This is the error I am getting when I try to build

mbarak@ruby-debug-ide-dev:~/nixpkgs$ nix-build -I . -A ruby-debug-ide --show-trace 
these derivations will be built:
  /nix/store/qz5d4ag3gxxcdyrk4n5488a2lx8i5zf3-ruby2.6.5-ruby-debug-ide-0.7.0.drv
  /nix/store/4jvldsnnknxshz3fwxwh49rhgw8pj3ij-ruby-debug-ide-0.7.0.drv
  /nix/store/ck76j91x4n0s0s7wv3xcw1414kji1nmn-ruby-debug-ide-0.7.0.drv
building '/nix/store/qz5d4ag3gxxcdyrk4n5488a2lx8i5zf3-ruby2.6.5-ruby-debug-ide-0.7.0.drv'...
unpacking sources
patching sources
configuring
no configure script, doing nothing
installing
buildFlags: 
WARNING:  You build with buildroot.
  Build root: /
  Bin dir: /nix/store/lbm5aw087kk9g05bv1fixj5020frc7rc-ruby2.6.5-ruby-debug-ide-0.7.0/lib/ruby/gems/2.6.0/bin
  Gem home: /nix/store/lbm5aw087kk9g05bv1fixj5020frc7rc-ruby2.6.5-ruby-debug-ide-0.7.0/lib/ruby/gems/2.6.0
Building native extensions. This could take a while...
ERROR:  Error installing /nix/store/xsayw8pjzjiycsfn7wiagrlj7pz62n8s-ruby-debug-ide-0.7.0.gem:
        ERROR: Failed to build gem native extension.

    current directory: /nix/store/lbm5aw087kk9g05bv1fixj5020frc7rc-ruby2.6.5-ruby-debug-ide-0.7.0/lib/ruby/gems/2.6.0/gems/ruby-debug-ide-0.7.0/ext
/nix/store/a3wafb77xrn5wcz21sxg8l40kdlm2wny-ruby-2.6.5/bin/ruby mkrf_conf.rb
NOTE: Gem::DependencyInstaller#find_gems_with_sources is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::DependencyInstaller#find_gems_with_sources called from mkrf_conf.rb:6.
NOTE: Gem::DependencyInstaller#find_gems_with_sources is deprecated with no replacement. It will be removed on or after 2019-12-01.
Gem::DependencyInstaller#find_gems_with_sources called from mkrf_conf.rb:7.
Installing base gem
Unable to resolve dependency: user requested 'debase (> 0)'
/nix/store/a3wafb77xrn5wcz21sxg8l40kdlm2wny-ruby-2.6.5/lib/ruby/2.6.0/rubygems/resolver/installer_set.rb:72:in `add_always_install'
  /nix/store/a3wafb77xrn5wcz21sxg8l40kdlm2wny-ruby-2.6.5/lib/ruby/2.6.0/rubygems/dependency_installer.rb:394:in `resolve_dependencies'
  /nix/store/a3wafb77xrn5wcz21sxg8l40kdlm2wny-ruby-2.6.5/lib/ruby/2.6.0/rubygems/dependency_installer.rb:301:in `install'
  mkrf_conf.rb:36:in `rescue in <main>'
  mkrf_conf.rb:29:in `<main>'

rake failed, exit code 1

Gem files will remain installed in /nix/store/lbm5aw087kk9g05bv1fixj5020frc7rc-ruby2.6.5-ruby-debug-ide-0.7.0/lib/ruby/gems/2.6.0/gems/ruby-debug-ide-0.7.0 for inspection.
Results logged to /nix/store/lbm5aw087kk9g05bv1fixj5020frc7rc-ruby2.6.5-ruby-debug-ide-0.7.0/lib/ruby/gems/2.6.0/extensions/x86_64-linux/2.6.0/ruby-debug-ide-0.7.0/gem_make.out
builder for '/nix/store/qz5d4ag3gxxcdyrk4n5488a2lx8i5zf3-ruby2.6.5-ruby-debug-ide-0.7.0.drv' failed with exit code 1
cannot build derivation '/nix/store/4jvldsnnknxshz3fwxwh49rhgw8pj3ij-ruby-debug-ide-0.7.0.drv': 1 dependencies couldn't be built
cannot build derivation '/nix/store/ck76j91x4n0s0s7wv3xcw1414kji1nmn-ruby-debug-ide-0.7.0.drv': 1 dependencies couldn't be built
error: build of '/nix/store/ck76j91x4n0s0s7wv3xcw1414kji1nmn-ruby-debug-ide-0.7.0.drv' failed

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/package-ruby-debug-ide/6471/4

Why does it fail to build the native extension? Is there an error message for that somehow?

