The way in which Nixpks builds Ruby gems means that certain operations
by bundler *will not work*, namely `bundle install --redownload`.
According to the source the _cache/_ directory should have been kept,
however it seems through revisions to the file it has been purged.
Here was the comment from the original commit that introduced
buildRubyGem:
```
# Note:
# We really do need to keep the $out/${ruby.gemPath}/cache.
# This is very important in order for many parts of RubyGems/Bundler to not blow up.
# See https://github.com/bundler/bundler/issues/3327
```
Why is the _cache_ directory needed?
Bundler and RubyGems uses the cache as a source of truth.
When bundler executes `bundler install --redownload`, any gems it
discovers in the _GEM_PATH_ it assums must have their _.gem_ file
present in the cache (unaware it was installed from Nix).
Rather than downloading the gem from RubyGems the bundler code forcibly
re-installs the gem from the cache directory instead and **fails** if it
does not exist.
I've opened https://github.com/rubygems/rubygems/issues/4088 to see if
this failure should be soft and not so explicit; or fallback to fetching
the gem from scratch.
Without this change the following is the error:
```bash
> [nix-shell:~/code/nix/playground/jruby-bundler-rake]$ bundle install --force
[DEPRECATED] The `--force` option has been renamed to `--redownload`
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.jruby.ext.openssl.SecurityHelper (file:/nix/store/fis6nzrpw9pmcivr84qh5byfgm07qn10-jruby-9.2.13.0/lib/ruby/stdlib/jopenssl.jar) to field java.security.MessageDigest.provider
WARNING: Please consider reporting this to the maintainers of org.jruby.ext.openssl.SecurityHelper
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Fetching gem metadata from https://rubygems.org/.
Using bundler 2.1.4
Installing hello-world 1.2.0
Bundler::GemNotFound: Could not find hello-world-1.2.0.gem for installation
An error occurred while installing hello-world (1.2.0), and Bundler
cannot continue.
Make sure that `gem install hello-world -v '1.2.0' --source
'https://rubygems.org/'` succeeds before bundling.
```
Wth the fix the following no woccurs:
```bash
[nix-shell:~/code/nix/playground/jruby-bundler-rake]$ bundle install --redownload
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.jruby.ext.openssl.SecurityHelper (file:/nix/store/69wjlj4yirp48rv1q03zxgd4xvf0150d-jruby-9.2.13.0/lib/ruby/stdlib/jopenssl.jar) to field java.security.MessageDigest.provider
WARNING: Please consider reporting this to the maintainers of org.jruby.ext.openssl.SecurityHelper
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
Fetching gem metadata from https://rubygems.org/.
Using bundler 2.1.4
Installing hello-world 1.2.0
Bundle complete! 1 Gemfile dependency, 2 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
```
```
[nix-shell:~/code/nix/playground/jruby-bundler-rake]$ ls -l /nix/store/cwl9n5073hqgpfhnw4wic13nrrgg9dn8-gem-env/lib/jruby/gems/2.5.0/cache/
total 8
lrwxrwxrwx 1 fmzakari primarygroup 102 Dec 31 1969 bundler-2.1.4.gem -> /nix/store/ifc8a0gsfkrhkv953rd4rz8bcspahi8y-bundler-2.1.4/lib/jruby/gems/2.5.0/cache/bundler-2.1.4.gem
lrwxrwxrwx 1 fmzakari primarygroup 110 Dec 31 1969 hello-world-1.2.0.gem -> /nix/store/xi9ln6n1mz2is5ppykjxqhhkpjq9zm6i-hello-world-1.2.0/lib/jruby/gems/2.5.0/cache/hello-world-1.2.0.gem
```
I have a minimal project that demonstrates this issue at https://github.com/fzakaria/jruby-bundler-nix-failure
4af8bc084a