Fix asset host support & improve output path

[javan]

• Fixes duplicated asset hosts (Fixes javascript_pack_tag seems to add path twice with asset host #320, Closes Fix asset host duplication #335)

• Fixes that asset hosts would be ignored when defined as Proc or when set on ActionController::Base.asset_host directly

• Fixes that webpacker:compile and bin/webpack would generate dev server URLs in development mode. This should only happen when using bin/webpack-dev-server.

• Changes paths.output so that it's:

  1. Relative to public/
  2. Distinct from paths.entry
  3. Not broken if you change it. Previously webpacker would compile into paths.output/paths.entry, but wouldn't include paths.output in generated URLs. So, if you changed paths.output, you'd end up with busted URLs:

  Before:

  >> Webpacker::Configuration.paths[:entry]
  => "packs"
  
  >> Webpacker::Configuration.paths[:output]
  => "public/assets"
  
  >> Rails.application.config.action_controller.asset_host
  => "https://cdn.example.com"
  
  >> Webpacker::Manifest.lookup("application.js")
  => "https://cdn.example.com/packs/application.js"
  
  # That ^ URL would not work because the compiled file isn't there
  >> Rails.root.join("public/packs/application.js").exist?
  => false
  
  # Because the compiled file is here
  >> Rails.root.join("public/assets/packs/application.js").exist?
  => true

  After:

  >> Webpacker::Configuration.paths[:entry]
  => "packs"
  
  >> Webpacker::Configuration.paths[:output]
  => "assets"
  
  >> Webpacker::Manifest.lookup("application.js")
  => "https://cdn.example.com/assets/application.js"
  
  >> Rails.root.join("public/assets/application.js").exist?
  => true

[guilleiguaran]

Looking good

[gauravtiwari]

How about we set ASSET_HOST to dev server host in development when it's enabled? That way we don't need to replace manifest plugin.

if (env.NODE_ENV === "development") {
  env.ASSET_HOST = `//${host}:${port}` // relative to origin
}

OR

if (env.NODE_ENV === "development") {
  env.ASSET_HOST = `${devServer.https ? 'https': 'http'}://${devServer.host}:${devServer.port}`
}

We would be able to do this in development config without if, but guess we can't since we have much shared logic here.

[javan]

I described the reason for not doing it this way in the PR:

Fixes that webpacker:compile and bin/webpack would generate dev server URLs in development mode. This should only happen when using bin/webpack-dev-server.

That said, I cleaned things up in 2a94cdf by constructing and setting ASSET_HOST in bin/webpack-dev-server.

[gauravtiwari]

If we consider using ASSET_HOST env in development then I guess we don't need any of the changes here :)

[gauravtiwari]

Awesome 👍 been thinking to do this

[gauravtiwari]

🍰 🎉

[dhh]

Looks good to me!

[mltsy]

You guys are awesome! Thanks for working on this feature 👏