Allow for dynamic build_url (and maybe other @config settings)

[tammersaleh]

Problem

We need to ensure some links are fully qualified URLs with scheme, host, etc.

This works when deploying to a small number of known locations by hardcoding build_url in each environment inside nanoc.yaml. However, we deploy PRs to dynamic environments that have generated hostnames via Netlify's build preview feature.

Suggestions

Some solutions that come to mind:

• Allow some targeted configuration values to be defined as environment variables, such as BUILD_URL
• Parse nanoc.yaml as ERB before consuming, so we can build_url: <%= ENV["BUILD_URL"] %>
• Reconsider implementing the original request of adding a --config flag to the nanoc CLI --config option on commandline #676

If this is already possible somehow, please let me know.

Attempted solution

I tried a hack of including this snippet at the top of Rules:

@config[:base_url] = ENV["BUILD_URL"]

This works a charm for nanoc compile, but fails with frozen errors on nanoc check:

Captain! We’ve been hit!

FrozenError: can't modify frozen Hash
You attempted to modify immutable data. Some data cannot be modified once compilation has started. Such data includes content and attributes of items and layouts, and fil
nts.

  0. /usr/local/bundle/gems/nanoc-core-4.11.14/lib/nanoc/core/configuration.rb:119:in `[]='
  1. /app/Rules:3:in `parse'
  2. /usr/local/bundle/gems/nanoc-4.11.14/lib/nanoc/rule_dsl/rules_loader.rb:26:in `instance_eval'
  3. /usr/local/bundle/gems/nanoc-4.11.14/lib/nanoc/rule_dsl/rules_loader.rb:26:in `parse'
  4. /usr/local/bundle/gems/nanoc-4.11.14/lib/nanoc/rule_dsl/rules_loader.rb:15:in `load'
  5. /usr/local/bundle/gems/nanoc-4.11.14/lib/nanoc/rule_dsl/action_provider.rb:20:in `for'
  6. /usr/local/bundle/gems/nanoc-core-4.11.14/lib/nanoc/core/compiler_loader.rb:18:in `load'
  7. /usr/local/bundle/gems/nanoc-core-4.11.14/lib/nanoc/core/compiler.rb:31:in `new_for'
  8. /usr/local/bundle/gems/nanoc-4.11.14/lib/nanoc/checking/check.rb:35:in `create'
  9. /usr/local/bundle/gems/nanoc-4.11.14/lib/nanoc/checking/runner.rb:101:in `block in run_checks'
  ... 22 lines omitted (see crash.log for details)

A detailed crash log has been written to ./crash.log.

[denisdefreyne]

Hey Tammer,

I believe that you’d be able to achieve this by sticking the @config[:base_url] = ENV["BUILD_URL"] line into the preprocess block. For example:

preprocess do
  @config[:base_url] = ENV["BUILD_URL"]
end

… and then you can use @config[:base_url] as usual.

Would this work for your use case?

[tammersaleh]

Thanks, @ddfreyne, that sure seems to have done the trick! I guess check calls @config.freeze after the preprocessing has happened? Do you think this is something we should add to the docs in some way?