The issue is that the ruby-debug-ide gem doesn't have an explicit dependency on debase, which means that it doesn't become available at build time. Try patching the dependency tree manually to see if it fixes the error.

Try patching the dependency tree manually to see if it fixes the error.

I tried using a local version of ruby-debug-ide where I explicitely set debase as a dependency in the Gemfile. That didn't work.
Do you have anything else I could try?

Also, it seems like ruby-debug-ide tries to install its dependency twice: once with a Gemfile and once in mkrb_conf.rb here:
https://github.com/ruby-debug/ruby-debug-ide/blob/master/ext/mkrf_conf.rb#L31

This might be part of the issue.

Indeed. debase probably needs to be added to the ruby-debug-ide buildInputs. This scenario is not super well supported. Basically you want package debase, then inject it back into the bundlerApp useing the gemConfig attribute.

{ bundlerApp, defaultGemConfig
, debase // Packaged separately.
}:
bundlerApp {
  gemConfig = defaultGemConfig // {
    ruby-debug-ide = attrs: {
      buildInputs = [ debase ];
    };
  };
}

Since I added debase to the default set of gems I tried doing this in pkgs/development/ruby-modules/gem-config/default.nix

  ruby-debug-ide = attrs: {
    buildInputs = [ ruby.gems.debase ];
  };

That still didn't work...

Another question, how do I set an environment variable before the build?

I tried doing this:

bundlerApp { <snipped> } // { NO_EXT=true }

I tried doing this

bundlerApp {
...

  preBuild = ''
    export NO_EXT=1
  '';
  postBuild = ''
    export NO_EXT=1
  '';
...
}

(i added a preBuild step locally just for this)

Edit: even faster if you can find some documentation I guess.

The fastest way until someone replies might be to start digging down into the source of bundlerApp.

If it's mkDerivation based, then any extra variables you set in the arguments of bundlerApp should be passed as environment variables, and will be there for the entirety of the build process (modulo and mutations that happen inside of course). I guess this might be easier to test empirically than to dig in the source though.

Furthermore, I think function application binds stronger than //, so you're actually merging in the "environment variable" to the result of the function application, which is not what you want. You want it to be merged into the parameters.

You can play in nix repl to see whats going on:

nix-repl> let f = {...}@args: { inherit args; }; in f ({c = 1;} // { d = 2; }) 
{ args = { ... }; }

nix-repl> let f = {...}@args: { inherit args; }; in (f {c = 1;}) // { d = 2; }
{ args = { ... }; d = 2; }

nix-repl> let f = {...}@args: { inherit args; }; in f {c = 1;} // { d = 2; }   
{ args = { ... }; d = 2; }

I've been reading a lot of the internal code to figure things out.
bundlerApp uses runCommand from trivial-builders which uses the stdenv.
Unfortunately, bundlerApp doesn't support passing arbitrary values down to mkDerivation. That's why I tried // on the results of the bundlerApp call.

Thanks for the hint re nix repl. I didn't know that existed!

Hmm, yeah, as far as I can tell it looks like you're right. I'm not sure why it's like that. Maybe there's another way, or its due for a fix :) or it's not supposed to be used like that for some reason?

Actually, can you .overrideAttrs the result?

I tried that, no luck.
Is there a way to confirm that the environment variable got applied?
I don't know if NO_EXT will work. I'm trying it because based on reading the code of ruby-debug-ide it might work.

Well, you can inspect the resulting derivation with nix show-derivation or you can try printing it with a pre or a post phase or somesuch.

show-derivation will take an output path as an argument, you don't have to figure out the .drv (though you can just get it from nix repl)

• I think.

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