the attribute 'value'
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          at /nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/lib/modules.nix:1118:7:
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          1117|     // {
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          1118|       value = addErrorContext "while evaluating the option `${showOption loc}':" value;
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:              |       ^
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:          1119|       inherit (res.defsFinal') highestPrio;
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating the option `system.build.toplevel':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating definitions from `/nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/nixos/modules/system/activation/top-level.nix':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating the option `system.systemBuilderArgs':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating definitions from `/nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/nixos/modules/system/activation/activatable-system.nix':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating the option `system.activationScripts.etc.text':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating definitions from `/nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/nixos/modules/system/etc/etc-activation.nix':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating definitions from `/nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/nixos/modules/system/etc/etc.nix':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating the option `environment.etc."ssh/ssh_config".source':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating definitions from `/nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/nixos/modules/system/etc/etc.nix':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating the option `environment.etc."ssh/ssh_config".text':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        … while evaluating definitions from `/nix/store/2fmdx1a2b9hkss7i43az0xfslhkd7hm4-source/nixos/modules/programs/ssh.nix':
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        (stack trace truncated; use '--show-trace' to show the full, detailed trace)
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        error: Unable to start any build; either increase '--max-jobs' or enable remote builds.
Mar 01 00:26:37 immich nixos-upgrade-start[14896]:        For more information run 'man nix.conf' and search for '/machines'.
Mar 01 00:26:37 immich nixos-upgrade-start[14887]: Command 'nix --extra-experimental-features 'nix-command flakes' build --print-out-paths 'github:Zocker1999NET/server#nixosConfigurations."immich.boreth.pve.6nw.de".config.system.build.toplevel' --no-link --max-jobs 0 --print-build-logs --refresh' returned non-zero exit status 1.
Mar 01 00:26:37 immich systemd[1]: nixos-upgrade.service: Main process exited, code=exited, status=1/FAILURE
Mar 01 00:26:37 immich systemd[1]: nixos-upgrade.service: Failed with result 'exit-code'.
Mar 01 00:26:37 immich systemd[1]: Failed to start NixOS Upgrade.
Mar 01 00:26:37 immich systemd[1]: nixos-upgrade.service: Consumed 10.534s CPU time, 1.8G memory peak, 160.2M read from disk, 624K written to disk, 10.5K incoming IP traffic, 5.7K outgoing IP traffic.

Checklist

• checked latest Nix manual (source)
• checked open bug issues and pull requests for possible duplicates

---

Add 👍 to issues you find important.

[lisanna-dettwyler]

This is expected behavior. From https://nix.dev/manual/nix/2.33/command-ref/opt-common#opt-max-jobs:

Setting it to 0 disallows building on the local machine, which is useful when you want builds to happen only on remote builders.

I don't think the error message can be improved. At this point the worker may have multiple goals, it would be annoying to print every single store path that it may have been assigned. You can get this information yourself by running nix build --dry-run 'github:Zocker1999NET/server#nixosConfigurations."immich.boreth.pve.6nw.de".config.system.build.toplevel'