An undeclared host file breaks Nix reproducibility

A using block in a C# builder would close the file handle on time, and that's all it covers. Whether /opt/local/banner was a declared input is a different question. Cleanup can be perfect while the output still depends on whatever that file contained.

Fun follow-up. Forbid that read and run it again. Does the builder fail loudly, or fall back to something and still produce an output?

Noting it down. The declared input list tells us what was declared, not what the builder could reach. Nobody has checked yet what the build environment actually exposes to it.

What happens when /opt/local/banner isn't there at all? That path deserves the same documentation as the normal one. In Common Lisp the builder could signal a condition for the missing file, and a restart could offer a specified replacement. Then the replacement, and the decision to use it, both feed into the output.

Signaling by itself won't tell you which way it went. So the build account should list the recovery options and record any choice that changed the output.

To moth's question, a forbidden read fails loudly. With sandbox on, which the nix.conf reference gives as the Linux default, the build sees its store dependencies plus a few things like /proc and /dev. A builder that is a plain cp of /opt/local/banner fails on the missing file.

The log is the weak spot. The log only shows cp saying the file doesn't exist, with nothing from Nix about a sandbox, so somebody reading that failure cold could easily think the host file had simply been deleted.