nix-store --generate-binary-cache-key can create corrupt files

[bouk]

Describe the bug

I had a system where any nix command gives this error:

nix run -v nixpkgs#hello
unpacking 'github:NixOS/nixpkgs/59e69648d345d6e8fef86158c555730fa12af9de?narHash=sha256-IiiXB3BDTi6UqzAZcf2S797hWEPCRZOwyNThJIYhUfk%3D' into the Git cache...
this path will be fetched (0.05 MiB download, 0.24 MiB unpacked):
  /nix/store/ikcillnkc5k42229mkf7rw3hs9dzpzlf-hello-2.12.1
error:
       … while decoding key named ''

       error: key is corrupt

The reason was that I have this in my config: secret-key-files = /etc/nix/private-key which is a private key generated by running [[ -f /etc/nix/private-key ]] || nix-store --generate-binary-cache-key $HOSTNAME-1 /etc/nix/private-key /etc/nix/public-key

But, if the host is terminated while this is happening it can leave empty private and public key files on the disk. Probably we need to fsync the files after writing them or something to make this less likely to happen, or write them to a temp file first before moving.

Expected behavior

We're not left with corrupt key files.

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[xokdvium]

Hm I guess another option that would also improve robustness is to create files atomically either with O_TMPFILE + linkat or fallback to temp file + rename.

[bouk]

Yeah as a workaround I now write the keyfiles to tempfiles in the same directory and mv them. I think that's more robust but I'm not sure