nix-build: --add-root ignored

[lheckemann]

What I did: nix-build '<nixpkgs>' -A hello --add-root test
Expected behaviour: test symlink to exist afterwards, or perhaps an error message complaining that --add-root is not a recognised option, or pointing me to -o
Actual behaviour: result symlink, no test symlink, no error message

[ElvishJerricco]

Same thing with nix-shell. -o test doesn't create any symlink either. I think this is pretty serious in the nix-shell case, since now a common technique just silently fails.

[bdesham]

Is there any kind of workaround for this? Is it possible to create a GC root manually?

(I’m running into this issue in the context of commercialhaskell/stack#4673. I have to run nix-store --gc every so often, because I don’t have that much drive space left, but then the next build of my Stack-based Haskell project takes 10 minutes of maxing out all of my CPU cores.)

[lheckemann]

@bdesham yes, nix-build will create a result symlink and make it a GC root by default. You can change the name of the symlink with the -o option. The issue I'm describing here is that the --add-root option is silently ignored instead of erroring or similar.

You have multiple ways available for creating GC roots:

• Installing paths in your user profile, system profile, or home profile using nix-env or configuration.nix or home-manager respectively
• using nix-build's default behaviour, with or without -o
• using nix-store -r /nix/store/$hash-$name --add-root ./root-name --indirect
• manually by making symlinks in /nix/var/nix/gcroots — I used this in a terribly ugly script at one point to root an entire build closure: https://github.com/lheckemann/dotfiles/blob/master/gcroots.sh

[jakubgs]

I just wasted like an hour trying to figure out what I was doing wrong. Turns out nix-build just silently ignores --add-root. Not cool. It appears that nix-shell also ignores it.

[zimbatm]

lol, @jakubgs. I guess the thousands of hours that other people spent on the project don't count in the balance.

[jakubgs]

How did you read that from my comment?

[stale]

I marked this as stale due to inactivity. → More info

[bdesham]

This is still a problem.

[stale]

I marked this as stale due to inactivity. → More info

[jakubgs]

No.

[burdiyan]

I was trying to create a gcroot for my nix-shell configuration, and the problem is that nix-build doesn't work with nix-shell because it explicitly fails inside the buildPhase. So I thought that nix-instantiate would do the trick if I use it with --add-root <something> --indirect. It created the symlink for the derivation of the nix-shell, but this didn't work and everything was garbage collected.

I know that nix develop creates gcroots automatically, but creating one for nix-shell should be easier (or at least possible without trial and error).

[vincentbernat]

For me, nix develop does not create a root either.