When deploying Hydra different than hydra.nixos.org one may encounter a problem as building any job that uses IFD fails with:

May 22 19:41:07 hydra hydra-evaluator[6960]: error: "attempted to realize '/nix/store/1jm02mfiv58rpy8zrx95cpqxzsp64ssh-source.drv' during evaluation but 'allow-import-from-derivation' is false" May 22 19:41:07 hydra hydra-evaluator[6960]: error: "attempted to realize '/nix/store/av3jr8ix4qcadq2wm3y3hplvxwzlhl4y-source.drv' during evaluation but 'allow-import-from-derivation' is false" May 22 19:41:07 hydra hydra-evaluator[6960]: error: "attempted to realize '/nix/store/2jm02mfiv58rpy8zrx95cpqxzsp64ssh-source.drv' during evaluation but 'allow-import-from-derivation' is false"

The recent change enforced passing --no-allow-import-from-derivation to hydra-eval-job unconditionally. This change makes it configurable and defaults to NOT PASSING IT – most of the deployments allow IFDs.

The configuration option is called allow_import_from_derivation and defaults to true. It is interpreted as a boolean, with only true option being true.