Debugging a Gem version issue in Nix


A user pointed this out, and I realized I’m getting the same thing: When I try to do anything in Core that loads the bundle, I get this: (at master@a47e0c1) is not yet checked out. Run `bundle install` first.

Switching to various versions of core and dev from the past two weeks (then rerunning dev up) doesn’t seem to solve the issue.

Manually clobbering and regenerating the shadowenv doesn’t solve the issue, nor does restarting my shell.

Digging into the BUNDLE_PATH, I notice:

$ ls $BUNDLE_PATH/bundler/gems | grep rails-

Meanwhile, this is not the version specified by the Gemfile.lock:

$ grep -A1 rails/rails Gemfile.lock
  revision: a47e0c19e60b17c3d0dcae240ef85de52113ea57

So, what’s actually happening here is that the bundle contains the wrong version of rails! Let’s find the Nix store path of that output:

$ nix-store -qR $DEV_PROFILE | grep cdbce

And then figure out the dependency path that led to it getting included:

$ nix why-depends $DEV_PROFILE /nix/store/dribkna63fjlimqln6r0wzn9q6rliqj2-rails-cdbce03
╚═══nix-support/shadow.lisp: …")..; generated from /nix/store/6cvzpz3k9y6p5ymsws99nzf08qzxh0jm-gem-bundle-shopify/nix-support/…
    => /nix/store/6cvzpz3k9y6p5ymsws99nzf08qzxh0jm-gem-bundle-shopify
    ╚═══lib/ruby/gems/2.7.0/bin/rails -> /nix/store/5468x4qkgvqhbxngpvqwjsfs2fa5dzsa-ruby2.7.1-railties-6.1.0.alpha/lib/ruby/gems/2.7.0/bin/rails
        => /nix/store/5468x4qkgvqhbxngpvqwjsfs2fa5dzsa-ruby2.7.1-railties-6.1.0.alpha
        ╚═══lib/ruby/gems/2.7.0/bundler/gems/rails-cdbce035a5b0 -> /nix/store/dribkna63fjlimqln6r0wzn9q6rliqj2-rails-cdbce03
            => /nix/store/dribkna63fjlimqln6r0wzn9q6rliqj2-rails-cdbce03

Let’s tidy that up to make it more readable…

$ nix why-depends $DEV_PROFILE /nix/store/dribkna63fjlimqln6r0wzn9q6rliqj2-rails-cdbce03
╚═══nix-support/shadow.lisp: …")..; generated from (gem-bundle-shopify)/nix-support/setup-hook
    => (gem-bundle-shopify)
    ╚═══lib/ruby/gems/2.7.0/bin/rails -> (ruby2.7.1-railties-6.1.0.alpha)/lib/ruby/gems/2.7.0/bin/rails
        => (ruby2.7.1-railties-6.1.0.alpha)
        ╚═══lib/ruby/gems/2.7.0/bundler/gems/rails-cdbce035a5b0 -> (rails-cdbce03)
            => (rails-cdbce03)

shadowenv-ruby-env-shopify -> gem-bundle-shopify -> ruby2.7.1-railties-6.1.0.alpha -> rails-cdbce03.

So, since the Gemfile.lock has a different version, one of these links is doing something wrong.

My first thought from this point is, let’s find the thing that’s actually consuming a Gemfile.lock to build the bundle.

So let’s find the lockfile(s!) used to construct this:

$ nix-store --query --requisites $(nix-store --query --deriver $DEV_PROFILE) | grep gemfile-and-lockfile

Two of them!

$ nix show-derivation /nix/store/j59wxgq3rd3jh02n5fslc65lm1qkl8f0-gemfile-and-lockfile.drv | jq -r '.[].inputSrcs[]' | grep lock
$ nix show-derivation /nix/store/g9n53dg0q197vpxr777236wvlzvmhky3-gemfile-and-lockfile.drv | jq -r '.[].inputSrcs[]' | grep lock

Ok, well it turns out one of those only contains rake and must be for some internal tooling thing (probably just packaging bundler), but the other one:

grep -A1 rails/rails /nix/store/nv9qiah5wvvqwh8yc5yjbnhz36ysn84x-Gemfile.lock
  revision: cdbce035a5b0ebc779dd445888ab5670f945603c

We wanted a47*, but got cdb*! This should be fixed automatically for us when we run dev up, but it doesn’t seem to be. How is this not busting the cache?

I’m going to try adding a blank line to the Gemfile.lock to see if that triggers a rebuild, but first, I’m going to back up .dev/nix-profile and .shadowenv.d so that I can trivially restore the broken state if that fixes it.

$ mkdir bak; mv .dev/nix-profile .shadowenv.d bak
$ echo >> Gemfile.lock
$ dev up

Interestingly, the build widget doesn’t seem to be aware of the build as it’s happening (nothing active in the “in progress” field, probably due to output format changes in the latest Nix version that we haven’t updated the state machine for yet, but unrelated). However, the debug logs indicate that it did build a new gem bundle.

As expected, now, we get a47*:

ls $BUNDLE_PATH/bundler/gems | grep rails-

So, the problem seems to be that Nix isn’t detecting the Gemfile.lock change. Is this an mtime issue? Maybe it’s not actually checksumming files from local paths to determine whether there’s work to do.

Let’s restore the broken state and try again:

$ rm -r .dev/nix-profile .shadowenv.d
deactivated shadowenv
run dev up to generate this project's shadowenv.
$ cp -a bak/.shadowenv.d .
activated shadowenv (node:v8.12.0, ruby:2.7.1)
$ cp -a bak/nix-profile .dev
$ git checkout Gemfile.lock

Now, what does dev up do?

$ dev up
$ ls $BUNDLE_PATH/bundler/gems | grep rails-

That is highly curious. This should be the same state as before, other than that the times of the Gemfile.lock have now been bumped.

(…wip, still debugging…)