'nix build --dry-run' fails on new derivations, acts like readOnlyMode

[dtzWill]

Using nixpkgs master (shouldn't matter which, but):

$ git rev-parse HEAD
6887a0fc9a802331a2d77b499f9692548899c84c

Using nix build --dry-run on an installable we don't have a .drv file for, fails:

$ nix build -f . --dry-run android-studio
don't know how to build these paths (may be caused by read-only store access):
  /nix/store/0fp7f41a6s1zfha98qf6180qghc3858y-android-studio-3.0.1.0.drv!out

Which honestly is an error I've only seen when doing unusual things previously.

nix-build on the same works as expected:

$ nix-build --dry-run -A android-studio
these derivations will be built:
  /nix/store/0fp7f41a6s1zfha98qf6180qghc3858y-android-studio-3.0.1.0.drv
these paths will be fetched (645.11 MiB download, 1151.45 MiB unpacked):
  /nix/store/5h6zhfg68lzih2r8v4bj3dqi7bkwlfml-android-studio-fhs-env-fhs
  /nix/store/8ai9s4h4x1hwszaf8mp1dl1pbnx1cjxb-android-studio
  /nix/store/955pgx40p0iz4vss7yk0b7ahc7hi1k5n-android-studio-fhs-env-init
  /nix/store/d1q9aypz215rg59bb1fgymjr0qicnymf-android-studio-fhs-env-usr-target
  /nix/store/ihdlxc3gqvj1ak9pjg521yiv0a5niqpp-android-studio-fhs-env-chrootenv-etc
  /nix/store/jdpkx4dwr7sr95dfy93vvcl3fx59sfc4-profile
  /nix/store/lrxd00h7cq8ffqrrgx3c81bfyj2gzwyr-android-studio-fhs-env-usr-multi
  /nix/store/nq7bid5yn9gia84g43aiznrs2z8jxcl6-android-studio-fhs-env
  /nix/store/xwxcr2gjv23dy8svjpv2mrqnfynrdq69-fonts.conf

And since it instantiates the .drv file, nix build works again:

$ nix build -f . --dry-run android-studio
these derivations will be built:
  /nix/store/0fp7f41a6s1zfha98qf6180qghc3858y-android-studio-3.0.1.0.drv
these paths will be fetched (645.11 MiB download, 1151.45 MiB unpacked):
  /nix/store/5h6zhfg68lzih2r8v4bj3dqi7bkwlfml-android-studio-fhs-env-fhs
  /nix/store/8ai9s4h4x1hwszaf8mp1dl1pbnx1cjxb-android-studio
  /nix/store/955pgx40p0iz4vss7yk0b7ahc7hi1k5n-android-studio-fhs-env-init
  /nix/store/d1q9aypz215rg59bb1fgymjr0qicnymf-android-studio-fhs-env-usr-target
  /nix/store/ihdlxc3gqvj1ak9pjg521yiv0a5niqpp-android-studio-fhs-env-chrootenv-etc
  /nix/store/jdpkx4dwr7sr95dfy93vvcl3fx59sfc4-profile
  /nix/store/lrxd00h7cq8ffqrrgx3c81bfyj2gzwyr-android-studio-fhs-env-usr-multi
  /nix/store/nq7bid5yn9gia84g43aiznrs2z8jxcl6-android-studio-fhs-env
  /nix/store/xwxcr2gjv23dy8svjpv2mrqnfynrdq69-fonts.conf

This is breaks one of my primary uses for --dry-build: using it after modifying expressions to see what the fallout is. Anyway I don't think it's quite the intention of --dry-run anyway.

---

FWIW this seems to have been introduced in c36467a#diff-fc4f446fc21b1365be7f6339ceb8efe8L232 although I'm not sure why, haven't looked too closely.

[Ericson2314]

I haven't looked at diff, but I could see the temptation to reduce use less fewer privileges in a dry-run case. Agreed with @dtzWill this destroys one of the primary use-cases of --dry-run so we should fix by any means necessary, but more long term its more motivation to better support in-memory drvs

[dtzWill]

Beep boop!

• is this indeed a bug, and not desired behavior?
• maybe someone wants to take a look at fixing? :)

[danbst]

was hit by this as well! --dry-run should instantiate stuff, right? cc @edolstra

[madjar]

I looked into why it was behaving that way, and it looks like the build command sets the readOnlyMode (while legacy nix-build) did not:

https://github.com/NixOS/nix/blob/master/src/nix/installables.cc#L253-L254

If that's not the desired behaviour, then removing these two lines should be enough. It looks like no other command is calling that function, so it should not have any side-effect.

[anka-213]

I think I ran into this problem too. I was using callCabal2nix, which uses IFD and got the error

error: --- EvalError ----------------------------------------------- nix
at: (83:54) in file: /nix/store/h3qd1vw6cfppz75jf49fa3d73c0sgw06-source/pkgs/development/haskell-modules/make-package-set.nix

    82|       # info that callPackage uses to determine the arguments).
    83|       drv = if stdenv.lib.isFunction fn then fn else import fn;
      |                                                      ^
    84|       auto = builtins.intersectAttrs (stdenv.lib.functionArgs drv) scope;

cannot import '/nix/store/lf5277jq85fr89pp90r2rrzk2vdlyxrn-cabal2nix-gf', since path '/nix/store/lavrxplp0lhv3aa2s56qx47sr51q5gwj-cabal2nix-gf.drv' is not valid
(use '--show-trace' to show detailed location information)

After that, even if I tried without --dry-run I got

error: --- CachedEvalError --------------------------------------- nix
cached failure of attribute 'packages.x86_64-darwin.gf'

[Atemu]

Could the dry-run flag be split into a completely dry run (--arid-run?) and a dry run that still instantiates perhaps?

[anka-213]

Could the dry-run flag be split into a completely dry run (--arid-run?) and a dry run that still instantiates perhaps?

That would be especially nice if --arid-run could show what --dry-run would download and instantiate instead of just crashing.